EFFECT OF SUBSTITUENTS Oil .THE STABILITY OF SOI© LANTHANUM PHENOLATE ION PAIRS

by

Peter John Newman

A Thesis presented to the University of Surrey for the Degree of Doctor of Philosophy

Chemical Research Laboratories,University of Surrey,LONDON, S.W.ll. September, 1969.

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ABSTRACTThe objects of the present work were, firstly, to investigate the

34-effect of substituents on the stability of ion pairs of the La -ion with some phenolate anions, and, secondly, to test the applicability of the spectrophotometric method in determining the association constants of such ion pairs. The first objective requires a knowledge of the dissociation constants of the phenols and the association constants of the respective lanthanum ion pairs.

Most of the thermodynamic dissociation constants of the weak acids used in the present work have been determined with high accuracy• However, as there was some uncertainty regarding the reliability of the published data for 3“ and 5-nitro salicylaldehyde, the thermodynamic dissociation constants of these acids have been redetermined, using a modified form of the spectrophotometric method of Ernst and Menashi.

Experimental evidence for ion association in the lanthanum phenolate solutions was obtained by comparing the absorption curves recorded for two series of solutions, one of which contains the phenol only and the other,both lanthanum and the phenol. For the purpose of determining the association constants of the ion pairs studied in the present work, one new optimization and three new iteration procedures based solely on spectrophotometric measurements, were developed. By these means, the association constants of six lanthanum salicylate, three lanthanum/ salicylaldehyde and four lanthanum benzoate ion pairs have been determined.

It has been found that a rough linear correlation exists between the stability of the metal complexes and that of the corresponding proton complexes? the results for the lanthanum salicylates and benzoates falling on one straight line and the results for lanthanum/salicylaldehyde ion pairs

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falling on another. The plots of the logarithm of the association constantsfor these ion pairs, against the reciprocal of the Bjerrum distance ofclosest approach have been found to give very good straight lines, onefor the lanthanum salicylates and benzoates and another for the lanthanum/salicylaldehyde ion pairs.

It has been shown that the lanthanum salicylates involve only thecarboxylate group of the anion and that, ion-dipole interaction between

3+the La -ion and the -OH group is negligible. However, on the other hand,3+ion-dipole interaction between the La -ion and the aldehyde group plays'

some part in the stabilization of the La^+-/salicylaldehyde ion pairs.

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ACKm/LEDGEMITS

The work described in this Thesis was carried out in the

laboratories of the Chemistry Department, University of Surrey,

under the supervision of the Head of Department, Professor J.E.Salmon,

and the direction of Dr. Z.L.Ernst.

The author wishes to express his most sincere and deep

appreciation for the invaluable guidance and constant help he has

received from Dr. Z.L.Ernst during the course of this work.

The award of a Research Studentship by the Science Research

Council is also gratefully acknowledged.

Lastly, the author would like to thank his wife, who apart

from typing the text of this Thesis, has helped to support him

financially during the course of this work.

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C O N T E N T SPage number

Abstract . *. •»» »«• •»• •«*■ •«.Acknowledgements 0 0 0 0 0 0 0 0 0 0 0 0 • • • • * 000

0 0 0 • • • • I I • » • * 0 0

000 000 00 0 0 0 0 #

SECTION II. Experimental Part 1. MaterialsPart 2. Spectrophotometric measurements Part 3« Procedure used to obtain experimental

evidence for ion pair formation

• 00 « 0 0

0 0 0 0 0 0

SECTION III. The spectrophotometric determination of thedissociation constants of 3“ and 5“nitro salicylaldehyde

Theoretical ... ...Results and DiscussionSelection of wavelengthMeasurement of D and Dn o 1Dissociation constants of 3“and 5-nitro salicylaldehyde 000 000 000

SECTION IY. The spectrophotometric determination of the association constants of the ion pairs of the lanthanum ion with some substituted salicylic acids and salicylaldehydes ..

The lanthanum-salicylic acid system Evidence for ion pairing

0 0 0 0 0 0

SECTION I. Introduction ..• ... *»* ... ••• • • • 9

242529

32

34 3 6 38 38 38

41

... ... ... ...

4748

49

page numberTheoretical ••• •»• ... • <>» ..« •o • 55Results and Discussion . • • ... ........ 57Determination of Dq and D^ , 57Determination of the association constant •.. 62The lanthanum 3-methyl, lanthanum 5-chloro

and lanthanum 5-bromo salicylic acids;y stems .«» ... .. ■ ..« ••• 69

Evidence for ion association ... ... ... 69Results and Discussion ... ... ........ 76

Determination of Dq and D^ *.. ... .; * 76

Determination of the association constants i.. 83The lanthanum 3~, and 5-nitro salicylic acid

systems....... .. ... ... ... ... 84Evidence for ion association ... ... ... 84Estimation of D ... ... ... 94cDetermination of Dq and D^ ... ... ... 96Determination of the association constants of

lanthanum 3-> and 5-nitro salicylates ... 105The lanthanum salicylaldehyde systems .., 112Evidence for ion association ... ... ... 112The pH dependence of the absorption curves of

the lanthanum/salicylaldehyde system ... 113The pH dependence of the absorption curves of

the lanthanum/3-, and 5-nitro salicylaldehyde systems ... ... ... ... ... ... 115

Determination of Dq and D^ ... ... ... 115

CONTENTS (Contd.)page

Determination of the association constants of the La^+-/ salicylaldehyde system ion .pairs by means of the optimization procedure................... ...

Approximation procedures for thedetermination of association • ■constants ... ... .. * ...

Evaluation of the association constants of the La^+-/salicylaldehyde and the La^+-/3“, and 5-nitro salicylaldehyde ion pairs using iteration procedure No.l ...Iteration procedure No.2...........Iteration procedure No.3......... •

The lanthanum /4-hydroxy benzaldehydesystem ... ... ... ... ... ...

3+ /Evidence for ion pairing in the La -ion/4-hydroxy benzaldehyde system ... ...

The lanthanum/benzoic acid systems ... ...Evidence for ion association in the lanthanum

benzoate systems ... ... ... ...Determination of D and D.,...................o 1Determination of the association constants of

the lanthanum benzoate ion pairs by means of the optimization procedure ... ...

number

137

138

141149154

155

162162

164165

182

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SECTION V.

APPENDIX ..

REFERENCES

CQM I T S (Contd.)page number

Evaluation of the association constants of the lanthanum benzoate ion pairs using the iteration procedures .., 183Iteration procedure No.l ... ... 183Iteration procedures 2 and 3 • • • •♦• 191

Application of the iteration proceduresto the lanthanum salicylate ion pairs 191

Conclusions ... 208Review of the spectrophotometric methods

for determining association constants developed in the present work ... 209

226

231

S E C T I O N 1

ITOODUCTIOET

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A base may be defined quite generally as a nucleophilic species.It seems reasonable, therefore, to expect that the bases which have the strongest affinity for hydrogen ions, would also form the most stable complexes with a given metal ion. Since,stability constants are really measures of the standard free energy changes, it is not unreasonable to assume that some correlation should exist between the stability of a metal complex and that of the corresponding proton complex. Some workers [1,2,3*1 even suggested that a linear relationship should exist between the logarithm of the stability constants of the complexes (log If) which a series of closely related ligands form with a metal ion and the negative -logarithm of the acid dissociation constants (pK ) of these ligands. Theageneral form of this correlation

log K = apK +b, ...(l.1)ain which a and b are constants, was first used by Bjerrum [3] and byMartell and Calvin [2].

Empirical relationships of this kind have been found to hold bymany workers [l - 12] . For example, Van TJitert, Fernelius and Douglas[ 7 ] have shown that the logarithm of the stability constants of co-ordination

2 + 2-b 24-compounds of Cu , Ni , and Da with structurally similarySdiketones wasessentially a linear function of pK . Irving and Sossotti [9] alsoaobtained good linear correlation plots for divalent metal ions withsubstituted oxines. These studies were extended by Irving and Da Silva[ 10] to the complexes formed by a number of divalent metal ions withsubstituted imido-diacetates. They found that in every case the plot oflog K vs pK was approximately linear, a

For an adequate discussion of such correlation plots, it is clear that data of sufficiently high precision must be used. Unfortunately many

workers do not appreciate the importance of this fact and employ data of low precision and find good correlations where they do not exist and vice- versa. For example, Larsson[ll] in 1934 found a linear correlation exists for the complexes which a series of amines form with the silver ion. Bruehlman and Verhoek [ 5 ] also found that for the silver amine complexes, the plot of log vs pl^ gave two straight lines. One linewas obtained for the secondary amines and the other for the primary aliphatic amines and the pyridines. On the other hand, both Britten and Williams [lj] and Vosberg and Cogswell [14]* who also investigated the silver amines^ found no such correlation*

It was Irving and Rossotti [15] who first proposed such linear relationships oh the basis of the thermodynamic consideration for complexes of the type ML. Lebermann and Rabin [ 16] have extended the thermodynamic treatment to complexes of the type ML^. Jones and co-workers [12] suggested that linear correlations of this type are not generally valid and that even cases in which they appear to be obeyed are fortuitous. However, this view appears to haye been disproved by the extensive measurements of Perrin [ 17 ] on the complexes of the ferrous and ferric ions with amino acids and by Datta, Lebermann and Rabin [ 18,19] investigating the amino acid complexes of the cupric ion. These workers were able to show that, indeed, in the above cases a good linear correlation exists between the stability of the metal complex and that of the corresponding proton complex.A linear correlation plot of this type, with a correlation coefficient [20] as high as 0.998, was obtained by Ernst and Menashi [ 21 ] for a series of ferric salicylates. Similar results were obtained by Ernst and Herring [22] who investigated the effect of substituents on the stability of the complexes of the ferric ion with fifteen phenols. They found that the

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correlation plot was a reasonable straight line with a slope of 0.8*The correlation coefficient [20] of the plot was 0.85. The formation of the ferric phenolates may be represented by equation (1.2).

Pe3+ + HL FeL2+ + H+, ...(1.2)where HL is the phenol. By considering the free energy change for this reaction Ernst and Herring found

log K = pK& - [ (^°FeL2+ ~ M0^ ) + ( n V ” Mpe3+)]/ 2.305 HT, ...(l.3) where ji° with the appropriate subscript represents the standard chemical potential.

Equation (l.3) predicts that a plot of log K vs pK should yield aastraight line if, for a series of closely related ligands, the term ((-L°pej2-f- ~ H°HL) is either a constant or a linear function of pICn. The slope of this line will be unity in the first case but different from unity in the second. Hence, if the linear relationship holds for a given series of ligands, three cases may be distinguished,

1. Slope = lj substitution affects the stability of the metal complex to the same extent as that of the corresponding proton complexes.

2. Slope > 1; substitution affects the stability of the metal complexes to a lesser extent than that of the corresponding proton complexes.

3. Slope < I5 substitution affects the stability of the metal complexes to a greater extent than that of the corresponding proton complexes.

Since,in this case, just as in the case of the ferric salicylates? the slope was found to be less than one, these workers concluded that on the average, substitution affects tne stability of the ferric phenolate complexes to a smaller extent than that of the corresponding proton complexes*

It has been suggested that, in general, the magnitude of the slope of such correlation plots depends on a number of factors such as the

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ionization potential of the metal ion [8], nuclear repulsion between the metal ion and donor atoms [ 24-3, polarisation of the ligands by the metal ion [l2], tendency of the metal to formTf-bonds [12] and ligand field stabilizations [253. In most cases the slopes are different from unity, but are closer to unity for aliphatic’ than for aromatic ligands [ 12,25 3 . Jones, Poole, Tompkinson and Williams [l2] attempted to explain the deviations from unit slope solely in terms of -bonding effects. They concluded that with ligands whoseTT-orbitals can interact with suitable orbitals of the metal ion, slopes greater than unity should result if the central ion hasTt-acceptor properties and smaller than unity if it has

2+f^-donor properties. They also suggested that divalent ions such as Cuand NI^+ [26] acts asjT-electron donors while the Fe^+~ion [26,273 has theproperties of aTT-electron acceptor. Accordingly, for a series of closelyrelated ligands, the slopes of the correlation plots should be smaller thanunity in the former cases and greater than unity in the latter. Thesepredictions have been found to hold good in some cases but to fail inothers. Thus, Calvin and Wilson [4 ] showed that the slope of the correlationplots for both the cupric acetyl-acetonates and the cupric salicylaldehydesis less than unity. However, for the Ni^+ oxinates [9,29 3 and for the Hi^+and Cu^+ imidodiacetates [l,12,26], the slope of the correlation plots isgreater than unity. Also, as mentioned above, the slope of the correlationplot for both the ferric salicylates and ferric phenolates is less thanone. Thus, if Williams1 ideas are valid, it appears that, in some instances

3+the Fe -ion acts as aTt-electron donor.Since,it is difficult to assess directly the effect ofTT-bonding

on the magnitude of the slope of such correlation plots, it appeared desirable to extend these studies to complexes in which TF-bonding is

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absent. For this reason, it was decided to investigate the complexes ofa metal ion which is unlikely to enter into7T-bonding with a ligand such

3+as the phenolate anion* Accordingly, the La -ion has been selected for it has the xenon inert gas configuration. In addition,it does not undergo hydrolysis in acid medium nor absorb light in the ultra-violet and visible regions [ 30] , the spectral regions within which both the undissociated and singly ionized forms of the ligands used in the present work absorb light[ 31,32,33,34]. Both these factors serve to simplify the interpretationof the experimental data. However^ the fact that lanthanum salts precipitate at a pH of 8.25 limits the pH- range within which experimentswith this ion can be carried out.

Although lanthanum was first separated from the*complex earth ceria'by Mossander [ 35 ] as early as 1838, little interest was shown for the next 80 years or so in the lanthanum chemistry apart from the development of more efficient methods of isolating lanthanum. Since,the compounds are usually the most insoluble of all the rare earth compounds, fractional recrystallisation showed itself to be very useful in obtaining pure lanthanum salts. In this way, many lanthanum compounds have been prepared including lanthanum oxalate [ 36*37]» acetyl acetonate [ 38], and m-nitrobenzene sulphonate C 39J• However, the separation is so difficult that until comparatively recently lanthanum chemistry has been considered a 'fruitless time-consuming venture of little theoretical significance and no practical importance* C 40]• With the advent of modern separation techniques very pure lanthanum compounds have become readily available and since then, the interest in lanthanum chemistry has greatly increased. Most efficient among these separation techniques-'is solvent extraction and in particular the ion exchange method. The use of solvent extraction

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for the separation of the rare earths was pioneered by Selwood and Appleton [41] and by Templeton and Peterson [42,43]* who studied the distribution of La and Nd compounds between an aqueous and an alcoholic phase. However, the most effective tool for the resolution of the rare earths is ion exchange, a method largely pioneered by Spedding and co-workers [44*45*46]. This technique involves the selective elutriation of a mixture of rare earths absorbed on a cation-exchange resin by the use of complexing agents. Apart from being of great practical importance, these ion exchange researches have shown that of all the lanthanides, lanthanum appears to have the least tendency to form complexes [48,49?50,5l]•

3+ •The earliest work on the complexes of the La -ion with organic ligands arrears to be that of Schwarzeribach and co-workers. Using the method of pH titrations, they determined the stability constants of the complexes of the La^+-ion with the nitrilotriacetate (N.T.A.) [52] and with the hydroxy ethyliminodiacetate (H.I.M.L.A.) [53] complexones, and found them to be of the order of 10"^.

Peacock and James [54] in 1952 studied the complexes of the3+La -ion with a number of organic ligands such as malonate, succinate, and

fumarate by means of potentiometric measurements. They concluded the,t 3+the La -ion combines with these ligands through ion association. The

association constants of these ion pairs range- from 10^ (for lanthanummalonate) to 10 (for lanthanum fumarate).

Particularly interesting, on account of their exceptional stability,3+are the complexes of the La -ion with the amino poly-carboxylic acids,

which have been the subject of extensive studies. A very good example of complexes of this type is that formed by the lanthanum ion with 1,2. diamino cyclohexane IT TT H1H ‘ tetraacetic acid (LCTA), whose stability

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constant; according to Schwarzeribach et alia [55], is 1.8 x 10 • Even more34-stable are the complexes of the La -ion with di-ethylene triamino

pentaacetic acid (DTPA) [ 56 3 and triethylene tetraamino hexa N acetic acid(TTHA) [ 57 ] whose stability constants have been found to be 9«1 x 10"^

23and 1.2 x 10 »respectively.In 1961, Oefola et alia [ 58 ] undertook a potentiometric study of

34-the complexes of the La -ion with a number of chelating agents such asaminoacetic, malic, and salicylic acids. Of particular relevance to thepresent work is the lanthanum salicylate ion pair, which these authorsapparently formulated as a chelate and whose stability constant they foundto be 4.37 x 102.

The effect of substitution, on the stability of the complexes of 34-the La -ion with a series of salicylaldel13d.es was first investigated by

Postmus et alia [59 1 In determining the stability constants of thesecomplexes, the authors applied a method based upon spectrophotometric andpH measurements. Using the results of their measurements,they plottedthe logarithm of the stability constants of the lanthanum complexes againstthe negative logarithm of the dissociation constants of the correspondingsalicylaldehydes and obtained a reasonably good straight line.

Considerable amount of work has also been done on the complexes of 34-the La -ion with inorganic ligands. The earliest study of a complex of this

type appears to be that of C.W.Davies [6q] who estimated the. association constant of the LaSO^* ion pair using conductometric .measurements.

In 1948, Davies and James [6l] determined, also conductimetrically, the association constant for the lanthanum ferricyanide ion pair over a wide range of concentrations and at three different temperatures.. They found that their results were in accords with the .predictions of the

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Bjerrum theory [ 62 ] ?corresponding to distance of closest approach of about7-2 and concluded that in the lanthanum ferricyanide ion pair, at least

3+one water molecule is attached to the La -ion. Virtually the same resultswere obtained by James and Monk [63] and by Monk [64] for the lanthanumcobalt!cyanide ion pair.

Mattem I65] carried out a spectrophotometric investigation of 3+the complexes of the La -ion with the 'Carbonate, peroxide, thiosulphate,

chloride, chlorate, and nitrate anions 25°C in 1M NaClO^ solution. He found that, under these experimental conditions, very weak ion pairs are formed, their association constants being less than 1.

Monk [66] measured conductimetri'cally the association constants3+for the complexes of the La -ion with tri-and tetra~meta.phosph.ates,

5 6and obtained the values 5-0 x 10 and 4*6 x 10? respectively. On thebasis of the Bjerrum theory, he concluded that in these complexes the

3+La^ -ion is only slightly hydrated.Panckhurst and Woolmington [67 ] made a spectrophotometric study of

the lanthanum ferrocyanide ion pair and found it to be considerably more stable than that of the lanthanum ferricyanide ion pair.

Since, the lanthanum ion is unlikely to enter into covalent bond formation,it is reasonable to assume that all the lanthanum complexes?6Ven the most stable ones, ,are formed .through'ion association. An intimation as to the validity of this view may be obtained by evaluating the Bjerrum distance of closest approach ’a’ for the most stable complexes of the La^+-ion, i.e. those with I)CTA, DTPA, and TTHA complexones.

Let K represent the association constant of a lanthanum ion pair then, according to the Bjerrum theory (loc. cit.), in dilute solutions

1/K = (l-«)/o = (4rtVlOOO)(jzLa3+zL |e2/£Kr)3Q(b), ...(1.4)

in which K is the association constant of the ion pair, ®£the degree .of association of the electrolyte, c the concentration of the electrolyte,

3+N the Avogadro number, zTia3+ the charge on the La -ion, z^ the charge onthe ligand, e the dielectric constant of the solvent, (in the present case,water,e = 78*3)> k the Boltzman constant and e the electronic charge. Thequantity Q(b) is defined by

pb ,

Q(b) = J exp y. y . dy, ...(1.5)

e^/^ekT, ...(1.6)

and b =|z^3+Zjj|e /S'SkT .,.(1.7)

r being the distance measured from the central ion. Putting z^3+ = 3 and solving for Q(b) we have

Q(b) = IC/ 74.42 zL3. ...(1.8)

Values of the integral Q(b) in terms of the quantity b have been calculated and are given in standard text books [ 6s] . The value of b so obtained may then be used to find ’a’ for the ion pair by means of equation

(1.7).The results of these calculations for the three complexes are

summarised in the table (l.l). It can be seen from the table that, foreach of these complexes the calculations yield for 'a' a value of about o2A, which in view of the large size of the ligand molecule appears to be

much too small. It must be remembered, however that in the Bjerrum theory, the distance of closest approach is defined as the distance between the centres of two ions of opposite sign in contact, the ions being regarded

where y ~| z^a3+z

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TABLE 1.1.

LIGAHD K ZL log Q(b) b a

DCTA 1.8xl016 -4 12.3 43.5 1.97 ADTPA 9.lxl019 -5 16.04 55.0 1.95 2TTEA 1.2xl025 -6 18.79 59.5

02.16 A

as rigid spheres. Such a picture of an ion is certainly not a correctdescription of these three ligands in that they contain a number of chargedand highly flexible groups. Ligands of this type have the properties of

34-chelating agents and should be able to form with the La/ -ion highlystable complexes, through ion association alone. It is most likely that

34-in the course of such chelation, the La -ion and the ligand molecule suffersconsiderable dehydration which may greatly contribute to the stability ofthese ion pairs. Such a dehydration effect has already been suggestedby Monk in the case of the tri- and tetra-metaphosphates (loc. cit.).Further support for this conclusion may be found in the work of Betts andLahlinger [69] who showed that the formation of the complex of the La^+-ionwith EDTA results in a large increase in entropy. Such an entropy increasecan be best accounted for by assuming that in the course of the chelateformation, the ligand displaces the water molecules from the hydration

3+sphere of the La -ion.Since, the present work is primarily concerned with the ion pairs 34-of the La^ -ion with some phenols, a brief account of the Bjerrum theory

of ion association is perhaps desirable at this point.An. important part in the Bjerrum theory plays the quantity ’q*

called the ‘critical distance of separation’and defined by

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q = ]z+ 2i. je2/2elcTo ...(i.9)

It is equal to the distance of separation of the two ions at which the mutual potential energy of the two ions is eq.uaJ. to 2kT and represents the position of minimum probability of finding an ion of opposite charge on a sphere of radius q surrounding the central ion. Although the existance of ion pairs, predicted by Bjerrum, has been experimentally well established, the Bjerrum theory cannot be regarded as generally valid, at least not without suitable modifications. One defect of the Bjerrum theory is undoubtably the inadequacy of the model of rigid spherical ions which must fail in certain cases, as for example the case of the amino poly- carboxylate anions. Another objection to the Bjerrum theory is the use of the bulk value for the solvent dielectric constant, which is unjustifiable in the region close to the ion. However, in spite of its limit ations', the Bjerrum theory is still very useful in the discussion of ion association.

-There are several methods’available for the . determination of the association constants of ion pairs, of which the most widely used are conductivity, solubility, potentiometry, spectroscopy and the study of reaction rates.

The conductivity method is particularly useful in dealing with ion association in a symmetrical electrolyte, in which the ion pair will be uncharged and thus, will not contribute to the total conductance of the solution. With unsymmetrical electrolytes the position is more complicatedy since the ion pair is now a new, charged entity, contributing to the measured conductivity. Under suitable conditions the conductivity method is capable of providing very accurate values for the association constants of ion pairs as evidenced by the work of Davis and James (loc. cit.) with lanthanum ferricycanide.

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The measurement of the solubility of a sparingly soluble salt in the presence of another electrolyte was first used by Davies [?o] forthe determination of ion pair formation. It was found in order for the activity coefficients to be the same in any solution of the same ionic strength, independently of the nature of the added salt, incomplete dissociation of the electrolyte had to be taken into account. Many solubility data are difficult to interpret since considerable variation often occurs in the composition of the aqueous phase and, hence, in the stoichiometric solubility product. Calcium and thallous iodates have conveniently low solubilities and have been widely studied [71,72,73] for ion paring.

Numerous potentiometric methods have been applied to the problem of ion pair formation, the commonest being the measurement of pH, usually involving the glass electrode, Bjerrum and Schwarzenbach [ll3-,52] werelatgely responsible for the early development of this method. Several workers [74,75] have extended this method to the study of incomplete dissociation in salts of weak acids and -bases, The results' obtained by this method are often inaccurate due to the uncertainties associated with the liquid junction potential.

The changes in the optical absorption of a solution on the addition of other ions are analysed in order: to.derive the extinction Coefficient of an assumed new species (the ion pair) and thence, to evaluate its association constant. The optical density D , (in a cell of unit length), for such a system is given by

and where e and square brackets denotes the extinction coefficient andis the metal cation, X the ligand and MX the ion pair

the concentration of the species, respectively. The situation is even more complicated in cases in which the ligand is formed by the dissociation of a protic acid for then,both the undissociated and the anionic form of the acid may absorb. The analysis of the absorption spectra for such a system is not simple. In fact, according to Rossotti and Rossotti [76J, there appears to be no general method, reported in the literature, for the evaluation of the association constants for ion pairs from spectrophotometric data alone, when three species contribute significantly to the total absorbance. The difficulty of dealing with a system invdlving three or more absorbing species can be greatljr simplified if a suitable wavelength can be found at which either the metal ion or one or more form of the ligand does not absorb, thereby reducing the number of unknowns [773. Unfortunately, such a wave­length cannot always be found. Another way of overcoming the problem is to combine spectrophotometric techniques with pH measurements - the methodrecently employed by Postmus and co-workers (loc. cit.) in their study of

34-ion association between the La -ion and some substituted salicylaldehydes. The disadvantage of this method lies in the fact that it produces an additional source of error associated with pH measurements.

The measurement of reaction rates provides a useful method for estimating the degree of dissociation of salts in solution. Mien an ion of a strong electrolyte takes part in a reaction with a neutral molecule, the reaction velocity is usually more closely proportional to the concentration of the ion than to its activity [78]. Thus, when the ion is present as a salt which is incompletely dissociated, the reaction velocity may be used as a measure of its true concentration. The method, of course, can only be used in cases in which the rate of reaction is sufficiently slow that it may be conveniently measured. When this is

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possible j the association constants found by this method are in general agreement with those derived by other techniques.

Of the methods described above, the spectrophotometric procedure appears to be the most promising and that, for the following reasons -

1. Modern spectrophotometers are capable of measuring optical density with high precision; moreover, optical density measurements made at very low concentration are relatively as accurate as those made at higher concentration [79].

2. It dispenses with pH measurements.3. In many cases a wavelength can be found at which one or more

species do not absorb.Since, no general method for the determination of association

constants of ion pairs by purely spectrophotometric measurements has been reported in the literature (loc. cit.), it is clear that a new approach to this problem is necessary. In this connexion it appeared desirable to explore, in the present work, the applicability of the spectrophotometric method to the quantitative study of ion association as widely as possible.

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S E C T I O N II

EXPERIMENTAL

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PART 1MATERIALS

Unless otherwise stated, standard analytical procedures were carried out as described by Vogel C 8o3i All glassware used was either of Grade 'A1 standard or of carefully calibrated Grade ’B f standard. All reagents were of AnalaR1 grade - with the exception of the benzoic acids, phenols, sodium perchlorate, and’ lanthanum oxide.Purification of the Benzoic Acids and Phenols.

The benzoic acids and phenols - with the exception of salicylaldehyde itself which is a .liquid at room temperature - were purified by decolourising with activated charcoal (where necessary), and repeated recrystallisation from conductivity water with subsequent drying in vacuo over P2Cy Special care was taken to remove all traces of the 5-nitro- isomer from J-nltxo salicylaldehyde. This involved dissolving the impure sample of 3-nitro salicylaldehyde in HaOH solution, from which the5-nitro isomer crystallised out on standing [ s i ] . The purity of each benzoic acid or phenol was tested by melting point determinatiom and by comparing the absorption curves for solutions of the ligand with those reported in the literature. The following table (2.1) gives the origin of the sample, the colour, and the observed and literature values; of the melting points of the benzoic acids and phenols, used in the course of the present work.Purification of Salicylaldehyde.

Salicylaldehyde (Eastman Chemicals) was purified by the copper acetate method [86 ]. The purified material was a very pale yellow liquid which after 3“4'days turned cherry pink, even though it was stored in stoppered bottles in the dark. No colour change, however, was observed

-26-

TABLE 2.1

Compound Origin of Colour and Melting Point . Reference)Sample Nature of Observed Literature j

Compound Value Value • ______ °C °C •

Salicylic Acid B.D.H. White needles 159.5 159-159.5 [82]5-Methyl-Salicylic Acid

B.D.H. White needles 161.75 161,5-162 [82]

5-Chloro-Salicylic Acid

EastmanChemicals

White needles 172.25 172-173 [82]

5-Bromo-Salicylic Acid

EastmanChemicals

White needles 168.5 167.5-168.5 [82]

5-Nitro-Salicylic Acid

EastmanChemicals

White needles 231.75 232-233 [82]

3-Nitro-Salicylic Acid

B.D.H. Bright yellow needles

145.0 144.9-145.2 [82]

5-Nitro-Salicylaldehyde

EastmanChemicals

Tan needles 109.25 109-110 [83]

5-Nitro-Salicylaldehyde

EastmanChemicals

Bright yellow needles

126.0 126 [85]

4-Hydroxy-Benzaldehyde

B.D.H. White needles 116.25 115-116 [83]

4-Nitro- Benzoic Acid

Obtained by Pale yellow ^hydrolysis of needles ’AnalaR' 4-nitro Benzoyl Chloride.

241.5 241.5 [83]

3-Bromo- Benzoic Acid

EastmanChemicals

White needles 155.5 155 [83]

Benzoic Acid B.D.H. White plates 121.8 122 [83]4-Hydroxy- Benzoic Acid

Hopkin and Williams

White needles 215.25 214.5-215.5 [84]

when the purified material wsts kept under nitrogen. The purity of the material was tested by measuring the refractive index and the absorption curves for solutions of the ligand. Good agreement between observed and literature values was found [87,88,89?9o].Sodium Perchlorate

Sodium perchlorate was purified by repeated recrystallisation from conductivity water, and the resultant crystals were dried at 13Q°C and allowed to cool in a desiccator over phosphorus pentoxide.Perchloric Acid

Standard perchloric acid solutions were prepared from a 70-72% perchloric acid solution and standardised against both borax and sodium carbonate. The borax used for this purpose was purified by recrystallisatio'n from conductivity water, as described by Manov, Dehollis and Acree [91J. Sodium Hydroxide

Standard solutions of sodium hydroxide were prepared by transfering a filtered 50ya sodium hydroxide into a nickel-bottle C 92j (fitted with a soda-lime guard tube) containing a requisite amount of carbon dioxide-free conductivity water. The resulting solution was standardised with potassium hydrogen phthalate and standard perchloric acid.Lanthanum Perchlorate

A stock solution of lanthanum perchlorate was made up by dissolving 99*5$ lanthanum oxide (Johnson and Mat they) in a dilute perchloric acid solution [ 93J * The resulting solution was analysed for lanthanum by titration with E.p.T.A. in the pres.enoe of xylenol .orange..as the indicator [94]« The E.L.T.A. used for this purpose was purified by re crystallisation from, conductivity water in the manner described by Blaedel and Knight [95]. The E.B.T.Ai solution was standardised [96] against spectroscopically pure zinc (Johnson and Matthey). The concentration of free perchloric acid

-28-

in the lanthanum perchlorate stock solution was found by passing an aliquot of the solution through a cation-exchange resin (Zeocarb 225) in the hydrogen form and titrating the eluant with a standard CO^-free sodium hydroxide solution using methyl red as the indicator.

-29-

PART 11SPECTROPHQTQMETRIC MSASUREMTS.

The Instrument.All spectrophotometric measurements were made by means of a

Unicam S.P. 500 spectrophotometer. The instrument consists of a deuteriumlamp, for the range 200 - 320 mp, and a tungsten lamp, for the320 - 1000 mu. region, a monochromator with a 30° quartz prism,.andinterchangeable blue, and red photocells. The photocell output current ismeasured by balancing the drop across a load resistance of 2000 megohmswith a slide wire potentiometer calibrated in both percentage transmissionand optical density. The sensitivity control varies the electricalsensitivity by approximately 10 to 1, allowing the user to set the slitwidth and operate at predetermined band widths. A switch is also providedwhich increases the scale sensitivity by a factor of 10, permittinggreater accuracy of reading when the optical density is above unity. Theinstrument is equipped with a thermostated cell holder, in which the cellsare kept at 25 - 0.1°C.

The wavelength scale of the instrument was calibrated initiallyo oby means of the hydrogen lines at 6563A and 486IA. It was then checked

every time optical density measurements were made using a holmium glassofilter which absorbs very strongly at 3609A, The optical density scale

was calibrated using an aqueous solution of potassium chromate [ 97] in 0.05 M KOE which was found to be a suitable standard. Measurements were made occasionally at a number of wavelengths to check the optical density calibration.Optical Density Measurements

Before any optical measurements were made the spectrophotometer

was always switched on for at least one horn* to allow the machine to become electronically stable. In all spectrophotometric measurements matched silica cells were used^ these were of 1,2 or 4 cm, path length, depending on the nature of the experiment. One cell always contained the solvent, and the other the solution whose optical density was to be measured. The cells were placed in exactly the same position every time measurement^ were made. The cells were calibrated, every time optical density measurements were made, with solvent in both cells. The measured optical densities were then corrected for these small cell differences. In measurements where sodium hydroxide was employed, the cell containing - the solution was washed with water, followed by alcohol, acetone and ether. The dry cell was quickly filled with the solution containing sodium hydroxide and stoppered.

In the measurement of optical density for calculation purposes, duplicate solutions were used in all cases. The concentrations of the absorbing species were adjusted, in such a way so as to give optical density readings in the preferred range 0.2 - 0.8. For measurements made at a fixed wavelength, the specified wavelength was always approached from one direction. This was done to prevent 'backlash1. The slit width was then set and all measurements at that wavelength were made using the same slit width.

Prior to any optical density measurements the experimental solutions were kept in a thermostat at 25°C for at least one hour. After the solutions had attained thermal equilibrium, they were transferred to optical cells and the latter placed in the thermostated cell holder 5 there they were allowed to stand until the temperature of the solutions in the optical cells reached 25°C. This usually took only a few minutes. The temperature

-31-

of the solutions in the optical cells was measured by means of a thermistorbridge. The optical density was then recorded as an average of at least

+ ;three successive readings, which agreed to within - 0.2c/o.

-32-

PART IIIPROCEDURE USED TO OBTAIN EXPERIMEIJTAL EVIDENCE POE ION PAIR FOBMATION.

In the present work experimental evidence for ion associationwas obtained by applying the following procedure} Two series of solutions;,(referred to as series A and series B), were obtained by Mixing fixedamounts of a standard La(ClO^)^ solution and a standard solution of theligand, with varying1 amounts of either a standard HCIO^ or a standard NaOHsolution. In some cases, the requisite amount of HCIO^ was very small? sollsdtto minimize the experimental error sodium hydroxide was also added.To each mixture, a calculated amount of a standard NaCIO. solution and a ’ 4requisite amount of water were added, so that, in each series, thestoichiometric concentration T^ of La(ClO^)^, the stoichiometricconcentration T^ of the ligand, and the ionic strength I of the finalsolutions were maintained constant. Throughout the present work the

_2concentration of lanthanum was kept at T . « 1 x 10” mole/l., and theionic strength at I ~ 0.1. It should be pointed out, however, that inorder to observe the lower pH convergence limit in a few cases, e.g. thelanthanum, salicylates, it was found necessary to use solutions whoseionic strength was slightly greater than 0.1. In order to keep the opticaldensity within the optimum region of 0.2 - 0,8, the concentration of the

-4 -5ligand was varied from case to case in the range 2,5 x 10 - 1 x 10mole/l. The stoichiometric concentration of La(ClO^)^ was thus always

3+40 - 1000 times greater? this large excess of La -ions being used to minimize the formation of ion pairs containing more than one ligand per metal ion.

The solutions of series B were prepared in exactly the same way except that the La(C10^)^ was replaced by NaClO^ - the amount of which

was adjusted so as to keep the ionic strength of the solutions constant at I - 0.1. Thu% to each solution of series A, there corresponds a solution of series B at exactly the same stoichiometric concentration of HCIO^(and/or NaOH). The hydrogen ion concentration of the solutions, in both series A and B, was varied in a pH range which depended on the nature of the ligand.

To obtain evidence for the occurrence of ion association between 3+the La -ion and a given ligand, the absorption curves of the two series

of solutions were recorded as a function of pH, The absorption curves of all the solutions of series A were then compared with those of the corresponding solutions of series B. In all cases studied in the present work significant differences between the two types of absorption curves we re observed, the magnitude of which depended on the pH of the solutions and the wavelength, (see for example figure 4«l)» These differences were interpreted as evidence for ion association.'

S E C T I O N III

SPECTROPHOTOMETRIC DETERMINATION OE THEDISSOCIATION CONSTANTS OF 3- AND 5-

NITRO-SALICYLALDEHYDES.

-35-

The determination of the association constants for the ion pairs to he studied in the present work requires a knowledge of the dissociation constant of the respective organic acids. The thermodynamic dissociation constants of most of these acids have already" been determined with high precision by other workers, [ 31 ? 98 s.99»100,101,102,103]* The values obtained are listed in table (3 *l)• There was, however, some doubt as to the reliability of the values of the dissociation constants for the 3- sood 5- nitro-salicylaldehydes. These have been determined by ; Postmus, Magnusson and Craig [59] using a procedure involving both spectrophotometric and pH measurements. Since,this method involves two sources of error, it was decided to redetermine these quantities spectrophotometrically. Por this purpose^a modified form of the procedure reported by Ernst and co-workers [.98,105] was used. The theoretical basis of this method in explained below.

Spectrophotometric methods of measuring acid dissociation constantshave been extensively used in recent years [ 97 ? 103»104-J. As alreadymentioned, the main advantages of this method lie in the fact that opticaldensity measurements made at very low concentration are relatively asaccurate as those made at higher concentrations, and that even for verysmall dissociation constants, quite reliable results can be obtained underfavourable conditions. Thus, for example, Ernst and Menashi [3l] have

-15shown that dissociation constants as small as 10 can be measured spectrophotometrically with sufficiently high precision (-2$).

TABLE 5.1

Compound DissociationConstant

Reference

Salicylic acid 1.064xl0“5 [31]3-Methyl Salicylic acid 1.135xlO”5 [31]5-Chloro Salicylic acid 2.23x10“^ [31]5-Bromo Salicylic acid 2.20xl(f3 [31J5-Nitro Salicylic acid 7.57xlO“5 [99]3-Nitro- Salicylic acid 13.4xlO"5 [99]

? Salicylaldehyde 4.23x10“5 [100]4-Hydroxy Benzaldehyde 2.43xicf8 [.100]Benzoic acid 6.295xlO“5 tioi]4-Nitro Benzoic acid 3,6lxl0”4 [102]3-Bromo Benzoic acid 1.553xlO“4 [l'02]4-Hydroxy Benzoic acid 2.692xl0“5 [32]3-Nitro Salicylaldehyde 5.870xl0”6 present work5-Nitro Salicylaldehyde 4.676xlO~6 present work

THEORETICAL

The thermodynamic dissociation constant of a weak acid of the type HL is defined by

- h [ X." ] fj2 / [ HL j , ...(3.1)where f^ is the activity coefficient of the singly charged species, h = [H+], and all the other-symbols have the usual significance.

The optical density of a solution of a weak monobasic acid of

-37-

concentration Tt, obeying the Lambert Beer law, can be shown to be given byij

D = (»o hfj2 + Kl)/(K1 + hfj2). ! ...(3.2)

In the above equation the quantities and B^ are defined by

Do = eH L TLl ...(3.3)

TL^? *••(3.4)in which eTTT and eT~ are the molar extinction coefficients ©i* the undissociatedH 1 > Jjand dissociated forms of the a.cid, respectively, and i the optical.path length.

Equation (3*2) may be rearranged to yield£ l/(K1 + hfj2) = (D - Do)/(D1-Do). ...(3.5)The concentration of the anion L , corresponding to any given

value of h is[L_j = + hq2). ...(3.6)

By combining equations (3*5) and (3*6) we find[iTJ = (b-bo)tl/(d1-do). ...(3.7)

Let a and b represent the stoichiometric concentration of HClCh

and BaOH, respectively, then the electroneutrality condition takes here the form

h = (a - b) + [iTJ, ...(3.8)or, in view of equation (3*7)

h = (a b) + (B - - Dq). ...(3.9)By combining the last equation (3*9) with, equation (j. 2) and

rearranging we find(a_b) + (d_i,o) Ti /(B1-Bo) = K ^ - D ) / f-f (B -Bq). ...(3.10)Thus a plot of y = (a-b) + (B-Bq) /(B^-B^) against

(B^-B) /f^ (B~Bq) should yield a straight line passing through the origin, with a slope equal to K ,

-38-

Accordingly, the determination of requires the measurement of the optical densities of solutions of the nitro-salicylaldehydes, under conditions of varying pH.

RESULTS AM) DISCUSSION SELECT!OH OF WAVELENGTH

To find the most suitable wavelength, the absorption curves for solutions of the two salicylaldehydes were recorded at various pH values.The results are presented in figures' (3.1) «and (3.2),fromwhich it can be seen that the spectra consist of two distinct bands, aad that the absorption curves intersect at . well defined isosbestic pointy In both cases, the band corresponding to the presence of the undissociated species occurs at shorter wavelengths than that of the ionized species. It can be seen that in both cases the effect of changing the pH is most pronounced at the wavelength corresponding to the absorption band of the anion. Accordingly, optical density measurements were carried out at these wavelengths, that is, at A. = 425 mp for 3-nitro salicylaldehyde and at X = 405 mH tor 5-nitro salicylaldehyde,

MASUPJIiEHT OF D AM) D 0The procedure based upon equation (3.10) requires a knowledge of

and at the selected wavelength. The quantity Dq was found by measuring the optical density of solutions of the two salicylaldehydes which were 0.001, 0.005, 0.01, 0.05, and 0.101 M ’in perchloric, acid. ‘The results of these measurements, which were carried out at the selected wavelength shew that as the concentration of HCIO^ was increased, at first a decrease in the optical density of the solutions took place but over the range 0.01 - 0,10 M HCIO^, no further variation: in D was observed.

oom2:O

oCD

h-3_JoCOo

oto>X 2w sQ LLJ< g

oino-<roCOCNCO

CO

ooo

CO oCDO00

ootooLOCOCN

o*<roCO

o•o'o oo

COLO'oCOoCD

COocotO ( j -«—

N (N O o

otoCN00

A1ISN3Q IVDIldO

~h (m

/j)

-4U-oLO

OCDOCO

O

> wX ^ LU OoCO

oLO< <3 or

oCO

oo

LO

CD(NCO

OGO

O

OCD

OLO

O

OCO

CO

o10 CD& '2 5 oo

COCNCO

LO^ XO C l CNt- sr 0 ^ CO o

COo oCD

LO O00" O

o-C OLLO

CN CO v j LO _ oCD

O -J LO OC NCN

A1ISN30 I V O U d O

- 41-

Accordingly, the optical density of the solutions which were 0.1 M in HCIO^ were adopted for Dq.

For the purpose of determining the quantity D^, solutions of the salicylaldehyde were prepared which were 0.01, 0.05, and 0.10 M in NaOH.The optical density of all these solutions over this concentration range of NaOH was found to have the same value, which was therefore identified as D^. The values of Dq and for 3-nitro salicylaldehyde are given in table (3*2) and for 5-nitro salicylaldehyde in table (3*3)•

MSASUREMT OF THE DISSOCIATION CONSTANTS OF 3- AND 5-NITR0 SALICYLALDEHYDES.

For the determination of the dissociation constants of 3- and 5-nitro salicylaldehydes, varying amounts of either a standard HCIO^ or a standard NaOH solution were added to a fixed amount of a standard solution of the salicylaldehyde. To each mixture, a calculated amount of a standard NaClO^ and a requisite amount of water were added, so that the stoichiometric concentration T^ of the acid, and the ionic strength I of the final solutions were maintained constant. In order for the pH variation of optical density to be as large as possible, solutions of the salicylaldehydes were used whose pH values were of the same order of magnitude as the corresponding value of pK^. Thus, for both salicylaldehydes the pH of the solutions ranged from about 4*5 - 5*5. In general, the effect of COg on solutions of pH-^5 is negligible. Nevertheless, to minimize this effect, boiled-out water was used throughout and all experimental work was carried out in an atmosphere of nitrogen.

The ionic strength of the experimental solutions is given by I « i {[Na4] + [H+j + [0H"J + [iTj + [C104"jj. , ...(3.11)

which, in view of the electroneutrality condition,

-42-

[Na+j + [h+] = [OH'J + [IT] + [C104-j, ...(3.12)■becomes

I = [Na+ J + [H+ J, ...(3.13)

where [Na+ J - d = concentration of NaClO^ added. Since,under the

experimental conditions employed [H+ J is negligibly small compared w ith

[Na+ ]fc:0.1, then

I = [Na+ J . ...(5.14)

The activity coefficient f^ and all other activity coefficients used in the present work, were obtained from the Davies equation [reference [77] page 4lJ in the form

- log10fa.~ 0.5z2 [l*/(l + x*) - 0 .31], ...(3 .1 5 )

with a possible error not exceeding - 2$.In accordance With equation (3.10),the requisite values of the

dissociation constants were .found b y plotting

(a - b) + (D -. DQ) Tl / - Dq) against ^ - D) / S * (l> - dJ,For both acids very good straight lines were obtained, which are

shown in figures (3*3) and (3.4) » The slopes of these plots were evaluatedby appljd-ng the method of least squares, n

The precision of the experimental data was estimated by calculatingthe standard deviation cr of y_ = (a -b) + (D - D ) Tt / (D_ - D ). Thisy 1 ' o' L ' v 1 o'is defined by

o = f£(Ay)2 / (n - I)]"8", ...(3.16)'y 1

where Ay = yca-c “ y0bs n num^er observations. The standarddeviation of the slope (K^) is given by

^ = CTy ...(3.17) .in which x = (D.. - D ) / f_ ( D - D ) .x 1 o' ' 1 v o'

-43-

The experimental data and the results of these calculations are given in tables (3*2) and (3*3)» In view of the small standard deviation (less than i 2$), found in the present work for the two dissociation constants, we may conclude that the procedure discussed here is reliable.It should be noted however that the values of found in the present work are about 10 o higher than those reported by Postmus, Magnus son and Craig C 59 J ®

- 44-

03ltnLAH

OH

<3!W0 ps &■* Hjz;1CAPSofa

03 * 0*i H

CA* 0

Hi J<5

Q• •*OnHO

©O!lCQ

aot!

C3*

=LsIA03IIr<

H©OII

H

IAIOHKCALAVO

LA0H 0 co 00 LA A- CO•ct1 VO H 03 O LAK O O H O O O• © • s • •tfT* O 0 O 0 O O<1 + 1 + 1 + 1

ON O -sp O COON 03 03 03ca ON H A- vo LA6 » • « . © ©H H CA vo CO

IAO O A- CA IA A- ONH LA VO LA 03 O A-H ON 00 LA CA HK • • • 0 0 «

rH H H 03 .4* vo COJA

H 03 H A- ON OO CA ON CO H COO A- IA CA 03 03 H

• 0 • 0 • •O O O O O O

03 •O H ON ON ON ON ON ONH \ ON ON ON ON ON ON

O ON ON ON ON ON ONW H • 0 0 • « •

0 ON ON ON ON ON ONn s

-M* •O H A- A- o- A- O CAH \ CO O 03 VO A-0 A3 CA CA CA CA CA

M H 0 « « * 0 •0 H H H H H H

& B

«O H -d* •4* -4*H \ vo VO vo vo VO VO

0 CA CA CA CA CA CAK H 0 « e 0 • 0

0 H H H H H His a

VOIoH

*Oo0o+1II

• H

VOioH

KOA-co

«LAIIHfcsd

FIG

URE

3-3

DIS

SOC

IATI

ON

CO

NSTA

NT

OF

3-N

ITR

O

SA

LIC

YLA

LDE

HY

DE

.

-46-

lavoHaca

ftl-rftMmfclftCaoHft<moHH&iIA

M

*3?*oH§Ks*H'ST*

ftH

QCNca03ftO

ft

2o03

zl2OvoH CAII

.<:

HftO

H

mft a

H\CA ©a HCA OR

ft 14-d<ft !ft o< HMA

ovooar~l

II

LAOHmtN

<3

OCNCVl0H1

HCNHao1

pH00Hao+

COonHao+

CNVOoao

HVOOao+

"kVOCO0

oCOa03H

COCAaH03

ofN»aCA03

03VOaA-CA

IA-d*aVA

LAoHX

CAvoLA•CA

VO A- i—I aVO

VOHCOaCN

OCNvoaCAft

ovovoaINH

CNIAVOaH03

QOVOCNao

ovoLAfto

o-d*-d*ao

•d1ooCAao

<r*IACAeo

03CACAao

03 o O H H \OX H'd 2

HOO0H

HOOftH

HOo0H

H0Oaft

ftOoaH

ftOoaft

-d< » O H H \0)

n0

ft 2

LAIACAaH

-d1HCA0H

CAr-03aH

CACA03aH

03ONHaft

ftLAHaf t

0O H H \OX H

0<3 0

•d*VOCAcH

•d*voCAaH

•d*voCA0H

VOCAaH

VOCAaH

CO03CNaON

VO •JoHXCACOOfto+1IIH

\o

voioHX

VOrsvo

iiH&

FIG

URE

3-4.

.

DIS

SOC

IATI

ON

CO

NSTA

NT

OF

5-N

ITR

O

SA

LIC

YL

AL

DE

HY

DE

.

oin

o

oco

l<N

oCN

O

Oo ooCN

OCO

+ (q-D)

THE SPECTROPHOTOMETRIC DETERMINATION OP THEASSOCIATION CONSTANTS OP THE ION PAIRS

OP THE LANTHANUM ION WITH SOME SUBSTITUTED SALICYLIC ACIDSAND SALICYLALDEHYDES.

Since, the stability constant of the lanthanum sa,licylate complex has been already determined potentiometrically by Cefola et alia [58]» the lanthanum - salicylic acid system appeared to be the most suitable starting point for the present work,

THE LANTHANUM - SALICYLIC ACID SYSTEM

EVIDENCE FOR ION PAIRING In order to obtain evidence for ion pairing in the present system,

the procedure described in the Experimental Section was used. Accordingly, two series of solutions, A and B, were prepared, in which the stoichiometric concentration of salicylic acid T^ and the stoichiometric concentration

—A —2of lanthanum perchlorate T^ was 2.5 x 10 mole/l; * and 1 x 10 mole/l., respectively, and the ionic strength, I = 0.1; The pH of the solutions of both series ranged from -"-1.5 - corresponding to a degree ofdissociation of about 5/ to about 99/ of the acid. The absorption curves of these solutions were then recorded over the wavelength region 250 - 550m|i, as a function of pH,

It was found that over this spectral region, in .which neither La(ClO^)^, NaClO^, nor HCIO^ absorb light, the effect of the lanthanum ions on the absorption curves of salicylic acid was small. The largest effect was observed at h° (the hydrogen-ion concentration of the solutions of series B) 1 x 10 mole/l. This is shown in figure (4.1) which representsthe absorption curves for the solutions of series A and B, at thishydrogen-ion concentration.

Inspection of figure (4.1) shows that the largest differences in the optical density of the two solutions occurs at X = 32Ding. Thiswavelength, therefore, was adopted for the study of ion association in the

FIG

URE

A-1

VARI

ATIO

N OF

D°

AN

D D

WIT

H W

AVE

LEN

GTH

-

SALI

CYL

IC

ACID

-

Tl

= 2*

51 x

1CT4

mo

le/L

,

D°

Tm =

1 x 10

“2 m

ole

/L,

Eo

V

oCO

oCOCO

oCMCO

oCO

ooCO

oCDCM

o00CM

oCM

oCO

O C MLO c o c m * - ' o o >. o o r ^ c o in• I I • • • « f • ft

AJLISN3Q lV O I ld O

ro c>i

A (m/j)

-51-

lanthanum salicylate solutions. -Another reason for adopting this particular wavelength is that, here, the pH variationoof optical density was found to be the greatest. It is perhaps worth mentioning, in this connexion,, that this was also the wavelength chosen by Ernst and Menashi [jl] for their spectrophotometric evaluation of the first dissociation constant of salicylic acid.

To obtain a qualitative measure of the extent of ion pairing in the present system, the difference (AD) between the optical densities ofcorresponding solutions of series A (p) and series 33 (D°), at X = 326m(i,were plotted against h°. The variation of AH with h° is presented in figure (4*2) and table (4.1). It will be seen that, at this wavelength and over the pH range employed, H is always smaller than H°, and that the difference, AD = D° - D, increases at first with increasing h°, reaches a maximum at h0^ 1 x 10 mole/l., and then decreases.

Since, in the pH region corresponding to this maximum, the AD values are significantly larger than the random experimental error involved in optical density measurement, the conclusion must be reached that some complex formation takes place in the lanthanum salicylate solutions.

On the right hand-side of the maximum, the quantities D and D° approach each other as h° decreases until at h°>- 1 x 10"^ mole/l., when more than 99f° of the salicylic acid has dissociated, they become virtually equal, at a value of about O.23O, On the right hand-side of the maximum,D and D° converge; as h° increases and approach the value O.69O. It will be seen later that these two limiting values coincide with the values obtained in the present work, at I = 0.1, for the quantities

Dl = ei tl ** ...(4.1)and Do = eq Tl I, ...(4.2)

TABLE

4.1

VARI

ATIO

N OF

AD WITH

h -

LANT

HANU

M SA

LICY

LAT

-52-

H— -- -—

e O o Oo O H 03 03 O

O AN rv_ fN- oII CO 0 0 0 0

o o o oH

NO 02-4* fH CN CN oNO 03 NO ND o

n ft 0 0 C eo 00 H o o o

•4* ON

i! CA ON cF03 03 00 A - OCA LA ND ND O

c 0 0 ft ftC n H o O o• r» -4 '

zLB o r^. CA

ND H 03 o H03 o 03 NO NO OCA 0 0 0 0 ft

o 03 o o oII H

01 VO LA HNO CA CO NO 03o IA IA Oft » 0 o 0ft NO 03 o o o

H\ CA 03 NO NOo ON NO -4 H 03

H IA NO IA IA O0 c c ft ft 0S CA 03 O O o

031 NO 0 - CN coO ON O o Is- 03H LA oo IA •4* o

0 0 0 0 tlX Ol 03 o o o

03 O NO ON r -o o O 03 ON 03o CA H CA o

e ft 0 ft ft ftH H CA o o o

I! 03 H 03 CN•4 1 O IA CA H

S NO CA CA O« 0 ft ft ft

o CA Q o O

oo H H oft 03 IA NO IA rt

H 03 CO 02 CCS O\ 0 0 ft ft ft0 O CA O o o

H0 CA 0 - 03 LAs 03 03 CA CA o

•CF H H 03 03 oI 0 0 0 ft 0O o -4* o O oH

o CO CO ourt H H 03 03 oO 03 03 03 Q

ON 0 0 0 ft ftO o IA o O oIA

o CA •03 O H

H \II 0X H P i C l Q Q

h 3 0 0 <3Eh C*j

oooftoIIQ<‘

CO03Cooo

Q

FIG

URE

4-2

VAR

IATI

ON

OF

AD

WIT

H pH

FOR

LAN

THA

NU

M

SA

LIC

YLA

TE.

Lf)

CO

CL

<f CMo *o2 CM EO ° (O“ ? ? CMCM m— 00

O00oo

oo

av

-54-

respectively, in which eQ is the extinction coefficient of the undissociatedsalicylic acid molecules, the extinction coefficient of the HlT.'ion,and £ the length of the optical path.

This type of behaviour can he best explained by assuming that, inthe lanthanum salicylate solutions, ion pairing takes place between the

3+La -ion and the singly ionized form of salicylic acid HL and that -theextinction coefficient of the ion pair formed is not appreciably differentfrom that of the HL ion. As will be seen later, this fact can be used inobtaining an approximate value for the association constant of the ionpair. At pH values, at which most of the salicylic acid (in the absenceof lanthanum) would be in the undissociated form, little ion pairing occursand hence the optical densities of both the solutions of series A and Bwill be approximately equal to !D ,,and AD will be very small. As h°decreases, the degree of dissociation of the salicylic acid increases,and consequently, the extent of ion pairing also increases; as a result,the concentration of both the undissociated salicylic acid molecules H^Land the HL ion in the solutions of series A will be less than in thecorresponding solutions which do not contain lanthanum (series B). Since,the extinction coefficient of the complex e is approximately equal to thatoof the HL ion e , the optical density of the A series of solutions willbe less than that of the series B solutions, by an amount roughly proportionalto the decrease in the concentration in H^L brought about by ion pairing.Finally, at pH values at which most of the salicylic acid is in the singlyionized form, the contribution of HgL to the total optical density willbe small and both B° and I) will be approximately equal to D^.

The conclusion can thus be drawn that under the experimental3+ -conditions employed, ion association occurs between the La - and HL ion,

-55-

2+leading to the ion pair LaHL . It appears, therefore, that contrary, to Cefola’s suggestion, no chelation occurs in this complex.

The fact that e ei, can explained by assuming that the selectedC J.wavelength, X = corresponds to an isosbestic point for the equilibriumbetween the HL~ ion and the ion pair.

THEORETICAL

In accordance with the spectrophotometric evidence discussed inthe forgoing section, it was assumed that in the lanthanum salicylate

3+ -solutions* ion association mainly occurs between the La - and HL ions.Calculations based on data reported C106 3 on ion association in salts ofsodium and univalent anions suggest that at the total concentrations ofHa - ions and salicylic acid employed in the present work, the extent of

+ —ion pairing between the Na and HL ions would be much too small to be spectrophotometrically detectable5 the species HaHL can thus be disregarded. Since, in the pH range 1 . 5 - 5 adopted in the present work, the terms [OH"], [l2~J and [LaOH2+] are negligble, the only equilibria which need to be considered in the case of the lanthanum salicylate solutions are

H^L -— J L H+ + HL~, (4.3)■7 <~+

La + HL“ LaHL . ... (4.4)• • • (4.4)

The respective equilibrium constants are given by

where h = [h+], c = [LaHL +], and f., f9, and f., are the activity coefficients

-56-

of the singly, doubljr, and triply charged species, respectively.The concentration terms c, U a 5+], [HL”], and [h2L] are related

to one another by the expressionst l = O + [h l “] + [lyj, ...(4.7)

Tm = [La3+] + c3s[La3+], ...(4.8)since, in the present case, T ^ » T^, and hence, a fortiori, T^^c.

By combining equation (4*7) with (4.5)» it follows that[hl~] = (f^c^/^+hf.,2), ...(4.9)

[H2L] = (TL-o)hf12/(KL+hf12). ...(4.10)

The electroneutrality condition here takes the formh * (a-b) + c + [lIlT], .,,(4.11)

where a and b are the stoichiometric concentrations of perchloric acid and sodium hydroxide added, respectively.

For the solutions of salicylic acid which do not contain lanthanum, series B, equations (4.7)> (4*9)» (4.10), and (4.1l) simplify to

^ « [hi-]0 + [HjL]0, ....(4.12)

[hl“]° = TlK j/x , ...(4.13)

[h2L]° = ..,(4.14)

h° = (a-b) + [EL-]0, ...(4.15)where x = K.+ h°f. . ,..(4.16)

By means of suitable combinations and rearrangements of the above equations, it can be shown that

c = [H2l]° - [HgL] + [HL-]0 - [HL“], ...(4.17)Ah - 0 - [HL-]0 - [HL-] , ...(4.18)

-57-

Ah ~ [h2L]° - [h2L], ...(4.19)in which Ah = h -h°. •*.(4»20)

Substituting for [HL ] and [HL ]° from equations (4.9) and (4*13) into equation (4*18) yields

Ah2 x fx2 + Ah(x2+a?LK1f12-cxf12) - cxh0^ 2 = 0, ....(4.2l)which is a quadratic equation in Ah. Since, in the pH range employed in

:12 2the present case, the term Ah x f. is negligibly small, equation (4.21)

becomesAh = cxh°f12/(x2+TLK1f 2 - cxf-j2). ...(4.22)

Substituting for [hL~] from equation (4*18) into equation (4*6) and rearranging, it follows that

(Ahx + T-jX^/c =c*x, ...(4.23)

in which'1 + F/KTT/r, ...(4,24)rl

where F = f^/(^^ x ^3)° ...(4.25)

Thus a plot of y* = (Ahx + T^K^/c against x should yield a straight line with a slope equal to<x .

The optical density of the solutions of series B is given byD° « e^Hgl.]0! + ...(4.26)

In view of equations (4*13) and. (4* 14)» the last equation can be also written in the form

D°x = + D ^ f j 2. ...(4.27)

For the solutions of lanthanum salicylate (series A), the Lambert- Beer law takes the form

D - So[H2L]i + ei[ HL"]i + eoot, ...(4.28)2+where e is the extinction coefficient of LaHL , c

-58-

Upon substituting for [HL ] and [h^l] from equations (4*9) an&(4.10) into equation (4.28) and solving for c, it follows that

C = [ n ^ + hfl2Do - D^+hf^)] TL / [ n ^ + hf1\ -

' ...(4.29)where D = e Txi . ...(4.30)c c L

Combination of equation (4*29) with equation (4.27) gives

c = [ADx + Ahf,2(D ~D)] Tt / [ (D°-D )x + Alif 2(D -D )]. ...(4.3l)1 0 L C 1 0 C \ .

Finally, subtracting equation (4.28) from equation (4.26) and rearranging, we have

A h C D ^ ) - AECl + Dl0 = B,c, ...(4.32)from which it can be seen that if yn « Ah(Do-D^) - ADT^ + L^c is plottedagainst c, a straight line should he obtained with a slope equal to D ,c

RESULTS AND DISCUSSION-

' DETERMINATION OF D AND D_ o 1

:The evaluation of the association constant K requires a knowledgeof ec, eQ, and at the selected wavelength, X = 326mp. For the lasttwo coefficients, Ernst and Menashi [31] have found the values sq = 717

-3at I «s 0.8 and e . = 200 at 1 ^ 1 0 . However, as all solutions employedin the present work were maintained at I = 0.1, it was decided to re­determine these quantities by means of equation (4.27). This predicts that a plot of y = D°x against D°f^2 should give a straight line with a slope and intercept equal to Dq and D^K^ respectively.

The hydrogen ion concentration of the solutions of salicylic acid

-59-

made up for this purpose was calculated using equation (4.15) in the form h° = (a-b) + Kj!Tl /(K^ + h0^ 2), ...(4.33)

which can he transformed into a quadratic equation in h° and solved by standard methods,

The activity coefficients were evaluated by means of the Davies equation [ reference [77] pa-ge 4l] ? with a probable error Dot exceeding - 2% For I = 0.15. this yields the value f^ = 0,785* The results of these calculations are given in table (4*2), where d represents the concentration of NaClO^ in the experimental solutions. The plot cf y = D°x against h°f^ for salicylic acid is shown in figure (4*5)* In order to avoid confusion between the quantity x and the multiplication sign, in tables and diagrams where these symbols appear together, the multiplication sign will be ringed. The precision of the experimental data was estimated by calculating the standard deviation cr of y = D°x. This is defined by the equation

s RL (Ay)2/(n-l)P, .*.(4*35)where Ay = yca]_c " ^expt n ~ num^er observations. The standard deviations of the slope and intercept were found from the expression [107]

oy|n2 /(n-l)[nX x2-(l.x)2]| S, ...(4.36)

^Intercept = ^ ( 4 x)2-,K ! •••(4.37)

aSlope

inhere x = h°f^^.It will be seen from the table that the values e = 688 ando

= 228, found in the present work at I = 0.1, differ significantly from the corresponding values obtained by Ernst and Menashi (loc. cit.). Since, these differences cannot be completely accounted for by the inaccuracy of the Davies equation, it appears that eQ, and to a much greater extent vary with changing ionic strength. It ha,s been reported before that, with

TABLE

A* 2

DETE

RMIN

ATIO

N OF

D AND

D..

FOR

SALI

CYLI

C AC

ID

So-4li

<3

d„aVOCMAIIr<:

H

H

©rHOa

-4iorHMo\oA

CM

ft

OrH

VOa -a -

CMrHLA cr\

CM

o \ ir \ 4LA VO VO4 oo vorH A

ACOa -

rH

IA H OA O OA 00 VO

i—ICM CO

a- cm voo -4- voA A A(A O

o co vo -4*A co. A -4A CO A- VOA

rHrH

A+3

to

cocovoIIo©

AOoo+1<i

o

In• •yoAVO

ft

A ,+3

Ho

COCMCMIIi—I ©

AOO?!IIrHft

to

ACMCM

i—Ift

rHA

©u

CO

oIIH*P

A-rHA-II©

uO£Wdo•H>©£

AIOrHIIH*P©OOCM

©

FIG

URE

4-3

DET

ERM

INA

TIO

N

OF

D0

AND

D.

FOR

SALI

CYL

IC

AC

ID.

-61-

ro

COO

CN

OCO

certain charged species, the molar extinction coefficient is dependant on the species environment. Thus,Newton and Arcand [l08] showed that £q63+ and eQegQ^+ vary wi'kh. changes in the ionic strength of solutions in which they were measured. Gates and King [1093 have reported analogous effects concerning the absorption of CrfHgO^Clg"*" ion, end so have Coll and co­workers [lio], for the FeCl^+ion pair. The latter workers attribute this effect to the dehydrating action of the perchlorates, used for controlling the ionic: strength of the experimental solutions. Monk and his associatesE m ] have shown that the extinction coefficients of some uranyl ion pairsdepend not only upon the salt concentration, but also upon the nature of the salt used to maintain a constant ionic strength. Biggs [ll2] has emphasized also, that it cannot be assumed that activity coefficients remain constant in mixtures of constant ionic strength but of varying composition unless one of the components is always in large excess.

Since,the ionic strength of the experimental solutions employed in the present work was 0.1 throughout, the values Dq = O.69O and = 0.229 were adopted for the determination of the association constant of the lanthanum salicylate ion pair. It was also found in the evaluation of K, that use of the present values for eQ and gave more consistent results than those of Ernst and Menashi (loc. cit.)

DETEEMINATIOil OF THE ASSOCIATION CONSTANT

An inspection of equations (4*33)> (4*22), (4,23), and (4.31) showsthat if I>c were known, the values of Ah and c, corresponding to any given value of h°, could be calculated by solving equations (4*22) and (4«3l)«It would then be possible to evaluate the association constant K by plotting y ’ = (Ahx + TjK^)/c against x and measuring the slope a of the straight

line so obtained. Thus, for example, the procedure leads to a value of 21.20 x 10 for K, if D is taken to be equal to L, an assumption whichc 1

is justified by the experimental results discussed earlier. However, asthe exact value of D was not known, it was decided to obtain the bestcpossible value of K by the use of an optimization procedure which involvedthe evaluation of <x as a function of D # For this purpose, first the valuesof h° and x corresponding to any given value of (a-b) were obtained fromequations (4*33) and (4.16),respectively. The value of D was then variedcat 0.001 intervals over the range 0.21 - O.24 and for each value of !D ,oa set of Ah / c values was computed by means of equations (4*22) and (4*3l)>using the method of successive approximations. The values of y 1 were thenfound and the slope of the plot of y 1 against x determined by applying themethod of least squares. The goodness of fit at a given value of D wasoassessed by calculating the standard deviation [107] of <x . Thevariation of a^ with D is given in figure (4*4) from which it can be % oseen that as D increases, o, decreases, passes through a minimum andCthen increases.

The values of CX and D corresponding to this minimum, and which0enable the experimental data to be best fitted into equation (4*23) are<X= 4*597 and D = 0.226, From this it can be seen that, in accord with othe experimental evidence discussed previously, the value of I) is veryoclose to 3) . It will also be noted that the optimum value of K does not significantly differ from the one found by assuming Dc =

The results of these calculations, leading to K for the lanthanum salicylate ion pair, are summarized in table (4*3)> in which the precision of the experimental data is expressed in terms of the residuals Ay’ and Ay’1 and of the standard deviations o , ov and ou*. The plot ofiv G

-64-

oCNO

IDOOCNO

OCOCNCD

CN Q CNCD

OCNCNO

LOCNCD

OCNo

oCN

OCMo

TABLE

4*3

ASSO

CIAT

ION

CONSTANT

OP LANTHA

NUM

SALI

CYLA

TE

rH CM in o CT\ m oCO o tn -4 vo oK O o o O o rH« • • • • • •vo o o o o o OCM r*i 1 + + + iOJ

rH -4 o rH m m -4oX -4 -4 vo rH rH oPi -4 o n- VO o m• • • • • •

on CO in -4 -4 m•aoit rH r- tn rH n- I—1 CM

X rH CM CM i—i rH rHO o O o O O• • « • • •o o o o o o• «V rH 1 + + I + 1avoCMtn

©rHoa

KCMoo•rH11EH

rH\0HOa•4iorHXONOmCM

PIEh

VO

v.;voo

CMO

O

CMII rH n~ -4 4* -4 O 00co m o CM Is- rHn! X vo CO CM in Is- CM• • • • • ••» o o rH rH rH CM• *s i>>rHO11 moH H ON cr\ O VO O voo CM in -4 0\ -4X rH in m o n-••t • • • • • «• o -4 m CM CM rH rH

UNOrH O -4 VO VD CO CT\CM in vo O VO CO rH1 X o •4 in tn CM rHO • • • • • •rH Xl rH rH rH rH rH rH

Pi< 1

Q

CMOrH

tnOHX

©rHOa

©rHOa

ON n- CO VOrH CM CM CMO o O O* • • •o o O o

CM ON ON VDtn ON r~ rHtn tn -4 m• • • •o o o o

r- vo -4 CMtn •4 in tnCT\ CO n- vo• • • ‘ •m tn tn m

ON t— VO rHin n GN rH4 rH 4 inO • 0 ' •O rH CM rn

-4*CMO

O-4*in

oO N<-

rHCMO

cm m>4- VOm m• •o o

COcomm tn

vooo•vo

rHOo?lII«oto

COCMCM

X OPI

-4CMO•o + 1

I «to• *wr-CTNin•-4II

COm<JH.014CMOXIs-m•4u

B•d>ouPh

CMO

CO

8+i

to

CMo

COCO

TSo*§0co0fHP414

FIG

URE

i.-5

DET

ERM

INA

TIO

N

OF

THE

ASS

OC

IATI

ON

C

ON

STA

NT

FOR

LAN

THA

NU

M

SA

LIC

YL

AT

E.

un

ro

CO

CN

OCNCOzoi*

01® X

LUh-<_l>-O— J < CO

2 :32<X

(ZoLL

la

LLO

O<

a:LUf—LUa

CD>4-LU(Z3CDLL

LO

ro

CN

CN o00o

c x 1

0

y 1 = (Ahx + T^K^)/c against x, at Dq = 0.226 is shown in figure (4*5)*In computing the value of K from the best values of D and C< , the valuesoff = 0.-7B51 ?2 = °*3797> and f^ = 0,1132 (obtained from the Davies equation(loc. cit,)), were used. The standard deviation of K is seen to be lessthan Vfo thus indicating that satisfactory precision has been attained.In conclusion, it can be said that the optimization procedure employed inthe present case is quite capable of giving satisfactory results. However,it appears that an important condition for a successful application of thisprocedure is a knowledge of the approximate value of D ,c

2The present value for K is appreciably lower than the value 4*37- X 10obtained by Cefola, Tompa, Celiano and Gentile [58] at 30°C using theBjerrum method [113] . In determining the. K value, these authors made no

3+allowance for the activity coefficient of the La -ion and more-over formulated lanthanum salicylate as a six membered chelate. Since,this assumption is contradicted by the spectrophotometric evidence obtained in the present work, the reliability of these authors’ value must be regarded as questionable.

As a further test of the reliability of these calculations, thevalue of D was computed by means of equation (4*32) which predicts that oa plot of y" = Ah(D - D ') - ALTt + D-c. against c should give a straightO X X Xline with a slope equal to Dc. In this procedure the values of y" werecalculated using the values of Ah and C shown in table (4* 3) > and slopeof the plot of y M against c obtained by applying the method of least squares.The results of these calculations are also given in table (4« 3) • and theplot of y " against c in figure (4.6). The value 0.228 found by this meansand shown in table (4»3) as 3) X is in very good agreement with the valueo0,226 obtained from the plot of against D . The standard deviation

3E /of D is less than 0 , 5 thus indicating that the plot of y ” against c cyields a very good straight line#

THE IAFJI-IAMJM 5-MSPHYL, LAMTHAM3M 5-CHL0R0, AKD LAKTHAM 5-BR0H0-

SALICYLIC ACID SYSTEMS.

EVIDENCE FOR IOH ASSOCIATION

Evidence for ion association in the above systems was obtained using the same procedure as in the case of lanthanum) salicylate. Just as. in the latter case, the values of AD = D° - D were found to depend on both the wavelength and the hydrogen ion concentration and, in general, were small#

Figures (4*7)» (4*8)> and (4.9) show the relevant absorption curves for these three systems at the hydrogen ion concentrations at which the largest AD values were observed.

The wavelengths at which the pH variation of both D° and D were the largest were a,dopted for the evaluation of the association constants for these ion pairs. It is worthwhile to note that the wavelengths adopted for lanthanum 5-chioro and lanthanum 5-bromo salicylates were also those chcsai by Ernst and Menashi [ pi] for their spectrophotometric evaluation of the first dissociation constants of these salicylic acids. The variation of AD with h° at the wavelengths selected for all three systems is give::- in tables (4.4)7 (4.5), and (4.6). It will be seen that, in all cases, AD increases at first with increasing- h°, reaches a maximum at h°<3j£K , and then decreases, finally becoming zero at h°«olxlcT^ mole/l.

Since, in the pH region corresponding to this maximum, the AD

FIG

URE

4-7

AB

SOR

PTIO

N

CURV

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LAN

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TABLE

4.4

VARIATION

OF AD

WITH

h -

LANTHANUM

3.-METHYL SA

LICY

LATE

-71-

H

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FIG

URE

4-8

AB

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AD VD OO 00 CO OO n - OCO • • •

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03o «>ft 3 . 03 cn ONo a •d4 O CO Htl oo O VO tn Os cn « • • «o cn o o o O1 Hin ii

03 H HfS r< -cf4 n- in 03P cn in in O•3 • • • •< • *» 00 o o Offi •

H cn ON •d4a \ NO in 03 03< O cn in in OP H • • • •

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00 •d4 cl4 •d4 OQ ON • • • •<3 • CM O o O

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03 cn cn OS5 a • • *O H o o oH

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tl in cn cn oPQ K o 03 03 o<J • 0 » ft

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FIGU

RE

4*9

AB

SOR

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N

CURV

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LAN

THA

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-74-

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240

50 60

70

80

90

300

A (m/j)

TABLE

4.6

VARIATION

OF AD

WITH

h° -

LANTHANUM

5-BROMO

SALI

CYLA

TE

-O-

Heo

iiH• n

SoI!

A-ONo n

<<:

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- 76-

values are significantly larger than the random experimental error involvedin optical density measurement, it may concluded that ion associationoccurs in the present systems. The fact that with increasing pH, D tendstowards indicates that in each case, eQ is virtually equal to thismeans that just as the lanthanum salicylate complex, the lanthanum 3-methyl,lanthanum 5-chloro, and lanthanum 5-bromo salicylates are to he regarded

2+as ion pairs of the type LaHL .

RESULTS AM) DISCUSSION

DETERIdMTIOH OP L> Aim D.,______________ o 1

The quantities D and D^, for the three substituted salicylic acids were determined in exactly the same way as for salicylic acid i.e. using equation (4.27). This predicts that a plot of y = D°x against h°f^ should yield a straight line with a slope and intercept equal to Dq and ,respectively.

The results of these calculations are summarised in tables (4*7)»(4.8), and (4.9), where the values of Dq and found by /Ernst and Menashi[31] at the same wavelength, are also given, It can be seen that thestandard deviations cr and are less than 2 thus indicating that

0 1satisfactory precision has been attained. It is noteworthy that, just as in the case of salicylic acid, the present values of !D and differ significantly from those reported by Ernst and Menashi (loc. cit.). For reasons already explained in the treatment of lanthanum salicylate, the present values for Dq and have been employed, in preference to those of Ernst and Menashi, for the determination of the association constants for lanthanum 3-niethyl-, 5-chloro- and 5-bromo salicylates.

TABLE

4.7

DETERMINATION

OF D

AND

D, FOR

3-METHYL

SALICYLIC

ACID

-77-

oII

a03onon

H•OIIH

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0HOaoH

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onO in IN On in IV- VD ONH 03 H O 03 H H H oo O o O O O O oK ■ 0 • 0 0 0 e « 0o O o o o o o o>N + + 1 1 + 1 + +<S

r4O 3 CO on H on IN CMQ O o cm in 03 H H ON ONH VD 00 o tv- CO ON ON oII « • 0 • 0 ft e ft* M o o H . H 03 cn VD

onO 03 H tv- IV. VD o co Of=L ON 03 in VO CO o on -cr<n- ON VO co H on inV V t • a • ft • • •X H H H CM on in vo fv-

onOH tv. VD 03 03 H VD on inK in CO 01 on in VD o oon in 00 in tv. ON 0303 H d • » • • 0 • 0<H o o O H CM on in vo0A

in ON H VD on IV. CO co0 o IN • -tf' 03 V0 co oQ •ir1 in VD IN tv- tv- coe a • • • • ft %o o o O o o o o

03 •O H IN IN IV. -tf* IV­ ON IV. oH \ ■tf* O o ON co H H0 ON ON ON co in on CM Or4 H e • • « • • 0 00 CN ON ON ON ON ON ON ON•o a

on 0O H ON CO H in CO IV- IV­ inH \ ev­ IV. CO in CO rv- ON in0 en iv- ’ H cn on on on onM H * * • • • • •0 o o H 03 vo CO o<3 a H

II

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TABLB

4.8

DETERMINATION

OF D

AND

D, FOR

5-CHLORO

SALICYLIC

ACID

-79-

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II

H H »OIIHO•

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03 CA VD rs- COK

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TABLS

4.9

DETERMINATION

OF D

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5-

BROM

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4-12

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Figures (4*10), (4.H) ? and (4*12) represent the plots of y s D°x against h°f^ for the three salicylic acids which show that in each case, the experimental points lie on a very good straight line.

PETEHMMTION OF THE ASSOCIATION CONSTANTS

Since, in all three cases* the limiting value of AD at sufficiently high pH values is zero, thus indicating that D is virtually equal to D-,,

Q JL

the association constants of these three ion pairs were determined by theoptimization procedure already described. It was found that, in all threecases, the plot of the standard deviation q of 04against was verysimilar to that observed for lanthanum salicylate and gave a well definedminimum. The values of o< and D corresponding to these minima, and whichenable the experimental data to be best fitted into equation (4*23), sregiven in tables (4.I0), (4#ll), and (4.12). It can be seen that the valuesof D agree reasonably well with the corresponding values of D1#

O JL

The results of these calculations are ...summarised in tables (4*10),,(4.11), and (4.12). The standard deviation of each of these K values is : less than 2fo, thus indicating that also here satisfactory precision was attained. As no literature values for the three association constants appear to be available, no assessment of the absolute reliability of these results can be made. The plots of y ’ = (Ahx + T^K^)/c against x for the three ion pairs are shown in figures (4.13)> (4*14) > and (4.15)? from which it can be seen that the experimental points lie on very good straight lines.

The value of D , for each of these ion pairs, was also computed by means of equation (4.32). This predicts that a plot of y ,f - Ah(DQ-D^)-ADT^ + D^c against c should give a straight line with a slope equal to D^.

-84-

The results of these calculations are summarised in tables (4.10), (4.1l)?and (4.12). It will be noted that the values of D found by use of equationo(4.32) are in very good agreement with those obtained from the plots of

against D , The plots of y 11 against c are given in figures (4.16)?(4.17)? (4.18). Again, it will be seen that very good straight lines .were ..obtained. _ __________________ ___ _________

the lanthanum 5- and 5-nitroSALICYLIC ACID SYSTEMS

EVIDENCE FOR IOH ASSOCIATIONExperimental evidence for ion association in these systems was

obtained using the same procedure as in previous cases. It will be remembered that a successful application of the procedure requires;

1. The choice of a suitable concentration of the ligand T_, .L

2. The choice of a suitable wavelength, that is one, at which the pH variations of D°, D, and AD are the most pronounced and the values, of AD themselves are as large as possible. These conditions were found to bebest satisfied at X = 4I0mM- for lanthanum 3-nitro salicylate and at X = 390mM- for lanthanum 5-nitro salicylate, Figures (4.19) (4.20) represent theabsorption curves for the solutions of series A and B at the hydrogen ion concentration at which the AD values for the two systems, were found to be the largest. The variation of D°, D, and AD with h°, at the selected wavelengths, is given in tables (4.13) and (4.14). An inspection of these tables show that the lanthanum nitro salicylate systems differ from the

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previous cases, in two main respects. Firstly, in contrast to lanthanum salicylate, the value of AD at sufficiently high pH, tends toward a finite constant value and secondly, the value of AD, at "both very low and inter­mediate pH values, is zero.

Since, the value of D cannot be directly deduced from the limitingG

value of AD (AD^) as in previous cases, it would appear, at first sight, that the application of the optimization procedure, which requires a knowledge of a fairly accurate value of Dc, is not possible. However, as shown below, a rough assessment of Dq is possible by making use of the limiting value of D (d ) and, in particular, of the properties of the AD cross-over point, that is the point at which AD changes sign.

ESTIMATION OF Dc

Some idea of the value of D may be gained from the limiting valueGD^ which the optical density of an A solution reaches at a pH (pH^5), at which the dissociation of the first stage on ionization of the nitro salicylic acid is virtually complete but that of the second stage negligible. The optical density D^ of a solution of series A, at such a pH value, is given by

DL = ^H2L^Leo^ + ^Lel^ + •••(4*38)where the subscript L denotes the limiting value of the quantities under these conditions. Since, [hol]tC^O and [HL~] jt* (Tt-ct ), equation (4«38)c- u h Jj Jjbecomes,

d l = D i + CL (V D1 )/TL- ...(4.59)

On rearranging equation (4.55)> it follows that(^-DpAD.-np - cL / Tt, ...(4.40)

from which it can be seen that if the extent of ion pairing is large, Dt*2jD •J.j G

An accurate value of could be deduced from the properties of the cross-over point if the value of Ah/c corresponding to this point were known accurately. This can be seen from equation (4.32) which, for AD = 0, takes the form

eti/o = ...(4.41)or D = D.- — (D.-D ). ...(4.42)v J. Q 1 U

On substituting for Ah/c from equation (4.22), into the last equation, we find

Do = ...(4*43)

Since, in the present case, at the hydrogen ion concentration of the cross-2over point, the term cxf- is negligibly small, equation (4.43) simplifies to

Dc = B1-xh°f:L2(B1-I)o)/(x2+T:].K1f12). ...(4.44)

Thus, D can be calculated provided h° is known. Unfortunately, only a rough estimate of h° at the cross-over point is possible. Ref ©arcing totables (4.13) and (4,14)» we see that these values are h = 4*29 x 10”mole/l. for lanthanum 3“Uitro salicylate and h° = 3.34 x ICT^mole/l. for lanthanum 5-uitro salicylate. Substituting these values into equation (4*44) we find the values D = 0,356 for lanthanum 3-uitro salicylate andG

Dc = 1.290 for lanthanum 5-uitro salicylate. Thus, to both systems, the following condition applies

B1 > D o > D o. *..(4,45)This condition can be used to account for the pecularities of the variation of AD with h°, in the following way;

In very acid solutions,- that is at high values of h°, the degree of dissociation of the nitro salicylic acid (whether 3- 02? 5- ) is small

and very little ion pairing takes place; under these conditions. the optical densities of the solutions of series A and B will be virtually the same so that AD£bO. As the pH increases, the degree of dissociation of the nitro salicylic acid increases and consequently the extent of ion pairing also increases. Sinc^D^>Dc )> Dq and since,in addition, the concentration of HgL and HL~ in the solutions of series A are always smaller than those ; of the corresponding solutions of series B* I) is at first greater than D° and accordingly AD^CO. As the pH increases still further, AD decreases, passes through a minimum, and again becomes zero, At this point, the amount of complex formed in the lanthanum nitro salicylate solutions is large enough to bring down its optical density to that of the corresponding nitro salicylic acid solution. Beyond that point, the extent of complex formation in the lanthanum nitro salicylate solutions increases very rapidly with increasing pH and consequently its optical density falls' below that of the corresponding solution which contains nitro salicylic acid alone. At about pH 5, the first stage of ionization of the nitro salicylic acid is virtually complete and the extent of ion pairing reaches a maximum.Beyond pH 5, further increase in pH therefore cam have no effect on either D° or 1 and AD becomes constant,

DETERMINATION OE D and Dn o 1

These quantities for the nitro salicylic acids at the selectedwavelengths were found in exactly the same way as before i.e. by meansof equation (4.27). The results of these calculations are summarised intables (4*15) and (4.16), and the plots of y - D°x against h°f^, so obtained,shown in figures (4*21) and (4.22). It will be seen that, in each case,as the standard deviations and are less than the experimental

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URE

4-19

A

BSO

RPT

ION

CU

RVES

OF

LA

NTH

AN

UM

3

- NI

TRO

SA

LIC

YLA

TEIN

AQUE

OUS

SO

LUTI

ON

-97-

ocooino

orooCMooo

oooo

ocooLO

ooCO

oCN

E 'o oCN o oo

COoCN

oCDQ Oo00o

o00A1ISN3Q IV D I id O

(r/uu) v

TABLE

4.13 VARIATION

OF AD

WITH

h -

LANTHANUM

3-WITRO

SALI

CYLA

TEH•Oi!

H

aOcHII

$1*4oH

II

«"f\0HOa<NIOHKH

H\0HOaioHM03fN-r•03

ON ON OON ON O03 o o O• ft • • '

ON o o oONt'-

cn HH H O03 H H O

• « e ftON O o OON 1•4*

fN. 03 in-4< in O

VO H H o• « • ft

ON O O o•4* I03

H H ocn ct* H03 03 03 Oft • 0 0o o o oo 1

H

vo ON cn03 cn cn ocn cn cn o

• 0 « ftO o O oin 1

cn o cncn H

in vo vo Oft * « fto o o oHCO 03 ON cnin 03 ON oain r - VD o

t 0 « 0in o o oH co 03 voON cn O cnH co CO o

• « • ftH o o o

vo co COON in H cnin CO CO O• • ft •o o o orH 03 ca ovo VO 03 •4*ON CO CO o

• « e fto p o oCO 03 03 oco VD 03

CO c6 o« ft • fto o o o

03 03 ca ocn VO 03 -41H CO CO O6 0 ft fto o O o

cnO •H H\K 0 0

H Q Q Q0 0 <£ a *

OooIIQ<• ft*C3

03CO•OII>-3Q

..-*99-

ocoto O =>a: oI— LU

LD <

oCN

OOO><r2oCD

OCOO

O

oLf>

O

OCO

oCN

OOCO

oCN O00

oO ;oCO

Q Q oinCN

o

E

A 1IS N 30 IV O Ild O

TABLE

k.lk

. VARIATION

OF AD

WITH

h ~ LANTHANUM

5-NITRO

SALI

CYLA

TE

H0oII

H

aoeg

ii

=LaoONcaII

H\CDH-O.aegi .OH

MH

.a

H\©H0a•4*ioH-4?ONONeON

6*»

CA •sf HO O O

iA eg eg O0 o * Bo o o oo 1IA

CA NO CAH H O

-4‘ eg eg O« e 0 BO o o oLA 1CA

N H •4*CA O

IA eg eg oe « o •o o o oo ieg

N CA NOco O H

CA eg CA O• 0 0 Bo o O oo 1H

eg eg oCA IN CO H•4* CA CA O• 0 0 BO o O oIA 1

eg ON CACO LA CA HCO n N O6 « d «o o O OHCO n LA egO eg ON CAO CA CO o0 0 0 BNO o o oeg co eg NOCA eg ON CAeg H o O0 6 0 BH H H oO o o oNO CA ON•4-* CA eg oe • 0 eo H H oCA O CO egON NO H 4^O CA CA O• 0 e 0o H H oCO eg O eg

IN CA -4<o CA CA 0• 0 0 6o H H O-cH •41 O •4*O N CA -44O CA CA Oe « • Bo H pH O

CA oO HH X

0K H Pi Q Q

0 0 <1£ a

-4*O0oII*-3Q<

OCACA0H

II►3Q

TABLE

ka15

DETERMINATION

QF D

AND

D. FOR

3-NITRO

SALICYLIC

ACID

- io i-

soii

~LJH-4<II

H H •OII

H

0HQ

IOH

Xa-VDLA•Cl

C3O H o H H 03 Hp=4 O o O O O OO o O Q O OX 0 • • ' ft ft «O o o O o oI + + 1 +<

X .0 03 00 H 00 03 ON CAQ O O H 03 lA r-.H H H H H H HII • 0 • « • •H H H H H H H

03O O ON O CA ONH CA vO H VO 00/"S -4> VO n- 00 ON6 0 • * • • 0 •'O' H H H H H HX

CAOH O NO Is** ON IAG VO 00 O 03 00X ON 03 o C3 •4* ■« • • ♦ * •03 H O 03 •41 LA VO0£

, o : CM VO C— co un ;0 1 ON CM in ON UN c—0 in VO VO VO t— C-1 « « ft ft ft •i o o o o O O

03 «O rt v6 *cF o ON OH \ H 00 oo 00 c0 CN<33 O H CA IA K COX H c ■ « * • « •0 ON ON ON ON ON CA-0 a

CA • o CA O O o oO H LA ON O o 03 03H N CA 03 CA CA 00 0303 • « • * e- 0X H o CO VO -41 H H0 H

- a..a .

4>Pi0a •o VOB ONpCQ II00 0a 03

•po0 ONu ON•H o'O •o

0Q II

• •CA tv.

+ 1 + 1II II

0 HWVd,r.

■4* IV­H ONH Is-

11 II

0 HCO CD

• tCA A -o Oo o

» •o o+! +1II 11o H

. 0 0VQ b

00 00H HH 000 «O O

II II

0 H0 0

FIG

URE

4-21

D

ETER

MIN

ATI

ON

OF

D

0 AN

D D.

FO

R 3-

NIT

RO

SA

LICY

LIC

AC

ID.

-102-

go

CO

COo

CMM—

oo

e0 l ® x q

TABLE

ka\6

DETERMINATION

OF D

AND

Dn FOR

5-NITRO

SALICYLIC

ACID

-103-

aoH

oLaoCTscnII

H%OII

H

H\<DHOaioH

oncnCOCA0ON

onoH o i'*. ON 03 ON inHO

03O HO 03O cnO HOJn

•O •o «o .•O «o 0o<1 + 1 + + 1 +

oX 3 CO o «n VO H o

Q O 03 cn H 0303 cn in n- CO

II • & e e • »in in in in in in

2y

o on H CO IN- VO voH on ON O 03 VO

co CO O H 03 cn({*0 « « 9 « • svLV o o H H H HK

onOH ON o CO 03 H VO

in|N- cn

oin

OtN- ONco coO03 H fto •H «

03ft

cn0■41 ftVO

0£

0CO03 CO

ONE'­en

03ON

Hvo

VO03Q VO in in

03O 5H

•o

ON

0o

H

•o

03

•O ftO

in

ftO

voH \ ON O O o o o0 ON 03 ■cf VO CO ONK H s ft • • ft ft0 CO ON ON ON ON ON

V

onO

a

©H cn VO o IN* o cn

H \ 03 03 03 cn on 030 cn cn cn cn cn cnM H « « • » • fto H cn in n - CN(3 a

0ao3ra<s(i>a•PO0h•H73

«VO

+ 1II

03oH

CO

03OH

COto

03rHH

CO

VOOo0o

+ 1II<

03HH•OII<Q

O+ 1•II

rl_ W

oCOVO

HCO

oooco+ 1II

IJ Qto

HCOVO

Q

FIG

URE

4-22

D

ETER

MIN

ATI

ON

OF

D0

AND

D,

FOR

5-N

ITR

O

SALI

CYLI

C A

CID

.

-104-

go

CD

coO

CM

CM

LOCD\

01® xa

-105-

points lie on a good straight line,

DETERMNATIOK OF THE ASSOCIATION CONSTANTSop u m s m m . 5-. akd 5- nitro salicylates

In applying the optimization procedure to the determination of.: the association constants in the present case, the calculated values P =0.356 and P = 0.645 (l = 1cm.) for lanthanum 3-*» and 5- nit£o salicylsbesjO Grespectively, were used, (see page 95 )• It was found that, in each .case, theplot of against Pq gave a well defined minimum. The values of D< andP corresponding to these minima are given in tables (4*17) and (4*18). cThese show that, under favourable conditions, the procedure is capable ofdetermining association constants as small as 40, with a standard, deviationless than yfa. An inspection of the plots of y ! = (Ahx + T^K^)/c againstx for the two ion pairs, shown in figures (4*23) and (4*24), indicate thatthe experimental points lie on good straight lines.

The values of P , for each ion pair, was also computed by means of oequation (4.32). The results of these calculations are given in tables(4.17) and (4*18) which show that the values of P found by this means areGin perfect agreement with those obtained from the plots of against P^.The plots of y ,f = Ah(P -P.)-APT + P c against c, are given in figures

o i . l i i .

(4.25), and (4.26). These show that in each case, the experimental pointslie on very good straight lines.

In conclusion, it may be said the optimization procedure appearsto lead to a satisfactory value for the association constant, providingthat the value of P is fairly accurately known beforehand.c

17 ASSOCIATION

CONSTANT

OF LANTHANUM

3-NITRO

SALI

CYLA

TE

-106-

o« H

ONVDCA•O rA<IIQaoI!

<=»*rLS oH

H0oII

H

0HO2C2IoHH

fx3CQ<36h

♦r-i\©HOaioHMaVOLA•

03

VDOH

Or*IK!>><OHX

LAoH

ovo . oHun

< 1

Q<

P

03O H H \0r$ Hoa

cAOHrl0

H\0HO

A o 03 ON 03H o H H o OO o O O o O

• 0 o « 0 oo ° o O o o+ 5 + + + ' «

•H

ON CA CA CA CO + lH IA •4* 03 LA coIA H VD IA VD LA II

0 to e « • 0W

to

• v*

A A VO VD IA LA

CN ON o CA CA 03o CA 03 CO A CA •o O O O O O ON

0 to 0 • 0 e CAo o o o o o+ 1 I + 1 + II

«3* VD 03 o CA LACN CO CN C\ CA •VD A- ON CN 03 CA H

• 0 • 0 0 s OH H H H 03 03 O•

o

H o A LA LA + l-d4 CN VO CA H II

O ON A A IA IA« • * • 0 « Hi - o

■Qto

01 H H H H H

CA O H CA A .r.A- VD A CA 00 H ON03 VD O O CO ON vo0 • 0 e to » CAH H CA •41 to

O03 O CA o IA CA IICA CA 03 03 H H HiO O O O O O• • e Q e e oo o o O o o Q

a t— vo ! *=T CO ro CAa o m A CM HLA VO v.O vo A A 0

• . • * * to 9 oO . o o o O O

itVD co ON H 03

ON ON CN ON O OON H CA LA CO ON to• 0 • 0 » 003 CA CA CA CA CA • r*

HON

•O ro . rn r—1 o'- HLA CN o o CM CM Hm OJ ro no c0 CM jto • • • • a IIorH CO VO 'Vi' rH pH

<3

FIG

URE

4-23

D

ETER

MIN

ATI

ON

OF

TH

E A

SSO

CIA

TIO

N

CO

NST

AN

T O

FLA

NTH

AN

UM

3

- NI

TRO

SA

LIC

YL

AT

E.

-107-

oCN

co

O

LD

Ocn

01® X

FIG

URE

4-24

D

ETER

MIN

ATI

ON

OF

Dr

FO

R LA

NTH

AN

UM

3-

NIT

RO

S

ALI

CY

LATE

.

-108'IDCN

OCM

LD

to

O

ID

OIDO

e x1

0

4.18

ASSOCIATION

CONSTANT

OF LANTHANUM

5-NITRO

SALI

CYLA

TE

aCQ<

uncH ON cn cn o H m• o 03 H 03 03 ocn o O O o O oON • • • 0 • e03 *: o o o o o o• I + 1 1 + 1o <II ino oQ H H o o |N- 03 COON •d* cn cn H ON•«» H H H o CO -d‘2 « • « 0 0 •o 03 ca 03 03 H HH >NII oH ON H cn VO 03H r- 03 ON ON VD•« M H o O O o O=L • ft 0 • « •s •» O o o o o Oo In 1 1 + + + 1ON <1cnii OH 03 vo o ON in o<: 03 cn r- 03 in -d4X O O o H 03 in•** • « • • • •H «. H H H H H H• InOII mo ON -d< h- H ON OH H CO 03 H in Ocn 03 ON 03 H• r* X • « ft ft • «• [N- IS VO VO inH o\0H in0 o ON |N» 03 03a H cn in 03 ON ON 0303 vo o n- H cn 031 M • « • e • •O o H H 03 03 03H rCH <1H in cn CO •d* rt 0-II Q 03 03 H H H o

<3 o O O O O oIs4£nJ • 0 • « • •£■« o o o o O o

• cn in ON CO o ONH o 1S H in H\ Q vo m in •cb •d10 • ft • • e *H o o o o o o0a 03 •i O H CO o 03 cn in ino H \ CO ON ON ON ON ONH © ON H cn in IS COM M H • : • 0 • • -»cn o 03 cn cn cn cn cncn •o aCOON• cn •ON (-J l""1H \ rO vo o c— o mII CM CM CM <~n m CM 1

H H m « •m« m

•m♦

rO• rn : •►■3 0 o i—i ro in c— ON. <3 a

VOH+!it

Wto

• r*

cn

ii&

HOO

+ 1

W oCl

COONCQ«o

&oQ

cocncn•O+ 1II

vfo-d*COftoH

FIG

URE

4-25

D

ETE

RM

INA

TIO

N

OF

THE

ASS

OC

IATI

ON

C

ON

STA

NT

OF

.LA

NTH

AN

UM

5-

NIT

RO

S

ALI

CY

LA

TE

.

- H O ­

LD

ro

O

O

01® X

H I ­

LO

-J

COCNNT

<ro

Q t

O)

GO

inOX

CO u

LO

ro

CN

O

-112-

the lanthanum salicyialdehyde systems.

The experimental evidence which we have obtained leads to theconclusion that all the lanthanum salicylate ion pairs studied in the

34-present work, are formed through ion association between the La - and the HL ion and do not involve the phenolate group. Since, the values of the second dissociation constants of the salicylic acids employed in the present work, vary from lCf^ - ICf^ C^lJ, this type of behaviour is to be expected. It is possible, however, that the stability of these ion pairs is somewhat enhanced by ion-dipole interaction with the phenolate

34-group. The question now arises as to the conditions under which the La -ion will interact with the phenolate group. That such interaction ispossible has already been shown by Postmus et alia [59], who studied ion

34-association between the La -ion and some substituted salicylaldehydes.Nevertheless, to test the applicability of the optimization procedure tothese systems, it was decided to redetermine the association constants of

34-the ion pairs of the La -ion with the anions of some salicylaldehydes •

EVIDENCE POE ION ASSOCIATION

Evidence for ion association in the present systems, was foundby use of the procedure described in the case of the lanthanum salicylates.Accordingly, two series of solutions, A and B, were prepared whose hydrogenion concentration ranged from h°S£; 1x10” - 1x10"^ mole/l., for the La^+-ion/saJLicylaldehyde system, and from h ° ^ 1 x 1 - IxlcT^ mole/l., for theLa^+-ion /3~, and 5-nitro salicylaldehyde systems. Just as in the cane

34-of the lanthanum salicylates, the concentration of La -ions in all solutions of series A was made much greater than that of the ligand so as to minimize the formation of ion pairs containing more than one ligand per metal ion

-113-

and thus, to satisfy the fundamental assumption underlying the optimization procedure. The absorption curves of these series of solution were then recorded as a function of pH. Some of the absorption curves obtained are shown in figures (4.27)~(4.29), (4.30)-(4*33)» and (4.34)-(4.37) for the La^’Vs&licylaldehyde and the La^+/3-» and 5-nitro salicylaldehyde systems, respectively.

It will be noticed that, in each case, the absorption curves of the solutions of series A (the La^+/salicylaldehyde systems) are very different from those of the corresponding solutions of series B (salicylaldehyde only). The differences observed are discussed in greater detail below,

THE pH DEPEKDEITCE 0? THE ABSORPTION CURVES OF THE Mim-I/dMV'Si^IGYLiibbErlYHE SYSTEM

The effect of varying the pH in the range 1-3 on the absorption curves of the solutions of series A, has been found to be negligible. In this pH range, the absorption spectra of the solutions of series A exhibit a single absorption band in the wavelength region 270-450m(u, with a maximum at X = 325ni|' Above pH=3* increasing pH has a very profound effect on the absorption curves of solutions of series A in that the absorption band at X = 325m!-i gradually decreases while a new band seems to appear at X = 380m|i. At about pH=7, the absorption band at X = 325^ has disappeared and the development of the new band at X = 38O2111 is virtually complete. Above pH=8, further increase in pH has no effect on the absorption curves of the lanthanum/salicylaldehyde solutions, This type of behaviour can be best explained by assuming that, at this stage, the exten- of complex formation

-114-

has reached a maximum. In fact, due to the large excess of lanthanum ions, under these conditions virtually all the ligand has been converted into complex? the absorption hand of the A solutions can thus be regarded as being characteristic of the lanthanum/salicjrlaldebyde ion pair.

Since, the dissociation constant of salicylaldehyde is relatively small (IC = 10 ), the effect of increasing the pH on the absorptioncurves of solutions of series B is negligible up to about pH=6, In this pH range, the absorption curves of these solutions also exhibit a single absorption band at X - (see figure (4.27)). From about pli=6 onwards,considerable changes in the absorption curves of the B solutions take places the absorption band at X - 325m(-L gradually disappears while a new band appears at X = 380mn. Above pH--lQ the absorption curves consist of a, single band at X = 380mp, which does not change with further increase in pH. For the sake of comparison , the absorption curves of an A solution at pB=£* and of a B solution at pH.=13, are shown in figure (4.29), Although the single absorption bands exhibited by the two types of solution have maxima at the same wavelength (x = 380niji), the band for the A solutions is considerably higher than that of the B solutions. Since the pH variation of the optical density of the A series of solutions was the largest at A = 380my, this wavelength was adopted for the study of ion association in the La"/+-/salicylaldehyde system. The variation of B°, D and AD at this wavelength is given in table (4*19)#

The fact that the absorption maximum, for both the salicylaldehyde anion and the lanthanum-salicylaldehyde system, occurs at the same wave­length seems to indicate that complex formation occurs here, through ion association. The large difference in the heights of the two absorption bands, also suggests that the La^+-/salicylaldehyde ion pair should be .considerably

-115-

more stable than the ion- pair of the lanthanum salicylate system. It is rather surprising that Posterns et alia [59]? who also studied the lanthanum/ salicylaldehyde system, made no mention of the appearance of the charoctedstlc absorption band of the La^+/salicylaldehyde ion pair.

PEE pH DEPENDENCE OF THE ABSORPTION CURVES OP THEMTMHJM/5-, AED 5-NITRO SALICYL/iLDEHIDE SYSTEMS

The pH dependence of the absorption curves for both series of solutions in the present case, was found to be essentially the same as that observed for the lanthanum/salicylaldehyde system. In both cases, the limiting absorption bands of the ion pairs observed at sufficiently high pH values, (pH " ■'6), were considerably higher than those of the corresponding absorption bands of the anions. However, in contrast to the La^+-/salicylaldehyde system, the maxima of the limiting absorption curves of the ion pairs did not coincide with those of the corresponding anions : but were found to be shifted by about 20m|~i towards shorter wavelength (see figures (4*33) and (4»37))» The wavelengths at which these maxima occur were adopted for the determination of the association constants of these ion pairs. The variation of D°, D and AD, for the two systems at these wavelengths are given in tables (4.20) and (4.21).

PETERI'HMTIOH OP D AKD IL o 1

Por both 5- and 5-nitro salicylaldehyde, the quantities and were found in the usual way, that is, by means of equation (4.27). The results of these experiments are summarised in tables (4.22) and (4.23).

ABSORPTION CURVES FOR THE U M i m m V

, SALICYLALDEHYDE SYSTEM

LEGEKDTt = 1.031 x 10~4 mole/l.LT„ = 1 x 10“2 mole/l.

I = 0.1

SI ss 4 cm.

- - - ~ ------Series A --—— — - Series B

FIGURE 4.27. h° = 6.714 x 10”4 mole/l.FIGURE 4.28. h° = 4.187 x 10~5 mole/l.

Series A pH*v.8.FIGURE 4.29^

feeries B pH — 13.

FIG

UR

E U

-21

-117-

oCO

o' LO

ooCOooooocn

ooCD

oLO

ooCOoCM

oooCOocno00

ooCO

oLOCM O M

AJLISN3Q IV D I id O

A (m

/j)

FIG

URE

4-28

oIDai8. ocn

oooooCD/oID

oo00oCMoooocnocoooCO

oID

ooro

ooocooCD

OCO

ooCD

CN

A1ISN30 IVOIldO

FIG

URE

4-29

-119-

ooin

o00

o

oCD

oID

o*<roCO

oCM

Ooo

cn oCD

o

oCD

oID

oroCO

oCM

oooCO

ocnoCD

oCM

AJLISN30 " lVDIidO

CO

TABLE

4.19

VARIATION

OF AD

WITH

h -

THE

LANT

HANU

M/SA

LICY

LALD

EHYD

E SY

STEM

-120-

HeoI!

H

ao03

1!

r laoCOcnII

H\oHOS03IOH

H\OH0a-a*Io

HHHcnO

VO 03 1

O O • H

H «

COoofto

Ovoofto

Hoo

•oJ

•4* cn H i H O

• Hin M

COoo 0o

r-Ho fto

cKo0fto1

4 < cn O J O O

• HH M

cooo

fto

03orH

fto

1•4 1ONo •ot

H -4«cn i03 O

• Hm K

cooo

0o

•4*

fto

vocnCM

•of

H *4* Ov 1 O O

e Hcn «

COoofto

vovo03

0o

COinCM

•01

VO -4< O Io o

o rH H M

COoo

fto

0 -03cn

fto

ICVHcn

ftot

03 in CO 1 t '- o

• H cn x

COoo

fto

03COin

0o

1chn -in

•o]

vo vo 1

cn o • H

OV M

oHo

fto

cncnr -

fto

1cnCMn -

•o

O voH 1 03 O

o H H M

1

H03oftH

i

r-. ioHMH2

1

CO-4 103ftH

1

COIOHuftin2

1

00inCM

6 IH

CO1oHwH2

1

COinCM•H

1

h°m

ole

/l.

0P P p<

03COinoIIpQ<3

VOn-VO«oIIHP

COincm•H

IIPP

ABSORPTION CURVES FOR THE LAMARUM/ 5- NITRO SALIOYMLDEHYRE SYSTEM

W M D

T^ s= 4*653 x 10“5 mole/l. oT^ = 1 x 10” mole/l.

I » 0.1

£ es 2 cm.

- - - - - - - - Series A~Serie s B

FIGURE 4.50. h° = 5.015 x 10~4 mole/l.FIGURE 4.31. h° = 2.407 x 10~4 mole/l.FIGURE 4.32. h° s= 1.444 x 10~4 mole/l.

o —6^Series A, h - 8.61 x 10~ mole/l.FIGURE 4.33J

FIG

URE

4-30

OcnooooocooLO

ooCOoCN

ooo

o00ooCO

oLO

oorooCN

oooCOocno00ooCO

coinoo coA1ISN30 "IVDIldO

(r/uj) \£

FIG

URE

4-31

LO

Ocnoooo

oCD

otoo

oCO

oCM

o

oo

ocno00

o

oCD

o

oCO

oCN

o

ooCO

ocno00

o

oCD

o -1 inO C Nvr

A1ISN3Q IVOIldOcooo CO in

A (m/j)

oLO-I24.oCDO00O

oCO

oLO

o

oCO

oCM

o

o^ r <o00

o

oCD

oID

o

oCO

oCM

oooCO

CMc?

o

oCD

oLOCMCOCD LO

A1ISN30 IV O I ld O00

FIG

URE

4-33

oolo

oco

o

oCO

oIDo3*oCO

oCN

o

OOOCDo00

f<o

oCO

o

o

oCOoCN

o

ooCOocn

oCOo

oCO

COl o *<r

A1ISN3Q I V O I l d O

00 CO

TABLE

k.20

VARIATION

OF AD

WITH

h - THE

LA

NTHA

NUM/

3-NXTRO

SALI

CYLA

LDBH

YDE

-126-

o IN cnIN cnH O o o« ft • c• co o o oH cn 1• HOII co cn incn 03 COH O o H o• 0 0 •n- o o oB vo 1o03 vo ON cnII in in o inIN o 03 H *

<=*» • 0 • • t'.CO o o o VO03 I H=5L •s OIA 03 03 Io O r- H •c«03 o cn 03 11« 9- • ft11 n- O o O *-3H i Q<3... ON 03 cn0 n- in r- •*»H -4< o in ON\ • • • • H0 o o o VOH H i •0 O203 in CO o 03 111 H VO CO Ho ON n- cn HH • • • • QM 03 o o oH 11! H m cn CO VOVO o CO n- cow CO vo n- He-i • • • • •o o o O o•*% !• itH 03 vo\ CO0 in I n- 1 QH • •O o oami IN voo O coH H ] IN 1K • •cn O Oinvo♦ in ••sh O HH %ti 9 oM H a Q Q0 0 <1E-i £ s

-127-

ABSORPPION CURVES OP THE M H A l l /

. 5- NITRQ SALICYLimSHXDE .SYSTEM

IEGEMDTl « 1.074 x 10~4 mole/1.

TM = 1 x 10"2 mole/l.

I = 0.1

£ = 1 cm.- - - - - - - - Series A— ■— — —— *“ *— Series 3FIGURE 4.34 h° = 4.005 x 10~4 mole/l.FIGURE 4.55 h° = 2.214 x 10“4 mole/l.FIGURE 4.56 h° = 1.623 x 10~4 mole/l.

,Series A h° « 1.424 x 10 mole/l.FIGURE 4.37.I

'Series 3 pH 'X/13

FIG

URE

4-34

-128-

oCD

OID

O

OCO

OOOOcno

or<

oCD

OID

O

OCO

OOOCO

OcnoCD

o

_1 o o 1CO CN o cn to IDCD CNCO

AiisN3CI IV O I ld O

FIG

URE

4-35

-129.

o

oCDOino

oCO

ooo

oCDoCO

o

oCDoIDO

oCO

oCM

OOOCO

oCD

O00O

J o oCO1— O 07 00 a>> CD IDA1ISN3Q IVOIldO

A (m/j)

FIG

URE

4-36

-130-

o

oCOoIT)O

OCO

oCM

ooo

oCD

oCO

o

oCOoLO

o

oCO

oCMoooCO

oCD

o00o

J oLO XS- CDo CO

AllSNaa IVOIldO

FIG

UR

E

-131-o00

o

oCD

OIf)O

Oco

o

oooCD

COO

OCD

OLO

Oroco

oCN

OooCO

oCD

O00

O

1 ■ I - ■ - ■ » 8____« » — ■ I-----1--- OCN O CO CO. NT 'CM O 00 CD vj- CN• • • • • # | • I •CN v-A1ISN30 "IVOIldO

,a

Q<3faO15oH

<iHPi<

H03•

§

H

aopH

aoVOcoII

H\0H0a03IOH

H

i!

Eh

0HOaiorHMnro

o CO -3< Ho CM 03 . Oo H O.03 o

• ■O O1

O IN- CO[v. 03 03 oo H H qH «O

•O 01

H H oCO «4> HO H H O• « • •

o O O oIV. lIA

CO ONco CO

CO H H oe • • •

tv. O O oH !H

VO 03 VOin co CO ONo H CO H• « •o o o O

i .

CO co oCO COo H co

« • • «CO O O O03 H

in ONin CM VO

H H IN. in• • • 0

03 O o o03 1

n - vo . ONCO VO CO HCM H o ON

• • • •VO O H OH s

ON 03 coin ON O o

H CO vo• e • 9

o o H HH •H CO 03 ONO H H ONH CO CO• • • *in o H H

1

-=H H tv.CM o CM HEv. CO H

• • • «H o H H1VO 03 CO Hin VO 03 VOH •sH CO CO• • « 0o H pH 0

1

03 CO CO voH VO CM inO in CO 03• . • « •O pH pH O

1

in Ho \H O 0K H Q Q Q

0 0 <1,£3 s

H03CM•OII

Q<CMOVO

«HIIHQ

co 03 CO • ■HIIQ

TABLB

4.22

DETERMINATION

OF D

AND

D, FOR

3-NITRO

SALI

CYAL

DEHY

DE

-133-

IA "" ... II, oH vo o IN CA VD CMH CA H CA •4* O •pM O O O o O Hft 0 0 • 0 ft ©* O o o o o O a<3 1 I + + 1 1 ©hpwH C3ft 0 IA r- ON CA IA H cF ©

a Q O H IN- ON H CA CM ao H C3 03 CA CO IN coII • 0 0 0 * 0 -p* H H H H 03 CA oII ©Pi•Hts<F w• c* o IA LA IA H CO LA 0

3 . H r- ON IA CM r- 03 QST@

A- CO O VO VO OIA e « 0 • ft 0o o o H H 03 -sF<F K 0

IAII+ 1cFO IIH• <* vo vo vo 03 ON IN 0H H CA ON VO H vO v Wft N- CO ON IA VO ON 0o 03 H • 0 • 0 0 0

<H O o o H 03 CAII 0 ■»voH CA•iF« r* !N CA 03 03 03 IA II• 0 IA •cF CA r O ONH Q H H H pH H O 0\ O 0 0 0 0 0 to© O O O o O oH0a 0

IA 03 • H1 O H ON ON ON ON ON ON OO H \ ON ON ON ON ON On oH © ON ON ON ON ON ON 0H • 0 0 0 ft 0 o* 0 ON ON ON ON ON ON

•d a +1CAIA IIVOe <F » JO-iF O H co ON £N- LA ONH \ ■iF <F co LA CA kiII © ON in •li4 IA CO VOr4 H « 0 0 0 0 00 IS* IN sn VO •cF 03& .fi S HCOO0

cF • oO H IA IA IA LA IA IAH \ h- «n En. in r- IN II© o o O o o oK H 0 0 • 0 0 ft 00 ON ON ON ON ON ON Q© a

oca

ocoo

*pPi©©w©©13•Po©k•H'OQ

A-H03+ 1

W

IAONCAIAIIHW

H<FO

+1

VOHO

Q

03IANOVO

W

COOA03

FIG

URE

4-38

D

ETER

MIN

ATI

ON

OF

D

0 AN

D D,

FO

R 3-

NIT

RO

S

AL

ICY

LA

LD

EH

YD

E

-134-

/

CO

CM

OCO

01® x q

h°f,2

x104

TABLE

k.23

DETERMINATION

OF D

AND

D. FOR

5-NITRO

SALI

CYLA

LDEH

YPS

-135-

soC3It

<3

rlsoVDcnII

r<!

H «0II

0 H

tHsQH0ain1oHcooin

in

CO

COr-cocncoVO

in-5mLncn

ONcninON

coOnON COin03 H

rHCMmCM

ONrHvo CMCM

tv-VOON

voONONOnON

ONinGN

CMCMmoCMH \ COCO

cnvocn

Pr4o£10u2ca(3oSpoQ)h•H'O

r-+1ii

cw\o• **

in03HHIIOU

HOo«o+111<Q

'OONvoH•oIIoQ

03OHH

to

invoH

P3Q)S<D

POO•H

cno\03+1II

<w\oCOcnCO•»H

rH10

o«o+1tl

rQ\0CO0303003

Q

OON

H

to

COcn03»CN]

FIG

URE

4-39

D

ETE

RM

INA

TIO

N

OF

D„ AN

D D,

FO

R 5-

NIT

RO

S

AL

ICY

LA

LD

EH

YD

E

-I36-

cn

CM

0 1 ® XQQO oCMCD ID CO

h° f,2

x10

-137-

Judging by the standard deviations and cr and also by the plots of y =o o 2 o 1D x against h f^ shown in figures (4«38) and (4»39)> the precision attained

here is seen to be good.Since, as already mentioned, the dissociation constant of

salicylaldehyde is relatively small, the determination of Dq and forthis ligand by this means would require the use of suitable buffers. Forthis reason, it was decided to determine these quantities directly bymeasuring the optical density of salicylaldehyde in sufficiently acidsolution and in sufficiently alkaline solution,respectively. The valuesof Dq and D^, so obtained, are given in table (4.24).

BETEItMMTT OK OF THE ASSOCIATION CONSTANTS OFTHE La^+-/SALTCYLALhEHYhE SYSTEM ION PAIRS BY MEANS OF TI1E OPTMZATIOK PROCEDTIRE

On the basis of the cases studied so far, it appears that asuccessful application of the optimization procedure requires a knowledgeof a fairly accurate value of D . Since, no direct information regardingthe magnitude of D was available in the present cases, it was decided oto employ for this purpose, the highest D values (hj) at the selectedwavelengths. Accordingly, the value of Dc was varied around the value of

and the standard deviation of£X (cr ), corresponding to any given valueof Dc, calculated. In all three cases, however, the plots of cr^ againstthese trial values of D failed to show a minimum from which the bestcvalues of<* and could be obtained.

In view of this failure, a new procedure for determining association constants was developed. The theoretical basis of t'lis procedure is discussed in the following section.

-138-

APPRQXIMATIOK PROCEDURES, FOR. THE DKPERMIUATION OF AS S PCIATIOIT COHSTAHTS

In the following discussion the symbol HL will he used to representthe salicylaldehydes. Under the experimental conditions employed, ion

34-pairing between the La -ion and the anion of the salicylaldehydes can thus be represented by the equation

La5* 4- L~ LaL2*. ...(4.45)34-Since, in the process of associating with the La -ion, the salicylic acids

act as mono-protic acids, all the equations derived for these systems are applicable in the present case after suitable modification. Thus, the thermodynamic association constant for the La5*-/salicylaldehyde ion pair is defined by

K = cf 2/TjJ] L*~] f-j_f t ..#(4.4^)

and. the concentrations of L and HL being given by[iT] = (Tk-c^/CJtj+hq2), ...(4.47)

[hl] - (TL-o)hf12/(K1+hf12). ...(4.48)

On substituting for [l~] from equation (4*47) into equation (4.46) and rearranging we get

c = ^ ^ / [ ^ ( E T j j + P j + P h f ^ ] . . . . ( 4 . 4 9 )

As the pH increases indefinitely, that is, as the hydrogen ion concentration tends toward zero, the above equations simplify to

cL = ra^AKTjj+F), ... (4.50)

[l"]l = Tl-ol, ...(4.51)

[hlJl sS'0, ...(4.52)

-139-

the subscript L, refering to the limiting values of these quantities, reached at sufficiently high pH values.

Since, under these conditions the concentration of HL is seen to be vanishingly small, the limiting value of the optical density of solutions of series A is given by

H = L~kei® + 0Lec^ ...(4.53)On substituting for [l J^and c^ from equations (4.51) and. (4.50),

respectively, equation (4»53) becomes

MThe last equation forms an essential part of the new procedure

developed for the spectrophotometric determination of association constants of ion pairs. Some idea about the usefulness of this equation can be obtained by considering it under various conditions.

1. When Dl - and consequently the limiting value of AD (AD^) zero, the equation simplifies to D = I)T, This type of behaviour was observed

O Jj

in the case of lanthanum salicylate and lanthanum 5“bromo, lanthanum 5“ chloro and lanthanum 5-bromo salicylate.

2. The condition also obtains for values of K sufficientlylarge to make the second term negligibly small compared with DT, It will

1j

be seen later that this condition is indeed satisfied in the case of the lanthanum/salicylaldehyde system.

3. Inspection of the equation also shows that, in some cases, it should be possible to reduce the value of the second term by increasingthe concentration of lanthanum as much as possible. However, the usefulness of this procedure is limited by the requirement of maintaining the ionic strength of the solutions at I ** 0.1.

- 140-

The validity of this equation can also he tested by calculating from it the values of D^, for the two lanthanum nitro salicylate ion pairs, For this purpose, the abo.ve equation can be written in the form

Do = d l - _ J L _ (a d t ), ... (4.55)ET„Mwhere AD^ is the limiting value of AD, at sufficiently high pH. On substituting for DT, ADT, and K, the values given in tables (4.13) andJj 1J

(4.17)» (4*14) and. (4*18), the values D = 0^386 and D = 0.448 3X0c cfound for the lanthanum 3~> amd 5-nitro salicylate^ respectively, in fairly good agreement with the values calculated by the optimization procedure .

There are a number of ways in which equation (4*54) c,an be employed in developing an iteration procedure for the spectrophotometric determination of association constants of ion pairs. The simplest way of utilizing this equation is to incorporate it into the optimization procedure after suitable modifications. Accordingly, an iteration procedure was developed involving the following steps*

First a reasonable value for F/KT^ is assumed and a rough value of Dq is obtained by means of equation (4.54). The latter value is then used to compute Ah and c values for the experimental solutions, using equations (4.21) and (4.31)5 respectively. An improved value if F/KT^ is then obtained by means of equation (4.23), that is, by plotting y=(Ahx+T^ V / o

against x, calculating the slope by applying the method of least squares.The improved value of F/KT^s introduced into equation (4.54) and a new cycle of approximations started. The process of successive approximations is continued until the values of F/KT.. found' from, two successive cyclesi'-iagree to within ± 0,01$.

This procedure, (which we shall call iteration procedure No.l), was programmed in terms of the algol code and all calculations were carried

-141-

out using either an Elliot 503 an I.B.M. 1905E computer. A scheme forthe evaluation of the standard deviation of the association constant was also incorporated into the program. The flow diagram showing details of the program is given in the appendix.

Before proceeding to the discussion of the application of this procedure, a few remarks about the general principles underlying approx­imation methods seem to be in order.

Iteration methods have been widely used in the determination of the dissociation constants of weak acids, association constants of ion pairs, and stability constants of complexes [ 105, 108, 2ll The essential condition for the successful application of any iteration process is the judicious choice of the starting value for the process of successive approximations.

There are a number of conditions which must be satisfied in order for the process of successive approximation to converge. A succinct account of the general principles -underlying approximation procedures and their limitations can be found in n Handbook of Numerical Methods for Solutions of Equations”, by V.L.Zaguskin, [114],

EVALUATION OF THE ASSOCIATION CONSTANTS OF THE La5+-/SALICYLALDEHYDE AND THE La5+-/3-,AND, 5-KITRC SALICYLALDEHYDE ION PATHS

USING ITERATION PROCEDURE No.l

Although the intense absorption bands found, in the present work, for these three ion pairs suggested that their association constants are considerably higher than those reported by Postmus et alia C 591 > it was decided to employ the latter values in starting the iteration procedure.

-142-

However, it should he mentioned that in calculating the association constants of these ion pairs, thesejauthors ignored the activity coefficient' of the ion pair and of the La5*-ion. Tables (4.24), (4.25), and (4*26) give thes^authors values after correcting for these activity coefficients.

In all three cases, the iteration process was found to converge repidly towards a definite value of K, The results of these calculations are summarised in tables (4.24), (4.25), and (4.26). The results are also illustrated in figures (4.40)? (4»4l)> and (4*42) which show the plots of y* = (Ahx + TtJLlfrom the tables that in all three cases, the standard deviation of theassociation constants is less than 3 thus indicating that satisfactoryprecision has been achieved. It will be noticed that there is very goodagreement between the present value of the association constant of theLa5+-/5-nitro salicylaldehyde ion pair and that reported by Pontmus etalia and only fair agreement in the case of the La5+-/3~nitro salicylaldehydeion pair. There is, however, no simple explanation for the inordinatelylarge difference between the present value of the association constant ofthe lanthanum/salicylaldehyde ion pair. It is possible that in determiningthe association constant for this ion pair, the authors worked at a pH

•“8 ?7range (pH^ 4)» which in view of the small dissociation constant (lcf ) for the salicylaldehyde, is much too low for any reliable pH variations of the optical density to be observed.

However, to verify the results obtained by iteration procedure No. 1, and also to simplify this procedure, it was decided to explore other possible iteration processes. The results of these considerations are given in the following sections.

K_)/c against x, for the three ion pairs. It will be seen

TABLE

4.24

DETERMINATION

OF THB

ASSOCIATION

CONSTANT

OF THE

LA

NTHA

NUM/

-143-

VOrv-vo•o

of£lBQOoPift&oHEh<ft63EhH

ftH<dPiftoH

raQrHftftQftaoHft<U1

11a Ho Q03

It COO

<=» o0

« #* o3-s 11o00 0cn Q

ti cor<: in

CM•

H H«o 1111 O

QH

• r» 00ft in

H 03\ •0 HH0n itLi

031 QOHXH I

OII H

H\Ooa»oHur«HcnO

X03

G)2H>fcOPi•H+>CtS

•p«

ft

inoH H H o o H H

O O o o O OX O O o o o oft 0 ft e c ft

a. o o o o o o>> I + \ 1

<

inOH co 00 H vo VO rHiv n - cn cn CA oX tn CM cn IV A - 03

• • • ft « •a. o H CM cn <F VO

ino CO O cn IV- cn HH 03 - cn 03 CO cn

ON CM IV- in CA voX 6 • e ft a «in in -sF -ti* cn CM

O

inoH o IV. CM VO os O

cv- H cn CM CO cnX ON CM tv. in ON vo• « • o • < ft£ in m -=F -ti1 <n CM<

cn cn •ti* ON cn ONCM cn r - •ti* co H

Q IV vo in in •ti* cn< « « « • e »o

I0

1oi oi 0

1o

1

cn H CM tv- H rvQ cn -ti< CO m Os 03

tv. vo in in •ti* cn• • « e « «o O o o O o

CM «O H IV IV t '- tv. tv. IVH \ o o o O o o

© o o o o o oX H « ft ft « ft ftO *ti‘ -ti* •ti”1 •ti* •cF •ti*

*o a»

O H cn ON CO O CO oH \ o CO H ON H CA0 ON VO cn 03 CAX H 0 • « e « •0 NO vo VO VO vo in.a a

0O H VD VD VD vo vo VOH \ ON Os ON CA CA ON0 ON ON CA CA CN CAX H ft • « • e 6

0 vo VO VO VO VD VOrs a

CA&

OrHXtvo

q2H(3>W3o

•H>0Uft

-ti* •IO H XH VD

ft

OO

+ 1

oHXcnOO

ft +iEhftft

VD-ti*1 O HOHImIr>i03inCMin

cn

©

<3!>HG5Pi

•H

fc“)

0PiH<3>

•{JPi0moUft

ft ft

FIG

URE

4-40

D

ETER

MIN

ATI

ON

OF

TH

E A

SSO

CIA

TIO

N

CONS

TANT

OF

TH

E L

AN

TH

AN

UM

/S

ALI

CY

LALD

EH

YD

E

ION

PA

IR---

--IT

ERA

TIO

N

PRO

CED

UR

E N

o1

CD

LO

in

CO

OLOCO

01® X

TABLE

k„25

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF THE

LA

NTHA

NUM/

-145-

H • ra

t o0S3 1!

1Q .r.=1O ao in

ft o-d«

h> 1!OH «< •— cncn Hea 0 VOo •hi

iiH

HII

cd OH Q< 0cn H\ OJ3 0 0-O H inH 0 •

a HcaB

031O IIffi H ►aca K QQ H< II 01H oo £hH IIt-3 • *»<4 • y—sH o\ 2O 0 Hft h ag >

O «HH *P« hcn c3tn +>vo to

•if*O r-iH VO oo oW • 0o o+ +<3

•4*OH o vo

H oM CO oe 0o H

>>

ino in cnH 01 Hn- in• 0

03 Hoino -=t< in

H ON ON-4* cn

X • •03 H

£<3in -d*VO CO

Q CO •4*<3 • e

01 01

03 IN.03 03Q O VO

0 •H o

03 «O H in inH \ o o0 o o

H * «0 •4*'d a

•O H CO ONH \ -4*0 ON IN-

H e *0 n- [N-

,a a

«O H in inH \ n- n-

0 o oM H • 0

0 ON ON<3 S

03 cn HO H OO o O

• « eo o o+ + +

03 r > 03ON •ii* cnH co o

• « 0H H cn

VO 03 cn03 cn incn ON VO

« • eH o o

ON •d1cn ON cn

00 vo• e •

H o o

CO VO vo03 o H-=h cn 03

0 « •0

1o

iO

1

o CO oovo H Hin •d* cn« « 0

o O o

in m ino o oo o o

0 • •-d* -4*

tN. inCO m cn-d* m co• 6 «IV VO •if*

m in intN- r - . r -o o o

0 e 0ON ON On

ONtni_ioH cnO o• HO M+ CO

•if*

in•-d>itX»NVOin U0 0

•d* £W20•Hin >•d* O0 Uo P<

IN-

Kf*

••4* t cn-4* H O• O HO o0 n-o HH + 1 O

0in o

vo-5*03

ftt>e •i-o .«■> cn03 ocn HH M

« cnin O •iho 03o II •

• cn/“N

0 1!2H(3

ON > fH•4* OVO H

0 C303 Pi

•H+*FJiH o>«✓ CQ0

in u|N- p)o 4 Vw'e \ON

FIG

URE

U-U1

DET

ERM

INA

TIO

N

OF

THE

ASS

OC

IATI

ON

C

ON

STA

NT

OF

THE

LA

NT

HA

NU

M/

3-N

ITR

O

SA

LIC

YLA

LDE

HY

DE

ION

PA

IR

—IT

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TIO

N

PRO

CED

UR

E No

1 in

co

oin

01® X

TABLE

£.26

BETERMINATIOH

OF THE

ASSOCIATION

CONSTANT

OF THE

LA

NTHA

NUM/

-147-

H•6&ft

SoCM

QftOoftftft§§13H

OVD

Rl4*. COR ©* VO

b i6iit

iift*•» dQ•H ■ •4\ SO0 mH ind i.a 03

031 iiOH .ftM bR

ii4ci

R- 0b

ftii• r**

R dN Pi0ft HC3

no

1>fcfltn•1b 1H8 Mco 0o #in tn6 wKii

&ft. ft ft\ft ft

=d*..OX

IV­ in m CO 63 ONON in -4 1 R VO cnur i o O O O o o

• 4 • 0 » •«• b b b O b b

+ + + + i i<3

OX

ft On SO On 03H cn f t 63 cn

038 R cn ON in« * • 0 • •

*» f t ft 03 C3 cn iit*

inb cn CO b iNi ON 63ft ON m VO b ON cn

b co in in8 0 « • ft • *

63 ft R o b b0

inoR ON R b ON

cn co cn ON 66 03M VO cn b in in

« 4 » • ft£ 63 ft H b b b<1

CO cn in b ONin cn in On so

b co cn 03 R H<] • e • • « ft

bi

bi

oi

©i

bi

bi

c6 b b 03 R in .R CO m 'VO ON vo

b R VO in cn cn• • « ft • •

R b b b b b

03. * O f t R \

ft R R R R RR R R R R R0 b b b b b ©

M r i 0

e » 4 • 4 44 * tft*

•d 2

cn •b h

■ H \ 'cn $ o ON COo 03 in CO co

0H f t

63 R b ON CO IS i' C • • « 4 •0 r R R b b b

b a

cn • o R 03 03 63 03 63 03H s» VO VD SO vo so so,■ 0 cn cn cn cn cn cn8 H © © 0 « • «0 f t R R R R R

....d ..D . ..........-

ONtn

mbH.8OS03•63

d£COGr>ouft•v**pH

03©O

+in

cn.oR.8in63©

*b+i

- ^ r ii14 «V 3 * 0

ir. cnOn bif tR

e ONb CO

iivOft

RiPH

03ii

r<Rft>

rtf,

ftftPi

•n0

ii> w

RdtjHft

.w\ft w

FIG

URE

U U2

DET

ERM

INA

TIO

N

OF

THE

ASS

OC

IATI

ON

CO

NSTA

NT

OF

THE

LA

NT

HA

NU

M/

5-N

ITR

O

SA

LIC

YLA

LDE

HY

DE

IO

N PA

IR

—.

ITER

ATI

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PR

OC

EDU

RE

No1

-148-/

co

o

oCO

-149-

ITERATION PRQCSDITRE No.2.

Both the optimization procedure and the iteration method No.l, make use of the optical density measurements carried out, not only on \ solutions of series A but also on those of series B. To simplify the iteration process a new procedure was developed which, apart from the quantities and 3) , relies solely on the optical density measurements carried out on solutions A* Por this, purpose, an expression relating the concentrations of the, complex directly to the optical densities of the solutions A was derived as follows:

The optical density of solutions of salicylaldehyde of series A is given by

D/Tl = D [HL] + L~] + Dcc. ...(4.56)

Upon substituting for [iT] from equation (4*47) and rememberingthat

Tl = [hl] + [l ] + o, ...(4.57)

equation (4.56) becomes,

(D-Do)Tl - ° ~ ( V V + (W ] • — (4'58>

Combining this equation with equation (4*54) and rearranging we finally obtain

O = (d-do)tl/[(dl-do)(i+p/ktk)]. ...(4.59)

Por the purpose of computing the association constant of a givenion pair, a reasonable value of P/KT^ is first assumed, and the corresponding concentrations of the ion pairs of the experimenta,l solutions calculated by means of the last equation. The approximate values of c, so obtained, are then used to evaluate the values of Ah from equation (4.21). Next, a

- 150-

better value of P/KT^ is found by means of equation (4.23), that is, by plotting y ’ = (Ahx + T^K^)/c against x and measuring the slope of the plot, so obtained, with the aid of the method of least squares. A second cycle of operations is then started using the so improved value of F/KD^. The process of iteration is continued until the values of F/l<T found from two successive cycles agree to within - 0.01%.

This procedure was programmed in terms of the algol code and all calculations were carried out using an I .P.M. 1905F computer* A scheme for the evaluation of the standard deviation of the association constant was also incorporated into the program* The flow diagram showing details of the program is given in the appendix*

In all three cases, the process of iteration was successfully completed leading to definite values of the association constants. The results of these calculations are summarised in tables (4.27),(4*28), and (4.29). Plots of y ’ « (Ahx + TjX^)/c against x, based upon these results, gave very good straight lines, which are not shown here as they are virtually identical with those obtained by the use of iteration procedure No.l, It will be seen from the tables that the values for the association

4

constants found by means of iteration procedure 2 are in excellent agreement with those obtained by iteration procedure No.l. It appears, therefore, that the present values for the lanthanum/salicylaldehyde ion pairs are

U- L.correct and consequently, that* of Postmus et alia for the lanthanum/ salicylaldehyde ion pair itself is in error.

However, in order to test the reliability of the present values for the association constants of lanthanum/salicylaldehyde ion pairs still further, a third iteration process was evolved. Like the second iteration process, this also makes use of the properties of solutions A alone,

TABLE

4b. 27

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF THE

LA

NTHA

NUM/

-151-

03

o

QP3OOPSPi&oH<PSHH

05HcPk6 oHQ,r-»WP3Q►3<ij>-1OH<!to

c*

coin03•H13O II03 OQIICOin03rL 0

0 HO •<CO 11CO AQII•<: VO• #» nVDH •0 OO III! HQH• »* CO• OH O\ •O OHO IIB03 01 QOHMH 1II 0Hs 03

IIt

H O\ 30 HH ca0rj >id

toIOfjiH•HH +>«H (3CO +>O Ui«

rH11 c?1£ih3 W\

&

Lot

x1

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000e0

HOO

0O+

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HOO4Ol

HOO•O1

HOO0OI

in0Hr'l

>>

CO0-in

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00r -030H

HCOCO•03

VOCOin.•co

VDONe

•4*

H001eVO

in0HK0

con-CNein

O0303«in

COCOIN.0

[N-03in0•4*

coCOON•co

HCOVO

«03

in0HK£<

HIN-ON

•in

n-H030in

03COn-

«■it*

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H<r‘VO«O

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INinin•O

HON•41«0

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03 ® O H H \

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0v a

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coHvo

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G)M H 0<3 a

VOONON

eVO

VOONON

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voCOON

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VOCP O HOHM03

coOOH O

+ 1CD2H<3>Hc3S3•H

\4

vooHL<ln-tf*03IA

tNN*f=4

CO

TABLE

A. 28

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF THE

LA

NTHA

NUM/

-152-

H<fasoHfanJHfaofa

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ino H O -=H CM co A-H O O -S* O o cnVD O CO VD in• • B ft B BCM H H o o oO

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cn «O H cn CM o ON -tf* CMH \ o CM in co CO0 03 H o on CO £v»M H e 0 B B B 00 H H H o o ofa s cn •O H CM CM CM 03 03 CMH \ VD VD VD VD VD VD

0 cn cn cn cn cn cnH B B B « B Bo H fa fa H H HC3 S

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-154-

(apart from 'D and D^), but differs from the former in that it replaces the quantity Ah by h.

ITERATION PROCEDURE Ho.5

In the present case, the electroneutrality condition takes theform

respect to h, enables the evaluation of h provided c is known.In developing the new iteration procedure use was made of the

last equation together with equation (4.59) and equation (4.49)> which for this purpose was put into the form

should be obtained with aslope equal to F/KP^.Just as in the previous case, the iteration procedure is started by

These approximate c values are then used to evaluate the corresponding values of the hydrogen ion concentration of the A solutions from equation (4,61). Next, an improved value of E/KT . is found by means of equation

the slope of the plot, so obtained, with the aid of the method of least squares. The so improved value of E/KT^ is then used to start a second

h = (a-b) + c + [l“], (4.60)

which in view of equation (4*47) becomes

It can be seen that equation (4.61), which is of the second order with

(Tjrc)/c = (F/ETm) (l+hf^/K^. .. .(4.622Hence, if y = (T^-c)/c is plotted against 1 + hf^ /K^, a straight line

assuming a reasonable value of E/KT^ and calculating the concentrations of the ion pairs in the experimental solutions by means of equation (4*59)*

(4.62), that is, by plotting y = (T^-c)/c against 1 + hf^ /K^ and measuring

-155-

cycle of approximations. The process of iteration is continued until the values of P/KT^ found from two successive cycles agree to within - 0,01°/o,

This procedure was programmed in terms of the algol code and all calculations were carried out using an I.3.M. 1905E computer. A scheme for the evaluation of the standard deviation of the association constant was also incorporated into the program. The flow diagram showing details of the program is given in the appendix.

The results of these calculations are summarised in tables (4«30) (4.51), and (4.32), and illustrated in figures (4.43)> (4*44)* and (4.45)•It will be seen from the tables that, with the exception of the La^+/ salicylaldehyde ion pair, all the results are in excellent agreement with those found by iteration procedures 1 and 2. However, the association constant found for La^+/salicylaldehyde ion pair by iteration procedure No.3 is about IQffo higher than that obtained by iteration procedures 1 and 2.All attempts to trace this discrepancy to errors in programming or in the paper tape which records the experimental data have failed.

THE mmhlM/4-HYTROXY BSEZALDEHYDE SYSTEM

Of all the ion pairs studied so far, the La^+-/salicylaldehydeion pair is the most stable, The high stability of this ion pair may be

3+ascribed partly to the strong ionic interaction between the La -ion and3+the salicylaldehyde anion, and partly to the La -ion chelating through

ion-dipole interaction with the oxygen atom of the aldehyde group. Eor the purpose of testing the latter possibility, ion pairing in the La')+- ion/4-hydroxy benzaldehyde system, in which such ion-dipole interaction is not possible, was studied.

-156-

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4-45

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EVIDENCE FOR I OK PAIRING IN THE La^+-IQN/4-HmOXY BEHZALDEHYDE SYSTEM

To obtain evidence for ion pairing in this system, the experimentalprocedure used in all previous cases was applied. Accordingly, absorptioncurves for both A and B types of solutions were measured as a function of. pH.A pair of absorption curves, obtained at pH^ 8, is shown in figure (4.46)*It was found that over the pH range 3-7 j "the absorption curves of solutionsA coincide with the absorption curves of corresponding solutions of seriesB. From pH=7 onwards, however, slight differences in the two types ofcurves were seen, thus indicating that some ion pairing occurs. However,attempts to obtain the limiting absorption curve of the A solutions byincreasing the pH failed, as at pH-8.25 lanthanum oxide started toprecipitate from the solution.

Since, not even an approximate value for D^ was known and since,the AD values observed, even under the most favourable conditions, weresmall, the determination of a reliable value for the association constant:

m,+was not possible. This type of behaviour indicates that the la'' -/4-hydroxy benzaldehyde ion pair is considerably less stable than that formed by the La" -ion with the anion of salicylaldehyde. Since,both acids have approximately the same dissociation constant, it appears that in the La-*’1"-/salicylaldehyde ion pair, some ion-dipole interaction of the La^+-ion with the oxygon atom of the aldehyde group occurs.

THE M M I M / B E H Z O I C .ACID SYSTEMS

It was mentioned earlier that ion-dipole interaction between the3+La -ion and the phenolic group may have a. stabilizating effect on the

lanthanum salicylate ion pairs. Although, judging by the magnitude of the

.163

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Xo

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P CD

oCM

OOCOLU

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< CO OCO

o to CO

CMCO

A1ISN3Q iV O lld O

A ( m

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association constants of these ion pairs this effect cannot be very large,nevertheless, it was decided to investigate it by studying ion association

3+of the La -ion with the anions of some benzoic acids. For this purpose, benzoic a.cid and 4-hydroxy, 3“hromo, 4-nitro benzoic a,cids were selected.

EVIDENCE FOR I OF ASSOCIATION IF THEm nthanum benzoate systems

To obtain spectrophotometric evidence for ion pairing in these systems, the experimental procediire used in all previous cases was applied. Accordingly, for each system, two series of solutions, A and B, were prepared. As the di sociation constants of all these acids are about 10 the hydrogen ion concentration of these series of solutions was varied from- 1 x 1 0 ^ - 1 x 1 0 ^ mole/l. The absorption curves of these solutions were then recorded as a function of pH. A set of typical absorption curves observed for each system, is shown in figures (4.47)? (4*48)? (4«49)> and (4# 50), respectively. The wavelengths at which the pH variation of D°, D and AD are the most pronounced and the values of AD themselves are as large as possible were chosen for the study of ion association in these systems.The wavelengths, so selected, are given in tables (4»33)> (4•34)? (4»35)» and (4.36). These tables also show the variation of D°, D and AD ab the optimum wavelength for these systems. It will be seen that in the ca,se of lanthanum 4-nitro benzoate, in the pH range 2,5 - 4«5> AD increases rapidly with increasing pH, Beyond that pH range, the rate of increase of optical density with pH slows down and by about pH 5-6, AD attains a constant finite value. Very similar behaviour to this was also observed with lanthanum 4-hydroxy benzoate except that as tie second stage of ionization of the 4-hydroxy benzoic a,cid starts to occur beyond pH^6.5,

-165-

the direct measurement of b^ was not possible. However, as the value of Ab was found to be constant over the pH range 5-6, this value of Ab was assumed to be numerically equal to AbT and the quantity bT was estimatedL iifrom the relationship

Dl = Dj - ADl. ...(4.63)

The variation of Ab with h° observed for lanthanum benzoate andlanthanum 3-bromo benzoate differs from that found for the other two lanthanumbenzoa,tes, in that, the value of Ab changes sign at an intermediate pH?value.This type of behaviour is similar to that observed previously with thelanthanum nitro salicylates. The values of the hydrogen ion concentrationat which the values c ? Ab change sign, were used 'to estimate the valuesof bQ for lanthanum benzoate and lanthanum 3-bromo benzoate by means’ ofequation (4*44) • The values of b , found in this way, were b. .« 0 . 603o cfor lanthanum benzoate and b - 0 • 753 for lanthanum 3“ omo benzoate.o

Since, the values of Ab observed with all the lanthanum benzoates. studied in the present work, are significantly larger than the error associated with optical density measurements, it may be concluded that ion association occurs also here.

HEHEPJOTATIOIT OP b AND b-,; _ _ _ 0 1

The quantities b^ and b^ at the selected wavelengths for the present systems, were found in the usual way, that is, by means of equation (4.27). The results of these calculations are summarised in tables (4.37)> (4*38)» (4.39) and (4»40) and illustrated in figures (4*51)> (4*52), (4*53)> and (4.54)« Since,the standard deviation of both bQ and b^, in each case, is less than 2/o, the precision attained in this work must be regarded as satisfactory.

. FI

GUR

E 4-

47

AB

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LA

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WITH

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BENZ

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-167-

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URE

4-49

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ON VO vo oIN IN IN ovo ON ON o03 « 0 e

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4-50

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oo

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< CDIq «No o

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A1ISN30 IV O Ild O

(rVuu) y

TABLE

4,36

VARIATION

OF AD

WITH

h -

LANTHANUM

(t-NI

TRO

BENZ

OATE

-173-

H»OilH

so03

OHc a

IIr<:

H\0HO03IOHur'lH

OHOaia1OHONONON

eON

VD co VO 03ON H H 0O •CM 0H 0 9 ftO 0 OCO03 03 OCO O0 • • ftO O O OONtAVO CO COa- H O 0IA tA IA 0• 0 • ftO O 0ON

VO COCO CO VO HON IA IA O• ft • 0ON O O O03 COA- •d4 03H VO VO O• ft ft ftO O O OH

tA 03 vo VOH co CAA* VO vo O« ft e 0H O O 0VO 03 03 0H ON IAO VO VO 0« e e ftH O 0 0A- ON 03 A-O ON LA cHVO VO VO O

0 ft 0 ftO 0 0 O

A- 0 03 COON 0 IA0 A- VD 0a 6 • 00 O O OeIA HO \H 0 0M H Q Q Cl0 0 <£ a

COoeoIIQ<3

03IAVO

Cl

k.37

DETERMINATION

OF D

AND

Dn FOR

BENZOIC

ACID

-174-

£3oC3II

£o03

H «<!

H•OII

H

0HOainIoHKHVDcnftVD

II

ino O ON ON tN. cn oH m - f 03 H o f Ho o O O o OK -ft e 0 • • «o o o o o o

<.1 + + + I +

o * . a H H VD cn 03 mQ O in in H H ON IN

H fw cn H ON !N coII • • • • e •

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- t

03 cn - t cF in VD

O • f vo cn in H cnH ON r - VD in in co

VD r - CO ON o H0 0 ft • « e • «v y

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inoH

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00 VD e'­ in cn ONK ct* VD en in H cn

VD - t cn 03 CM in03 H ft • e • ft •

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O H 03 cn in

03 CO co in 03 coVD H VD H O

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0O H 03 03 03 ca 03 03H \ O O O o O o

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0*0 s H rH H rH H H

ftO H r-i CO vo m i—! ' inH \ 1 On On o rH CNJ vo

0 'St CNJ rH ON* C— " t - -M H « ft a 0 • 0

0.a a

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a mo ON ON ON O n ON ON

II II•/“— r sP P3 £© ft 0 •a o t-4 VD© VD 0 inu I ' - £j ONs •* P ft*a VO a 03a ca0 ii 0eS IIa

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FIG

URE

4-51

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Q lh Ju i<c c

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FIG

URE

4-52

D

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cn

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OH

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X03 H <H0 * £

Q

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1 +

H H CO vo oO 03 LA H tN, 03r*. CO CA H 03• o e « t 0o O o H H H

ON 0- ON A- Os V0ON V0 03 CO LAIA [N- co O H CA• 0 ft « « «H H H 03 03 01

CA VO Os CA H VOVO CA LA CA VO 03-cr ON r-{ t'- CA O• « « • « »O H CA VO 00

LA 00 LA co o CO00 Os 03 C 03 03CA VO H LA co OLA LA LA VOe e « • • •

o o O o o o

H H H H H HO O O O O OO O O o O o« e « ft « «H H H H H H

CD CA CA •tf* -51 LAVO H ON [N- LA CAON JN- CA o IV •sf»ft • e ft • e

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ON Os ON ON ON ON

11 11/—N /*N*P •P

a «© • © COa o a CO© o © ou ON a *-2 •• a 03IQ taCvJ td II© u ©s a H0 LO•p to •p0 o© ©«r» a • r*•H 03 •H OVO rd HOs v«/0 ft H •o Q O

• 603 VOON 03

+ i + |II ’ II

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FIG

URE

4-53

D

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OF

D„

AN

D D,

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BENZ

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-179-

/

o

00

C O

inOCN

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TABLB

%o40

DETERMINATION

OF D

AND

D, FOR

A-NITRO

BENZOIC

ACID

-180-

i

•'If*o H o -«P -4-* -4* ONH CO o o o O oO o O o o oX e • « • © 0o o o o o oi + + + 1<3

0 X0 -4< 03 LA IV CA CO CAB P o VO CO -5P CA o ONo pi VO co H •4' IV. CA03 11 • ft • 0 ft ftX CO CO CA CA CA CA11

-41##» o H •ct* CA CO CA HzL H LA H VO VO CA HB ®

O VO CO CA VO CAO • • • e 0 »H ■4* IA IA VO tvCAII<<: <poH• r» H cP CA 03 CA HH X -£p o IA IA CO O• o VO CA o IVO CO H 0 © • * ft c<H O H H CO CA CAII 0£H• r* o CO O co O O0 rv IA CO tr CA tvH 0 IA CO CA tv. LA •Cp\ Q VO VO IA IA IA IAO « ft • • « «H o o O O O oOMIA1 03 •O O H iv tv IV IV IA IAH H \ tv IV IV IV CA CA

(!) ON CA CA CA CA ONX X H 0 a « o « e0 CA CA ON ON CA CAON 'd sONGN

ON •4^ *O H IA CA CA ON CA COII H \ VO <P ON £v CO tv© 03 CO o co IV. LA

p X H e • • « « 00 g \ CO IV LA -cP CA& B

«41 eO H o O o O O OH \ ON ON CA CA CA CA© o o o O O OX H 0 e 0 e 0 «0 ON ON CA CA ON <Aca a

II II•P •Pd d© © © eB o B O© CO © Hu ON k cr>d 0* d *»m H VI CAas ©© II © I!S fi

0 H•P to -P Ua o© ©u • c*•H VO •p, CO►d ON "0 cow CA w VO

0 e H ftQ o Q o

» •O OH H+ | +1II II

to•OIAO031!Cto

COOO0o+1It

to

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COooeo+ 1

. H Qto)IACOVO0oIIH

FIG

URE

4-54

D

ETER

MIN

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ON

OF

Dn

AND

D.

FOR

4-N

ITR

O

BEN

ZOIC

A

CID

.

7.181-

co

o

CMH—

CN

OCO

-182-

DETEBI-IIMTION OF THE ASSOCIATION CONSTANTS OF THE L m M A K m BENZOATE ION PAIRS BY MEANS OF THE OFJIMZATION PROCEDURE

As seen in the case of the Le?+-/salicyl aldehyde system, the optimization procedure is not generally valid, and the results obtained by this procedure should be treated with ca.ution. Nevertheless, an attempt to evaluate the association constants of the present ion pairs by this procedure was made.

In applying the optimization procedure to the determination of the association constants of lanthanum benzoate and lanthanum 3-bromo benzoate, the values of D , calculated an the basis of the cross-over point, wereQused. Since, no direct information regarding the magnitude of D wascavailable in the case of lanthanum 4-hydroxy and lanthanum 4-nitro benzoate,the values of at the selected wavelengths wsce employed for the purposeof the optimization procedure. Accordingly, the value of I) was variedoaround these trial values of D and the standard deviation of (X, G ,C Os.

corresponding to any given value of calculated, It was found that,with the exception of lanthanum benzoate, in ea.ch case the plot ofagainst these trial values of D gave a well defined minimum, A summaryoof these calculations is given in tables (4.4l)» (4*42), and (4.43)? whichalso show the values of <x and D corresponding to these minima. In eachocase, the standard deviation of the association constant is less than 2fofthus indicating that satisfactory precision was here attained. It willalso be seen that the value of D found for lanthanum 3-bromo benzoatecis in reasonable agreement with that obtained from consideration of the cross-over point. The plots of y = (Atoc + I ^ J / o against z for these three ion pairs are shown in figures (4*55)> (4*56)» (4«57)> from

-183-

which it can he seen that the experimental points lie on very good straight lines.

The values of Dq for these ion pairs were also computed hy meansof equation (4.32). The results of these calculations are given in tables(4.4I)t (4.42), (4.43)» which show that the values of D found by thisomeans (D X) are in very good agreement with those obtained from the plots of o

against IN. The plots of y" = Ah(Dq-D^)-ADT^+D^c against c (not givenhere) show that the experimental points lie on very good straight lines.

In the case of the lanthanum benzoate ion pair, however, the plotof against D failed to show a minimum from which the best values <X 0 cof «3<and b could be obtained. Use of the value of D , obtained from c cconsideration of the cross-over point, in the optimization procedure leads

2to a value of 5.22 x 10 for the association constant for this ion pair which, as will be seen later, is in only poor agreement with that found by means of the iteration procedures 2 and 3* In view of the failure of the optimization procedure in tliD present case, an attempt to evaluate these association constants by the three iteration methods was made.

-WALMTIOII OF THE ASSOCIATION CONSTANTS OF THE LANTHANUM BENZOATE ION PAIRS

USING THE ITERATION PROCEDURES

ITERATION PROCEBURE No.1.In applying the first * iteration method to the evaluation of

the association constants of the lantnanum benzoate ion pairs, only partial success was achieved. The iteration process failed to converge

TABLE

k.kl

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF LANTHANTJK

-184-

oin.oa0oII

c,Q2ocaI!

<=H• w+rLQIAtN-03

fxlg IIQ ,<:&ao •»o Hft •ft O£ IIoH Hft< • »*N »H HVH \H 0f t Hft 0O s03IO

rH

H

ft

<3?HOa

IAIoHcoH03«

VOoHK!>><i

VOoHa

<3OH

VOOHo

VOoH3<3Q<1

03OH

OHM,Q

H\OHOH\CDHOa

•cF HO \H Q>

H Oa

CO

-tf* ON VO CA VO oIA O o 03 o HH o o O o Oe • A A A Ao o o o o o+ + 1 + + +

IA r- CA ON Is- 00LA o IN- r- H VOVO o LA 03 H COA • A A • A03 H H pH o

CO 03 IA VO CA COVO 0- H CO VO LA03 H H o H O« • A A A Ao O o o o o+ + 1 i + I

ON O n Is- 03 IA 03co CA H CA O ONON CA CA CA IA• » A A A Ao 03 CA LA VO

03 IA CA IA 03H VO o IA H LACO CO VO H 03• A A A A AIN. 0- IA CAH03 LA O I- LA VOH CA VO LA ON IN-VO A- O VO ON• • A A A AON LA CA 03IA IA LA Q H OCA 03 CM A1 CT\ VO03 03 CM CM/*N H HO O o o O Oi « 6 ft 0o o O o O 6Oo o IA o r- IAH CO CO VO ON CA03 co H CA VOCA CA <r1 -4*« A 0 © A 0o Q o O o oo O o O o oCO 00 co CO co COON ON ON ON ON ON• A A A A ACA CA CA CA CA CA

IA 03 O r-•sH O O IN- VO VOVO CO ON CO On o» A A A A oCO A- VO LA

o O o O O oON ON ON ON ON ONo o o o o o• A A A A AON ON ON ON ON ON

03OO•o+1IIKQ0

VO030oII

A03Owin Ho KQ VOIs-oA ft

03 oCAo +1AO II

+1II

.ct o

03OfH• r*

ON 0003 CAON IAA ftH Ci«II II

6

FIG

URE

4-55

D

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MIN

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OF

TH

E A

SSO

CIA

TIO

N

CO

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AN

T OF

LA

NTH

AN

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-4

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DR

OX

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BEN

ZOAT

E —

OPT

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P

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DU

RE

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•18 5'

co

o

CN

oCO

TABLE

kA2

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF LA

NTHA

NUM

- 136-

I

<oNmPQoSortcqion

voC3r-o

Q0oC3II

03-3>eaE)§ JJQ i .

«<:fx}O «*•*o HPh •& o& I!oH H<N ••H HNH ©rHPW 0O 0

03JOHXHII£

HX©HOainjoHXINHCO•ON

inoH

<3inoHX

O

< 1

oHX

inoHXo

VOoHX£< 1

Q<

Q

03OH

OHM' M

0HO

H\©HOs

HvO \H ©,': X Hj © O13

03— --j. VD 03 ON CO n-

H 03 o o o oO O o o o o

e 0 « « 0 0o o o o o o+ 1 — +) + + 3

03 CA 03 03 H ON03. eo cn 6- coO ON cn vo in• • « « e 003 H H H H

in CA VO rH CA INin cn VD H -4< HH 03 vo O H H• • • » » 0o O o O o o+ + 1 1 + +CO cn CA CA 03 o «tN H -tf* -41 vo CO 03ON 03 H in H o

e e 0 • « • om in IN A- CO »

o03 CA 03 O in H +1o CO VD INCO VO H 03 H O 11• « • « a •03 03 03 03 03 03 X GQen H in H 03•4* O m O ON -4*V£> o in ■4< H

0 e e • e •o 03 cn in VO VD

03‘n-o 03 o rs. in 03 •cn 03 H vo O oCO 1N in •4* cn cnO O o O o o !! •

0 e e « • » 03o o o o o o X o1 i 5 I 1 1 O HP X

in o in in in o cnH 03 cn in co cn cn03 vo ON H cn Oin in in in vo vo • e• « 0 0 c e CA Oo o o o O o VOo +1CO CO co CO CO co O H■ r*. IN n- IN SN N-CA ON ON CA CA ON +1 i *4- « • • e t « bcn cn cn cn cn cn II

(8in 03co cn cn -4» in ^ o

vo H ON [V. m cn HON N- cn O tN. -4* X0 0 0 • e • 03 -=FON ON ON ON . CO CO 03 CAin vo

o ecn HO O I O o O . o II IION ON CA ON ON ONo o O •O O o• o ' ■ • • • •CA CN CA CA CA CA

FIG

URE

4-56

D

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MIN

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OF

TH

E A

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TIO

N

CONS

TANT

OF

LA

NTH

AN

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-3-B

RO

MO

SA

LIC

YLA

TE

O

PTI

MIZ

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P

RO

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DU

RE

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inCM

oCM

in

inO

in

CD ooo

TABLE

k.h3

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF LA

NTHA

NUM

— —

• vo IN C\3 o CO VD COLA O H o -4 o O o03 H O o O o O oCA i4 • • • • • «• O o o o O oo !a + 1 i ! I +II < i

VD -4 co ON H VD oo O 03 -4< VD H UN COH -4* 03 03 H CO CAM « • • • • «•*- S: CA CA CA CA 03 03so03 CA *4* ON LA CA 03 CAO -4 CO O CO VD H11 H IA CA O 03 CA OM • • • • • e«•>> O1 01 oI O+ O+ 01<3

zLS CA ON VD CO CA VD COo O •4 O co o O VDfcl H H ■4 0- IN O -4* 03CA w » • • • • • e

CA CA CA •41 -4 UN HQ II Om Oo «<: VD CO CO UN CO CO ON «o o CO co CA -4 VD -4 oft • r H IA ON ON LA IN CAft H X « • • » • • +1• O O ON ON ON co IN& O H 11oH 11 VD CO IN H VD o CA KH O 03 co H VD 03 CO o< H rH O ON ON IA CO LAN » * • • • c toH #<■* r£ H H 03 CA CA CAS • <3H H\ UNft 0 03O rt UN LA IN- UN o IN CA0 Q UN O Is- ■4 H VD •I a <1 CA CA 03 03 03 H O03 O O O O O O&a 1 « • • « • « llO O O o o O o< H Mo k<ln oN H LA In CA CA o CA QS H *4* O H CO o63 II Q 03 ON Is- UN CA CACQ VD IA UN UN LA UNe • • « 0 « »O o O o O o o VDft LA• • 03H • 03 H 03 03 03 03 o o 655'I H O \ CO CO CO CO -4* -4 o\ H 0 ON ON CA CN ON ON

+1-4* 0 M H * 0 0 « • •H TJ 0 CA CA CA CA CA CA0 2 11aUN 01 •4* H UN CA CA ON ON CO

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-190-

in the case of the lanthanum benzoate and lanthanum 4~&itro benzoate ionpairs, in spite of the fact that the initial value of P/KT^ was variedover a wide range.

On the other hand, the iteration process was found to convergerapidly in the case of the lanthanum 4-hydroxy benzoate ion pair and gavethe value 281 for the association constant with a standard deviation of~jfo. The results of these calculations, in which no difficulties regardingthe starting value of P/ET^ were encountered, are summarised in table(4,44) and illustrated in figure (4*58).

The case of the lanthanum 3-hromo benzoate ion pair is interestingin that it stresses the importance of a judicious choice for the startingvalue of P/KT-vr. Here, convergence was reached only after the initialvalue of P/ET j was pinpointed to the third decimal place. The resultsfor this ion pair are presented in table (4.45) and. illustrated in figure(4,59). It will be noticed that the values for K, found by this means,are much less than those obtained by use of the optimization procedure.

Thus, it appears that even when results are obtained by theoptimization procedure, they cannot be regarded as reliable, unless thecalculated value of D agrees reasonably well with that found experimentally,oor by other means.

The failure of the first iteration method in the case of thelanthanum benzoate and the lanthanum 4-nitro benzoate ion pairs, suggeststhat the choice of a suitable value for starting the iteration processis not the only condition that must be satisfied in order for a givenapproximation method to be successful. The second condition is concerned

Ow-lwith the properties of the matrix^which consists of the partial derivatives of the functions underlying a given approximation procedure and will be discussed in greater detail in the Conclusions.

-191-

ITERATION PHOCEDURES 2 AND 5Since,the application of these iteration procedures leads to

virtually the same results for the association constants, it will be convenient to consider them together. Just as the.first iteration procedure, these methods failed in the case of the lanthanum 4-nitro benzoate ion pair but were successful in the remaining cases. The results of the calculations obtained by the two procedures are summarised in tables (4*4^) - (4.5l) from which it can be seen that the values obtained

by the two methods agree very well with one another. The poor agreement between the value of the association constant for lanthanum benzoate obtained on" the basis of the cross-over point and the latter values, is clearly due to the difficulties of correctly assessing the hydrogen-ion concentration at that point.

Plots of y* = (Ahx + Tt3L)/c against x, based on these resultsJj xfor iteration procedure No.2, gave very good straight lines. Only the plot for lanthanum benzoate is given here (figure (4 ,60)), as those obtainedfor lanthanum 4-hydroxy and lanthanum 3~hromo benzoate are virtuallyidentical with the plots found by use of iteration procedure No.l. Plotsof y = (t t-c)/c against 1 + hf^ /K_ , based on the results for iteration

procedure No.3, also gave good straight lines, and are shown in figures(4.61) - (4.63).

APPLICATION OF TUB ITEPJLTION PROCEDURES TO THE LANTHANUM SALICYLATE ION PAIRS

To test the reliability of the iteration procedures still further, they were applied to the recalculation of the association constants of the lanthanum salicylate ion pairs. The results 01 the calculations are

TABLE

4.44

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF LA

NTHA

NUM

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4.46

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TABLE

h .51

DETERMINATION

OF THE

ASSOCIATION

CONSTANT

OF

-204-

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summarised in table (4.52) which gives the values of I) , the associationoconstants and the corresponding standard deviations. It can be seen that in the case of the lanthanum salicylate, and the lanthanum 3-niethyl, lanthanum 5-bromo and lanthanum 5-chloro salicylate ion pairs, all iteration methods yield results which agree not only with one another but also with the results obtained by the optimization procedure.

However, all three iteration procedures failed completely in the case of the lanthanum 5-nitro salicylate ion pair, while in the case of the lanthanum 3-^itro salicylate ion pair only iteration procedure Ho.l, gave a result; this is about 10% lower than that obtained by use of the optimization procedure. It is interesting to note that here, convergence occured only after the value of F/ET^ had been pinpointed to the second decimal place. The case of the lanthanum salicylate ion pairs again illustrates the importance of the two conditions for a successful application of iteration methods 5

1, The choice of a suitable starting value for the iteration procedure.

2. Suitable form of the equations which underlie the iteration procedure.

-207-

to£3

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HEh cn CO vo n-< o 03 cn n- HPS Q C3 03 C3 03 i i• e • eEh O o O oH63 in r» 03 cop* • • « «/"S o o O ou63 +1 +! + 1 +1 i 1o Wo cn CO 03 03ea • « ft ftA * CO ON r- co0 H H n- coIS S3 H HoHEH cn CO vo n-< o 03 cn r- HPS Q 03 03 03 03 i 163 • • • ft

O O O OHM VO 03 1- cn CO .K • e ft 0 »o H o H •chwmO bd +1 + | +i + 1 i +1wO IN- O in ON voPS H • • • ft 6P-. • n- o IN CO in0 H 03 oo cn•7» Hr H-i h H HoHEh cn CN VO E- 03< O 03 cn n- Hft P 03 03 03 03 i cn63 • t • • •E-«H O O o O o

•d 'd *d•ri •ri •d •ri 'd *dO O •ri O •H •ria <3 O (3 O OP (3 (3 <3H O O OO •ri •ri O •ri O O' <f rri H •ri H •H •riIn In H H HO O O In O InH d •ri o •ri o oH H •H H •ri •ri>■ <3 13 H (3 H HC to 10 (3 03 (3 (3H 10 03 CO

h: H 0<t In O u 0 0w a 0

+> o H +> -P0 u £ •ri •rit3! »Qi 01 £1 3i4cn in in tn cn

-208-

S E C T I O N V

CONCLUSIONS

-209-

To assess the significance of the results obtained, it will be convenient to recapitulate the main objectives of the present work, as set out in the introductions

1* To s-fcudy the efficacy and usefulness of spectrophotometricmeasurements in determining the association constants of complexes formed

3+by the La -ion with anions of weak acids.2, To investigate substituent effects and the correlation of the

stabilities of the metal complexes to those of the corresponding proton complexes.

The extent to which these objectives have been realised in the present work, will become apparent from the following discussion.

REVIEW; Off TUB SPECTRQPHOTQITITRIC I4ETII0DS FOR DETEHfflMKx ASSOCIATION CONSTANTS

DEVELOPED IK THE PRESENT WORK

All the procedures evolved in the present work for the determination of the association constants, rely on spectrophotometric measurements alone. It will be remembered that an essential feature of the experimental method adopted for this purpose, is the comparison of the absorption curves of a pair of corresponding solutions, one ox which contains both the ligand find the metal ion?and the other the ligand only. This method is very effective in detecting ion association, provided that the optical density of the experimental solutions is measured over a sufficiently vide wavelength range for significant differences in the two absorption curves to be observed.

Another important condition for the successful application of this procedure, is O judicious choice of the concentration of the metal ion

-210-

T^, that of the ligand T^, and the wavelength. To maximize the extentof complex formation and, at the same time, to minimize the formationof complexes containing more than one ligand per metal ion, it is important

3+that the ratio La -ions ligand in the experimental solutions, he aslarge as possible.

In some cases, e.g. lanthanum salicylate, the method enables thevalue of D to be estimated from the plot of AD against h°, with a fair 0degree of accuracy. If such is the case, it is possible to calculatedirectly sn approximate value of the association .constant from:equations (42L(4.3l) and (4.23). Sometimes, a rough value of D can be obtained bycmaking use of the cross-over point exhibited by the plot of D° and Dagainst h°. This type of behaviour was observed with the lanthanum nitrosalicylates and lanthanum benzoate and lanthanum 3-Lromo benzoate. Ingeneral, however, no direct information about D can be obtained fromthe variation of AD with h°. In such cases, it is necessary to resortto equations (4.54)» (4*2l), (4«3l)> and (4.23) which relate DT to DJj cand K.

It is possible to compute the association constant of a given ion pair using the data obtained for both series of solutions, (A and B); such treatment of data is employed in the optimization procedure and the first iteration method. Alternatively, the association constants of the ion pairs can be evaluated by using only the data associated wxth the solutions of series A. Such treatment of data lies at the basis of the second and third iteration methods. In view of the difference in the treatment of the experimental data, some discrepancy between the results found by the optimization and first iteration procedures on the one hand, and the second and third iteration methods on the other, is to be expected. For convenience,

the values of the association constants obtained in the present work by the various methods are summarised in table (5#l).

The results obtained from the optimisation procedure indicate that the method has serious limitations, in that, it requires a knowledge of a fairly accurate value of D . However, even if such is the case, the

G

results obtained by this method cannot be regarded as being reliable., unless the calculated value of 3) agrees reasonably well with that foundGexperimentally, or by other means. As already seen, this condition is satisfied in the case of all the lanthanum salicylate ion pairs.Consequently, the values obtained by this method for the association constants of these ion pairs can be treated with confidence. The reliability of these values is also enhanced by the fact that, with the exception of lanthanum 5-nitro salicylate,they all agree very well with the values obtained by the iteration methods.

The optimization procedure leads also to definite values for the association constants of lanthanum 4-hydroxy and lanthanum 3-bromo benzoate which however, differ considerably from those found by the thiee iteration methods, and have thus to be rejected. In the absence of confirmation from any of the iteration procedures, the value which the optimization procedure yields for lanthanum 4-ftitro benzoate must be also considered as suspect.

The optimization procedure totally fails xn the casj of thelanthanum/salicylaldehyde and benzoic acid systems, in that, for eachcase the plot of against Dq showed no minimum, which, could be usedto evaluate the best values of D and K,c

As already- mentioned, one of the fundamental conditions for a successful application of the approximation procedures, is that the

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5•1,

(Con

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CA V0 cn cn0 O o o

0 H H H*7? hz*f-H fH 1-!ft M M•AO /"N /°S /■=\H £3 in vo

o H HP « 0 a

K Q o OS3 03 +i i i

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Cm a e aPM cn Oo on

s—' v-/

03 vo cn cn 'e o O O

0 H H HH“»* I-? 1.1n n MO /-sH £3 o 03 03B g o O o<Z P 0 • a£5 Q o . o .03 03 + + +H O 03 CA -d«H O in VO 03

Pm 0 0PM cn 01 cn

'w' >—✓ w

H vo cn cno o O

0 H H H55 fd K K MO N *NH P O cn w^ s o o o<3j p « 0 0PJ Q o . o .£J 03 +l + t^ o cn OA -d-lH O in VO 01

$ * a ftPM cn 03 cn

w V/ w

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CM &O

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0 H HVj rA 1

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H w ta

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-214-

starting value of F/EF^ should he as close to the true value as possible.Just how precarious the choice of a suitable starting value for the iteration process may become, can be illustrated by the application of the iteration procedure No.l. to the case of 3antlianum 3“do?omo benzoate.Here, the iteration process fails to converge if the starting value of P/K3?k is greater than 3*04 02? less than 3 *03* However, convergence is attained after only one cycle if the starting value is equal to the average value of these limits.

Even if the condition regarding the choice of a suitable starting value is satisfied, the iteration process may still fail to converge unless a second condition is fulfilled. According to Zaguskin [114] this condition requires that the sum of the elements of every row and every column of the matrix consisting of the partial derivatives of the functions underlying the approximation procedure, be less than unity. In the present work, failure to converge has been observed in the case of lanthanum 5-nitro salicylate with iteration procedures 1,2,and 3* lanthanum 3-nitro salicylate with iteration procedures 2 and 3* lanthanum benzoate with iteration procedure 1 and lanthanum 4-nitro benzoate with iteration procedures 1, 2, and 3» (see table 5*l)» Since,in all these cases, the starting value for the iteration process was varied over a very wide range, the conclusion must be drawn that the failure to converge here, . is due to the second condition not being satisfied. As seen from table (5«l) in the majority of cases, howeverj studied in the present work, the iteration procedures lead to a successful conclusion , the results obtained by the three iteration methods agreeing remarkably well with one another.

In conclusion, it should be pointed out that whenever the conditions for convergence are satisfied, the iteration procedures, evolved in the

-215-

present work (on the basis of optical density measurements alone), lead to highly reliable results for the association constants of the ion pairs.The fact that in certain cases, one iteration procedure may fail whileothers succeed, shows that a further examination of the conditions for convergence is neccessary.

The stability of an ion pair is governed by a number of factors, of which, the most important are -

1. The nature of the substituent on the ligand..2. The magnitude of the electric charges on both metal ion and

ligand,3. The ionic radii of the metal ion and ligand,4* Chelation.5.- Dielectric constant of the medium.Of these, only the substituent effects, the size of the ligand and

3+chelation need to be considered here, as only the ion pairs of the La - ion in aqueous medium, are the subject of the present work.

It is reasonable to assume that the effect of a given substituent on the stability of the metal complex will be the same in nature as that on the corresponding proton complex but different in magnitude. This means that for a series of closely related ligands, (derived from weak acids), the stability of the metal complex should increase monotonically with increasing stability of the corresponding proton complex. This type of behaviour is, indeed, observed with all three groups of ion pairs studiedin the present work. This can be seen from table (5*2) in which the ionpairs are arranged in three groups and which, in addition, to the diss­ociation constants of these acids, also gives the average values for the stability constants of the ion pairs found by the various methods.

TABL

E-216-

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TABLE

5»2.

(cont

inued)

-217-

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-218-

In some cases, e.g. ferric salicylate [2l] the difference in the standard free energy of formation of the metal and corresponding proton complexes is virtually independent of the nature of the substituent on the ligand. This condition can be expressed mathematically, by writing

AG°T Tt - AG°HT[ = Constant, ...(5*1)

where and AG0^ are the standard free energies of formation of themetal and proton complex, respectively.

Since, by definitionAG°t _ = -RTlnK, ...(5.2)nail

and AG°hl = -RTlnl/l^ = RTlnK^ ...(5.5)

equation (5*l) becomes

' AG°LaL “ AG°HL 88 “RTlnI-/Iw = Constant. ...(5.4)

The last equation on rearranging takes the form. log K ss pK^ + Constant. ...(5*5)

Equation (5.5) predicts that a plot of log K against pK^ should give astraight line with a slope equal to unity.

Such a plot is given in figure (5.1) fo** the ion pairs of theLa^-ion with the salicylate and benzoate anions and in figure (5*2) for

5+the ion pairs of the La -ion with the sali^ylaldehyde anions. In the figures, the experimental points are numbered ir. the same way as the corresponding ion pairs are numbered in table (5*2). It can be seen tha,t, in both cases, a reasonable straight line was obtained. In calculating the slope and intercept of these correlation plots, the method of least squares was used. The slopes and intercepts found by this means are 0.386 i 0^083 and 0.847 * 0.210 for figure. (5 JL) ,ard 0.998 * 0..030 and

-219-

-■1.802 i .0.196 for figure (5*£)> respectively.The fact that,in both cases, the slope is less than unity indicates

that, on average, substitution affects the stability of the ion pairs to asmaller extent than the respective proton complexes. This type of behaviouris to be expected since the proton in the proton complexes is covalentlybound and, consequently, is subject to stronger substituent effects than the

3-1 -La -ion in the ion pairs.To find to what extent the variates, in this case log K and pK^,

are correlated to one another, the correlation coefficient r [20] was calculated using the formula

r = z . & o g K - i s r r i ( p V ^ ) 2 O ios k)2]s,...( 5.6)

where n is the number of experimental points and the bar denotes the meanof the respective variables. In the limiting case, when the variates arefully correlated r assumes the values +1 or -1, It is zero when thevariates are not related at all.

The correlation coefficients, found in this way, are 0.851 for the 3+ion pairs of the La -ion with the salicylate and benzoate anions and 0.996

3+for the ion pairs of the La -ion with the salicylaldehyde anions. It should be pointed out, however, that the coefficient of correlation for the La^+-/salicylaldehyde ion pairs ;3S ; based on three points only and hence, is not very reliable. The present values for the correlation coefficients, are slightly smaller than/Jfound by Ernst and Menashi [2l] for the ferric

•fc Vsalicylates but are slightly larger than/Jound by Ernst and Herring [22] for the ferric phenolates. The relatively high values of r obtained in the present work, nevertheless, indicate that also, in the cases studied in the present work, a rough linear correlation exists between the stabilities of the metal complexes and those of the corresponding proton

complexes.It appears, therefore, that linear substituent effects occur not

only with covalent complexes but also with ion pairs. We are also led to the conclusion that since*all the metal complexes studied in the present work, are due to ionic interaction alone, Williams1 theory [l2] regarding the significance of the slopes of correlation plots is not generally valid.

Further insight into the significance of the results found in the present work, may be obtained by considering them in the light of the Bjerrum theory of ion association [62],

The stability of an ion pair can be explained In terms of the standard free energy of formation by

AG° t =-BTlnK, ...(5.7)

where K is the association constant of the ion pair. Another measure of the stability of an ion pair is the interionic attraction energy

z^Zge /a 5*8)

where a is the distance of closest approach between the. ions. It appears, therefore, that a plot of log K vs l/a should yield a straight line.The distance of closest approach can be calculated using equations (1.4),(1.6) and (1.7) given in the Introduction.

The results of these calculations for the ion pairs are given in table (5.2). A plot of log E against l/a as shown in figure (5*3) for the ion pairs of the La -ion with the salicylate and benzoate anions and in-.figure (5*4) for the ion pairs of the La^+-ion with the salicylaldehyde anions. In the figures, the experimental points are numbered in the same way as the corresponding ion pairs are numbered in table (5*2).It can be seen the!, in both cases, the experimental points fall on good

FIG

URE

5-1

CO

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-221-

Lf)

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FIG

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5-2.

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-222-

oo

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FIG

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5-3

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-223-

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straight lines, thus indicating that the stability of the ion pairs studiedin the present work, is inversely proportional to the distance of closestapproach, as to be expected on the basis of the Bjerrum theory.

The correlation plot shown in figure (5.3) can be also used todraw some conclusions regarding the magnitude of the interaction of the 3+La -ion with the dipole of tlie-GH group in the lanthanum salicylate ion

pairs•It has been shown in the course of the present work that in the

3+lanthanum salicylate ion pairs, the La -ion associates with the carboxylate anion only, leaving the -OH group intact. This can be represented by the formula,

OH

COO” ... La5+nevertheless, in view of the proximity of the -OH group, it is still

3+possible for the La -ion to interact with its dipole. However, the factthat, in constructing the correlation plot given in figure (5.3)> itwas possible to group together the lanthanum salicylate ion pairs withthe lanthanum benzoate ion pairs suggests that in both types of complexesonly the carboxylate group is involved aid consequently that in the

3+lanthanum salicylate ion pairs, the La -ion dipole interaction is insignificant.

On the other hand, chelation through La^+-/dipole interaction3+ /appears to play some part in the formation of the La' -/salicylaldehyde

ion pairs. Some support for this conclusion can be drawn from the fact that3+ .the association constant of the La -/4-hydroxy benzaldehyde ion pair,

where chelation of this type is not possible, was found to be too small for reliable measurements to be made.

APPENDIX

PLOW DIAGRAM POR THE OPTIMIZATION PROCEDURE

Read in data5 I initial trial value ofD-, increment of '.0 , final trial value ..of D • c9 cy cSets of (a-b), D°, D values.

Calculate h° from equation (4*33)I Calculate x, h°f-^, D°x.| Apply least squares to equation (4.27)1j obtain D and Dn.I o 1| Calculate standard deviations of D and D_.| I o 1 jT

tI Use initial trial value of D ,i c

■ -fcU Set Ah = 0.-•^Calculate c from equation (4.3l) j Calculate Ah from equation (4.21)| Test to see if previous value of Ah | differs by ^ 0.01 $.

J .j, YESNO

Calculate (Ahx + TjK^)/xCalculate Ah(DQ - D^) - ^DT^ +Apply least squares to equations(4.23) and (4,32); obtain K andD respectively cCalculate standard deviations of K and D

tPrint h° x;, h0^ , D x, Dq, , cr^, .c, Ah, y*, ylf, D^,K, OC. , , D^ , sD<-

rAdd selected increment to D .cTest to see if new value of D greatercthan final trial vdue of D~ selected._________ c______________ jNO ,, YES

STOP

A A

Read in data.initial trial value of F/KT^,

selected increment of F/KT^, final trial value- of F/KT^. Sets of (a, - b), D°, D values.

"IE....Calculate h° from equation (4*33)Calculate x, h°f^^, D°x.Apply least squares to equation (4*27)5 obtain Dq and D .Calculate standard deviations of D and IL.o 1Use initial trial value of F/EP^. Print f/e P ,Calculate D from equation (4.54)* cSet Ah = 0.Calculate c from equation (4.31)*Test to see if c <" 0.

NO *V».

YESCalculate Ah from equation (4.21). Test to see if previous value of Ah differs by ^ 0.01$.

NO y YESCalculate (Ahx + T KL )/x.Apply least squares to equation (4.23)5 obtain F/EP^. Print F/KT^.Test to see if rate of change of F/KT^ with successive cycles is becoming smaller.

Calculate standard deviation of" f/k T .Print h°, x, h°f-, D°x, D , D , cr-p. , ,

7 7 ± 7 7 o 7 I 7 D0 Uqc, Ah, y ’j y,f5 Rc>

STOPAdd increment of F/KT^ to value of F/

YES f i NOTest to see if previous value of F/KT^ 1jdiffers by ^ 0.01$, 5ii.... NO 1 j- YES

M* -«*3-Test to see if new value of F/EP. is greater than final trial value of F/EP^.Print new value of F/EP^.

NO YESSTOP

FLOW DIAGRAM FOR ITERATION PROCEDURE No.2.Read in data.

initial trial value of F/KT^? incrementof F/EL1, final trial value of P/KT^, D^, Dn, DT.Sets of (a - b) and D values,I ... - ^ .

j Calculate h° from equation (4*33)'| Calculate x.

Use initial trial value of F/KT^. Print F/KT^,i -^Calculate c from equation (4.59) •

Calculate Ah from equation (4.21), j Test to see if c < 0. |

NO V YES

Calculate (Ahx + T A )/x.Apply least square to equation (4,25)5 obtain F/I{Tt<t, Print E/El™.i-i ' IITest to see if rate of change ox F/KT^ with successivecycles is becoming smaller.

YES V !. HOTest to see if previous value of F/KT. differs by £ 0.01$.

M

NO ^ YESCalculate standard deviation of F/lCT . Calculate D from equation (4.54)*„ . __ _ Q

±j Print h , x, c, Ah, Dq, K, F/KD?m,

I MSTOP

J

Add increment of F/KT-,. to value of F/KTj, Test to see if new value of F/KPm is greater than final trial value of F/KT,,M^ NO 1i YES

STOP j

-230-FLOW DIAGRAM FOR ITERATION PROCEDURE No. 3.

A A

jRead in data.initial trial value of F/KT^, increment

of F/ET.vj, final trial value of F/KT^, Dq, D^, D^. j Sets of (a - b) and D values.

rCalculate h° from equation (4*33)• Calculate x.

Use initial value of F/KT^. Print F/EE^, -5*-Calculate c from equation (4* 59) •

Test to see if value of c v 0.NO f

Calculate h from equation (4# 6l).Calculate (T^-c)/c and 1 +Apply least square to equation (4.62)5 obtain F/KT^. Pnint F/lT? .Test to see if rate of change of F/KT.. witho / nsuccessive cycles is becoming smaller.

YESTest to see if previous value of f/et.differs by 0.01$.' NO™

M

IYES

^Calculate standard deviation of F/KT^.Calculate D from equation (4.54) •C . ..

Print h, x, c, (T^-c)/c, 1 + hf^ /K^» Dc? K, F/KTm?

TSTOP

YES

NO

I Add increment of F/KT, to value of F/KT^,| Test to see if new val'e of F/KT^ is | greater than final trial value of F/KT^. Print F/KTm.

NO T YESSTOP

-231-

R S F E E E N C E S

-232-

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O ffp rin te d f r o m the Transactions of The Faraday Society, N o . 544 , V o l . 64, P a r t 4 , A p r i l , 1968

S P E C T R O P H O T O M E T R I C D E T E R M I N A T I O N O F A S S O C I A T I O N C O N ­

S T A N T O F L A N T H A N U M S A L I C Y L A T E

Spectrophotometric Determination of Association Constant of Lanthanum Salicylate

B y Z . L . E rn s t a n d P . J . N ew m an

D e p t , o f C h e m is t r y , U n iv e r s i t y o f S u r r e y , L o n d o n , S . W . l l

Received 21th June, 1 9 6 7

Spectrophotometric evidence has been obtained for the formation of the ion-pair LaHSal2+ in aqueous solution of lanthanum salicylate. A new spectrophotometric method of determining associa­tion constants of ion-pairs of metal ions with anions of weak acids is described. The method, which dispenses with pH-measurements, has been applied to determine the association constant K = {LaHSal2+}/{La3+}{HSal-} for the LaHSal2+ ion-pair; at 25°C its value is 1-19 x 102.

T h e s t a b i l i t y c o n s ta n ts o f t h e c o m p le x e s o f t h e F e 3 + - i o n w i t h s o m e s u b s t i tu te d

s a l ic y l ic a c id s h a v e b e e n r e p o r t e d 1 a n d t h e c o r r e la t io n f u n c t i o n r e la t in g t h e s ta b i l i t ie s

o f th e s e c o m p le x e s t o th o s e o f t h e c o r r e s p o n d in g p r o t o n c o m p le x e s w a s d is c u s s e d . I n

g e n e r a l , t h e f o r m o f s u c h c o r r e la t io n fu n c t io n s d e p e n d s o n m a n y f a c t o r s , s u c h as th e

i o n i z a t io n p o t e n t ia l o f t h e m e t a l i o n , 2 p o la r i z a t i o n o f t h e l i g a n d b y t h e m e t a l i o n , 3

a n d i n p a r t ic u la r T i-b o n d in g w h ic h p la y s s u c h a n i m p o r t a n t p a r t i n t h e f e r r ic s a l ic y la te

c o m p le x e s . I t s e e m e d d e s ir a b le , t h e r e f o r e , t o e x te n d th e s e s tu d ie s t o m e t a l io n s

w h ic h a r e u n l ik e ly t o e n te r in t o 7 t -b o n d in g w i t h t h e s a l ic y la te a n io n s , a s e .g . , t h e

L a 3 + - io n . S in c e t h e a b s o r p t io n s p e c tr a o f a q u e o u s s o lu t io n s o f s a l ic y l ic a c id h a v e

b e e n s tu d ie d 4 i t w a s d e c id e d t o a d o p t t h e s p e c t r o p h o t o m e t r ic m e t h o d . I n th is p a p e r

w e p r e s e n t a m e t h o d o f d e t e r m in in g a s s o c ia t io n c o n s ta n ts o f io n - p a i r s ( in v o lv in g

a n io n s o f w e a k a c id s ) w h ic h d is p e n s e s w i t h p H - m e a s u r e m e n ts a n d re l ie s s o le ly o n

s p e c t r o p h o t o m e t r ic m e a s u r e m e n ts .

E X P E R I M E N T A L

R E AG EN TS

A l l re a g e n ts w e re o f a . r . g ra d e un less o th e rw is e s ta te d . S a lic y lic a c id a n d N a C 1 0 4

( B .D .H . la b o r a to r y g ra d e ) w e re p u r if ie d as p re v io u s ly d e s c rib e d .4* 5 T h e m e th o d o f p re ­p a r in g s o lu tio n s o f N a O H , N a C 1 0 4 , H C 1 0 4 a n d s a lic y lic a c id h a s a ls o b e e n r e p o r te d .4* 5 A s to c k -s o lu t io n o f L a ( C 1 0 4) 3 w a s m a d e u p b y d is s o lv in g 9 9 -5 % L a 20 3 (J o h n s o n a n d M a t t h e y )

in a H C 1 0 4 s o lu t io n . T h e re s u lt in g s o lu t io n w a s a n a ly z e d f o r la n th a n u m u s in g a s o lu t io n o f E D T A 6 w h ic h h a s b e e n s ta n d a rd iz e d 7 a g a in s t s p e c tro s c o p ic a lly p u re z in c (J o h n s o n a n d M a t t h e y ) . T h e c o n c e n tra tio n o f fre e H C 1 0 4 in th e L a ( C 1 0 4) 3 s to c k -s o lu t io n w a s fo u n d b y

p a s s in g a n a l iq u o t o f th e s o lu t io n th r o u g h a c a tio n -e x c h a n g e re s in (Z e o c a rb 2 2 5 ) a n d t i t r a t in g th e e lu a n t w i t h a s ta n d a rd C 0 2-fre e N a O H s o lu t io n u s in g m e th y l r e d as th e in d ic a to r .

O P T IC A L D E N S IT Y M E A S UR E M E N TS

O p t ic a l d e n s ity m e a s u re m e n ts w e re c a r r ie d o u t a t 2 5 0 ± 0 T ° C b y m e a n s o f a U n ic a m S .P . 5 0 0 s p e c tro p h o to m e te r u s in g m a tc h e d s ilic a ce lls ( o f 4 -c m o p t ic a l p a th le n g th ) . T h e

in s tr u m e n t w a s c a l ib ra te d as p re v io u s ly d e s c rib e d .4 T o te s t th e r e p r o d u c ib i l i ty o f th e

m e a s u re m e n ts d u p lic a te s o lu tio n s w e re u s e d th r o u g h o u t . A l l c a lc u la t io n s w e re c a r r ie d o u t o n a F e r r a n t i S ir iu s c o m p u te r .

1052

Z. L. ERNST AND P. J. NEWMAN 1053

R E S U L T S

SPECTROPHOTOMETRIC EVIDENCE FOR I O N - P A I R I N G

T h e s y m b o l H 2L r e p re s e n ts s a l ic y l ic a c id . A ls o , t o d is t in g u is h s o lu t io n s o f L a -

s a l ic y la te f r o m th o s e o f s a l ic y l ic a c id , t h e s y m b o ls a s s o c ia te d w i t h t h e la t t e r s o lu t io n s

a r e w r i t t e n , w h e r e a p p r o p r ia t e , w i t h t h e s u p e r s c r ip t z e r o . F o r t h e f i r s t c o n s ta n t o f

s a l ic y l ic a c id t h e v a lu e 4 K ± = 1 -0 6 4 x 1 0 - 3 is u s e d .

T h e e x p e r im e n t a l p r o c e d u r e in v o lv e d m e a s u r e m e n ts o f t h e o p t ic a l d e n s ity o f t w o

s e rie s o f s o lu t io n s . S o lu t io n s o f s e ries A w e r e p r e p a r e d b y m ix in g f ix e d a m o u n t s o f a

s t a n d a r d L a ( C 1 0 4 ) 3- s o lu t io n , a n d v a r y in g a m o u n t s o f a s t a n d a r d H C 1 0 4- s o l u t i o n ;

t o e a c h m ix t u r e a c a lc u la t e d a m o u n t o f a s t a n d a r d N a C 1 0 4 - s o lu t io n a n d a r e q u is ite

a m o u n t o f w a t e r w e r e a d d e d s o t h a t t h e s t o ic h io m e t r ic c o n c e n t r a t io n TM o f

L a ( C 1 0 4) 3 , t h e s t o ic h io m e t r ic c o n c e n t r a t io n r L o f s a l ic y l ic a c id , a n d t h e io n ic

s t r e n g th / o f t h e f in a l s o lu t io n s w e r e m a in t a in e d c o n s ta n t t h r o u g h o u t , a t TM = 1 0 ~ 2

m o le /1 . , T L = 2 -5 x 1 0 - 4 m o le /1 . , a n d I = 0 - 1 . T h e s t o ic h io m e t r ic c o n c e n t r a t io n o f

( L a C 1 0 4 ) 3 w a s 4 0 t im e s g r e a t e r t h a n t h a t o f s a l ic y l ic a c id , th is la r g e exc e s s o f L a 3+-

io n s b e in g u s e d t o m in im iz e t h e f o r m a t i o n o f io n p a ir s c o n t a in in g m o r e t h a n o n e

l ig a n d p e r m e t a l - io n . S o lu t io n s o f s e ries B w e r e p r e p a r e d i n t h e s a m e w a y e x c e p t t h a t

t h e L a ( C 1 0 4 ) 3 w a s r e p la c e d b y N a C 1 0 4 t h e a m o u n t o f w h ic h w a s a d ju s te d so as t o

k e e p t h e io n ic s t r e n g th c o n s ta n t . T h e H + - io n c o n c e n t r a t io n o f t h e s o lu t io n s r a n g e d

f r o m 5 x 1 0 - 2 t o 1 0 ~ 5 m o le /1 , i n w h ic h r e g io n o n ly t h e f i r s t s ta g e o f i o n iz a t io n o f

s a l ic y l ic a c id o c c u rs a p p r e c ia b ly ; a c c o r d in g ly , t h e o n ly l ig h t - a b s o r b in g s p e c ie s w h ic h

a r e c o n s id e r e d a r e H 2L , H L ~ a n d a n y c o m p le x e s t h a t m a y b e p r e s e n t .

T a b le 1.— V a r ia t io n o f A D w i t h h°

7 l = 2 -5 0 9 x 1 0 " 4 m o le /1 . ; Tu = 1 -0 02 x 1 0 ~ 2 m o le /1 . ; X = 3 2 6 m / i ; / = 4 c m ; 7 = 0 - 1

h° x 1 0 3 0 -0 1 0 0 -1 2 3 0 -2 2 8 0 -6 4 2 1 -3 0 0 2 -5 9 6 3 -5 9 3 6 -0 6 2 1 0 -0 4 4 9 -3 2D° 0 -2 2 8 0 -2 3 7 0 -26 1 0 -35 1 0 -4 2 6 0 -5 0 7 0 -5 4 2 0 -5 8 6 0 -6 2 0 0 -6 8 3D 0 -2 2 8 0 -2 3 2 0 -25 1 0 -3 3 2 0 -3 9 9 0 -4 7 9 0 -5 1 6 0 -5 6 5 0 -6 0 7 0 -6 7 9AD 0 -0 0 0 0 -0 0 5 0 -0 1 0 0 -0 1 9 0 -0 2 7 0 -0 2 8 0 -0 2 6 0 -0 2 1 0 -0 1 3 0 -0 0 4

T o o b t a in s p e c t r o p h o t o m e t r ic e v id e n c e f o r t h e o c c u r re n c e o f io n - a s s o c ia t io n in th e

L a - s a l ic y la t e s o lu t io n s t h e a b s o r p t io n c u rv e s o f t h e t w o ty p e s o f s o lu t io n s w e r e r e c o r d e d

a t v a r io u s p H - v a lu e s ( in t h e r a n g e 1 -5 -5 ) a n d t h e w a v e le n g t h r e g io n 2 4 0 - 3 5 0 m ,u . I n

th is r e g io n , i n w h ic h n e i t h e r N a C 1 0 4 n o r L a ( C 1 0 4 ) 3 a b s o r b s l i g h t , t h e e f fe c t o f t h e

L a 3+- io n s o n t h e a b s o r p t io n c u rv e s o f s a l ic y l ic a c id w a s r e la t i v e ly s m a l l e x c e p t a t

X = 3 2 6 mji a t w h ic h t h e p H - v a r i a t io n s o f t h e o p t ic a l d e n s ity w e r e t h e m o s t p r o ­

n o u n c e d . T h is w a v e le n g t h , t h e r e f o r e , w a s a d o p t e d f o r t h e s tu d y o f c o m p le x f o r m a ­

t i o n in t h e L a - s a l ic y la t e s o lu t io n s . A t th is w a v e le n g t h a n d t h r o u g h o u t t h e p H -

r e g io n e m p lo y e d t h e o p t ic a l d e n s ity D o f t h e L a - s a l ic y la t e s o lu t io n s (s e r ie s A ) is

a lw a y s s m a l le r t h a n t h e o p t ic a l d e n s ity D° o f t h e s a l ic y l ic a c id s o lu t io n s (s e r ie s B ) .

T h e re s u lts o f th e s e e x p e r im e n ts , a r e g iv e n i n t a b le 1 w h ic h s h o w s t h e v a r i a t i o n o f b o t h

D a n d D ° w i t h t h e H + - io n c o n c e n t r a t io n h° o f t h e l a t t e r s o lu t io n s . T h e d i f fe r e n c e

AD = D° — D in c re a s e s a t f i r s t w i t h in c r e a s in g h°, re a c h e s a m a x i m u m a t h° ~ 1 0 - 3

m o le /1 . , a n d t h e n d e c re a s e s .

S in c e , i n t h e p H - r e g io n c o r r e s p o n d in g t o th is m a x i m u m , t h e AD v a lu e s a re

s ig n i f ic a n t ly la r g e r t h a n t h e e x p e r im e n t a l e r r o r in v o lv e d in t h e o p t ic a l d e n s i t y m e a s u r e ­

m e n ts w e c o n c lu d e t h a t s o m e c o m p le x f o r m a t i o n t a k e s p la c e in t h e L a - s a l ic y la t e

s o lu t io n s . O n t h e l e f t - h a n d s id e o f t h e m a x im u m , t h e q u a n t i t ie s D a n d D° a p p r o a c h

e a c h o t h e r a s h° d e c re a s e s u n t i l a t / z ° ~ 1 0 - 5 m o le /1 . , w h e n m o r e t h a n 9 9 % o f t h e

1054 ASSOCIATION CONSTANT OF LANTHANUM SALICYLATEs a l ic y l ic a c id h a s d is s o c ia te d , t h e y b e c o m e v i r t u a l l y e q u a l , a t a v a lu e o f a b o u t 0 - 2 3 .

O n t h e r ig h t - h a n d s id e o f t h e m a x im u m , D a n d D° c o n v e r g e as h° in c re a s e s a n d

a p p r o a c h e s t h e v a lu e 0 * 6 9 0 . T h e t w o l i m i t in g v a lu e s 0 * 6 9 0 a n d 0 *2 3 c o in c id e w i t h

t h e v a lu e s w h ic h w e o b t a in e d a t I = 0 *1 f o r t h e q u a n t i t ie s

r e s p e c t iv e ly , i n w h ic h e0 = e x t in c t io n c o e f f ic ie n t o f t h e u n d is s o c ia te d s a l ic y l ic a c id

m o le c u le s , ex = e x t in c t io n c o e f f ic ie n t o f t h e H L ~ - i o n , a n d / = le n g t h o f t h e o p t ic a l

p a t h .

T h is t y p e o f b e h a v io u r c a n b e b e s t e x p la in e d b y a s s u m in g t h a t in t h e L a - s a l ic y la t e

s o lu t io n s c o m p le x f o r m a t i o n t a k e s p la c e m a i n ly b e tw e e n t h e L a 3+- a n d t h e H L - - io n s ,

a n d t h a t t h e e x t in c t io n c o e f f ic ie n t o f t h e s p e c ie s L a H L 2+ is n o t a p p r e c ia b ly d i f f e r e n t

f r o m t h a t o f th e H L - - io n . A t p H - v a lu e s , a t w h ic h m o s t o f t h e s a l ic y l ic a c id is i n th e

u n d is s o c ia te d f o r m , l i t t l e c o m p le x f o r m a t i o n o c c u r s ; t h e o p t ic a l d e n s ity o f b o t h A -

a n d B -s o lu t io n s is t h e n a p p r o x im a t e ly e q u a l t o D 0 a n d AD is v e r y s m a l l . A s h° d e c re a s e s t h e d e g re e o f d is s o c ia t io n o f s a l ic y l ic a c id a n d , c o n s e q u e n t ly , t h e e x t e n t o f

c o m p le x f o r m a t i o n i n t h e L a - s a l ic y la t e s o lu t io n s in c re a s e s ; a s a r e s u l t t h e c o n c e n t r a ­

t io n s o f b o t h H L ~ a n d H 2L in th e s e s o lu t io n s w i l l b e less t h a n in t h e c o r r e s p o n d in g

s a l ic y l ic a c id s o lu t io n s w h ic h d o n o t c o n t a in la n t h a n u m . S in c e t h e e x t in c t io n c o e f f i­

c ie n t o f t h e s p e c ie s L a H L 2 + d o e s n o t d i f f e r m u c h f r o m t h a t o f H L - t h e o p t ic a l d e n s ity

o f t h e A - s o lu t io n s w i l l b e less t h a n t h e o p t ic a l d e n s ity o f t h e B -s o lu t io n s b y a n a m o u n t

w h ic h is r o u g h ly p r o p o r t io n a l t o t h e d e c re a s e i n t h e c o n c e n t r a t io n o f H 2L b r o u g h t

a b o u t b y c o m p le x f o r m a t i o n . F i n a l l y , a t p H - v a lu e s a t w h ic h m o s t o f t h e s a l ic y lic

a c id h a s d is s o c ia te d t h e c o n t r ib u t io n o f H 2L t o t h e t o t a l a b s o r b a n c e w i l l b e s m a l l a n d

t h e o p t ic a l d e n s it ie s o f b o t h A a n d B s o lu t io n s w i l l b e a p p r o x im a t e ly e q u a l t o D x.T h e r e f o r e , u n d e r t h e e x p e r im e n t a l c o n d it io n s e m p lo y e d , io n - p a i r in g o c c u rs

b e tw e e n L a 3 + - a n d H L ~ - io n s .

I n a c c o r d a n c e w i t h t h e s p e c t r o p h o t o m e t r ic e v id e n c e w e a s s u m e t h a t in t h e L a -

s a l ic y la te s o lu t io n s io n -a s s o c ia t io n m a i n ly o c c u rs b e tw e e n t h e L a 3+- a n d H L ~ - io n s .

C a lc u la t io n s b a s e d o n d a t a r e p o r t e d 8 o n io n - a s s o c ia t io n i n s a lts o f s o d iu m a n d s o m e

u n iv a le n t a n io n s s u g g e s t t h a t a t th e t o t a l c o n c e n t r a t io n s o f N a + - io n s a n d s a l ic y lic

a c id e m p lo y e d i n t h e p r e s e n t in v e s t ig a t io n t h e e x t e n t o f i o n - p a i r in g b e tw e e n t h e N a + -

a n d H L - - io n s w o u ld b e m u c h t o o s m a l l t o b e s p e c t r o p h o t o m e t r ic a l ly d e t e c t a b le ; th e

s p e c ie s N a H L c a n th u s b e d is r e g a r d e d . S in c e , i n o u r p H - r a n g e t h e te r m s [ O H - ] ,

[ L 2 - ] , a n d [ L a O H 2 + ] 8 a r e n e g l ig ib le t h e o n ly e q u i l ib r i a c o n s id e r e d f o r t h e L a - s a l ic y la t e

s o lu t io n s a r e

Do — sqT^I, D x — sxThl, (1.1), (1.2)

THEORY

h 2 l ^ h + + h l -K

L a 3+ + H L - ^ L a H L 2 +

T h e r e s p e c t iv e e q u i l ib r i u m c o n s ta n ts a r e g iv e n b y t h e e q u a t io n s

(2.1)

(2.2)

Z. L. ERNST A N D P. J. N EW MA N 1055

w h e r e h — [ H + ] , c = [ L a H L 2 + ] a n d f x, f 2, a n d / 3 a r e t h e a c t i v i t y c o e f f ic ie n ts o f th e

s in g ly , d o u b ly , a n d t r ip l y , c h a r g e d s p e c ie s , r e s p e c t iv e ly . T h e c o n c e n t r a t io n te rm s c, [ L a 3+] , [ H L - ] , a n d [ H 2L ] a r e r e la te d t o o n e a n o t h e r b y t h e e x p re s s io n s

T l = c + [ H L " ] + [ H 2L ] , ( 2 .3 )

r M = [ L a 3 + ] + c * [ L a 3 + ] , ( 2 .4 )

s in c e TM^>TL, a n d h e n c e TM^>c.B y c o m b in in g ( 2 .3 ) w i t h ( 2 .1 ) ,

[ H L " ] = (Th—c)K1/(K l + h f l) , ( 2 .5 )

[ H 2L ] = ( T L- c ) h f 2/(K x + h f 2), ( 2 .6 )

F r o m t h e e le c t r o n e u t r a l i t y p r in c ip le ,

h = a + c + [ H L " ] , ( 2 .7 )

w h e r e a is t h e s t o ic h io m e t r ic c o n c e n t r a t io n o f H C 1 0 4 a d d e d .

F o r s o lu t io n s o f s a l ic y l ic a c id w h ic h d o n o t c o n t a in l a n t h a n u m , e q n . ( 2 .3 ) ( 2 .5 ) ,

( 2 .6 ) a n d ( 2 .7 ) s im p l i f y t o

Tl = [HL-]° + [H2L]°, (2.8)[HL-]° = (2.9)[H2L]° = TLh ° f jlx , (2.10)

h° = a + [Hir]°, (2.11)w h e r e

H e n c e ,

x = K x + h ° f l ( 2 .1 2 )

c = [ H 2L ] ° - [ H 2L ] + [ H L - ] ° - [ H L - ] , ( 2 .1 3 )

A h = c - { [ H L " ] ° - [ H L " ] } , ( 2 .1 4 )

A h = [ H 2L ] ° — [ H 2L ] , ( 2 .1 5 )

w h e r e

A h = h — h ° . ( 2 .1 6 )

S u b s t i t u t in g f o r [ H L - ] a n d [ H L - ] ° f r o m ( 2 .5 ) a n d ( 2 .9 ) in t o ( 2 .1 4 ) ,

Ah2x f l+ A h (x 2 + TLK i f l —c x f l ) —c x h ° f l= 0 . ( 2 .1 7 )

S in c e , i n o u r p H - r a n g e t h e t e r m A h2x f f is n e g l ig ib ly s m a l l , e q n . ( 2 .1 7 ) b e c o m e s

A h = c x h ° f ll(x 2 + TLK l f f —c x fl) . ( 2 .1 8 )

S u b s t i t u t in g f o r [ H L - ] f r o m ( 2 .1 4 ) in t o ( 2 .2 ) a n d r e a r r a n g in g ,

(Ahx + Tt K 1)lc = ocx, ( 2 .1 9 )

w h e r e

a = l H f i lK T u f J s ) , (2.20)T h u s a p l o t o f y ' — ( A hx+ T ^K ^/c a g a in s t x s h o u ld y ie ld a s t r a ig h t l in e w i t h a s lo p e

e q u a l t o a .

T h e o p t ic a l d e n s ity o f t h e s o lu t io n s o f s a l ic y l ic a c id w h ic h d o n o t c o n t a in la n t h a n u m

is g iv e n b y

D° = £ 0 [ H 2L ] 0/ + e 1[ H L " ] 0/ , ( 2 . 2 1 )

1056 ASSOCIATION CO NS TANT OF L AN T H A N U M SALICYLATE

w h e r e / is t h e le n g t h o f t h e o p t ic a l p a t h i n c m . I n v ie w o f ( 1 .1 ) , ( 1 .2 ) , ( 2 .9 ) , a n d ( 2 .1 0 )

t h e la s t e q u a t io n c a n b e a ls o w r i t t e n i n t h e f o r m

D°x = D i K 1+ D 0h ° f l ( 2 .2 2 )

F o r s o lu t io n s o f L a - s a l ic y la t e t h e L a m b e r t - B e e r l a w ta k e s t h e f o r m

D = e 0 [ H 2L ] / + e 1[ H L " ] / + . f i ccZ, ( 2 .2 3 )

w h e r e ec is t h e e x t in c t io n c o e f f ic ie n t o f L a H L 2 + .

U p o n s u b s t i t u t in g f o r [ H L ~ ] a n d [ H 2L ] f r o m ( 2 .5 ) a n d ( 2 .6 ) in t o ( 2 .2 3 ) a n d s o lv in g

f o r c,D t K t + h f i D o - D l K t + h f p ^

D t K i + h f t D o - D A K t + h f l ) L ' '

w h e r e

Dc = ec7hl, ( 2 .2 5 )

C o m b in a t i o n o f ( 2 .2 4 ) w i t h ( 2 .2 2 ) g iv e s

A D x + A h fl(D 0- D )

(D ° —Dc)x ± A h f l(D 0—Dc)

F i n a l l y , s u b s t r a c t io n o f ( 2 .2 3 ) f r o m ( 2 .2 1 ) , a n d u s e o f ( 2 .1 4 ) a n d ( 2 .1 5 ) , g iv e s

A / ! ( D 0 - D 1) - A D T L + f ) 1c = D cc , ( 2 .2 7 )

f r o m w h ic h i f y" = Ah(D0 — D x) —ADTh ± D ±c is p lo t t e d a g a in s t c a s t r a ig h t l in e s h o u ld

b e o b t a in e d w i t h a s lo p e e q u a l t o D c.

D IS C U S S IO N

DETERM INATION OF D 0 A N D D 1

T h e e v a lu a t io n o f t h e a s s o c ia t io n c o n s ta n t K r e q u ir e s a k n o w le d g e o f ec, e0 , a n d

e1 a t t h e s e le c te d w a v e le n g t h X = 3 2 6 m / i . F o r t h e la s t t w o c o e f f ic ie n ts E r n s t a n d

M e n a s h i 'v h a v e f o u n d t h e v a lu e s e0 = 7 1 7 a t I — 0 -8 a n d S j = 2 0 0 a t 7 ~ 1 0 - 3 ;

h o w e v e r , as a l l s o lu t io n s e m p lo y e d in t h e p r e s e n t w o r k w e r e m a in t a in e d a t I = 0 -1 w e

r e d e t e r m in e d th e s e q u a n t i t ie s b y m e a n s o f e q n . ( 2 .2 2 ) . T h is p r e d ic ts t h a t a p l o t o f

T a b le 2 .— D e te r m in a t io n o f D 0 a n d D i

c = ( 2 - 2 6 )

21 = 2-509 x 10-4 mole/1.; I = 0-1; X = 326 m /i; / = 4 cmax 103 mole/I. 6X102mole/I. D° h°flx 103 X X 103 y = D°xX 103 Ay x 1030-459 9-950 0-351 0-395 1-459 0-512 ±0-0051-157 9-888 0-426 0-801 1-865 0-795 ±0-0022-496 9-736 0-507 1-600 2-664 1-351 -0-0033-511 9-644 0-542 2-214 3-278 1-776 -0-0044-490 9-553 0-566 2-809 3-873 2-192 -0-0096-006 9-400 0-586 3-735 4-799 2-812 ±0-009

D o = 0-690; a Do = ±0-003. Go — 688; c£o = ±3.D i = 0 -2 2 9 <7d i — ±0*005. £i = 228; Cgj = ±5 .

D°x a g a in s t h°f\ s h o u ld g iv e a s t r a ig h t l in e w i t h a s lo p e a n d in t e r c e p t e q u a l t o D0 a n d

D iK u r e s p e c t iv e ly .

T h e q u a n t i t y h° w a s c a lc u la t e d u s in g e q n . ( 2 .1 1 ) e x p re s s e d as a q u a d r a t ic e q u a t io n

a n d s o lv e d b y s t a n d a r d m e th o d s . T h e a c t i v i t y c o e f f ic ie n ts w e r e e v a lu a t e d b y m e a n s

o f t h e D a v ie s e q u a t io n , w i t h a p r o b a b le e r r o r n o t e x c e e d in g 3 % . 8 T h e s lo p e a n d

in t e r c e p t o f t h e p l o t w e r e f o u n d b y a p p ly in g t h e m e t h o d o f le a s t s q u a re s . T h e r e s u lts

a r e g iv e n i n t a b le 2 w h e r e b is t h e c o n c e n t r a t io n o f N a C 1 0 4 a n d A_y t h e r e s id u a l

Z. L. ERNST A N D P. J. N EWM AN 1057

^ c a ic .— obs.- T h e p r e c is io n o f t h e r e s u lts w a s ass e s se d b y c o m p u t in g t h e s t a n d a r d

d e v ia t io n s o f t h e s lo p e d a n d in t e r c e p t 9 a n d h e n c e t h e s t a n d a r d d e v ia t io n s aDo a n d

aD , o f D 0 a n d D u r e s p e c t iv e ly .

T h e v a lu e s e 0 = 6 8 8 a n d e x = 2 2 8 f o u n d i n t h e p r e s e n t w o r k a t I = 0 *1 d i f f e r

s ig n if ic a n t ly f r o m t h e c o r r e s p o n d in g v a lu e s o b t a in e d b y E r n s t a n d M e n a s h i .4 S in c e

(a s s h o w n b y o u r c a lc u la t io n s n o t r e p o r t e d h e r e ) th e s e d i f fe r e n c e s c a n n o t b e c o m p le t e ly

a c c o u n te d f o r b y t h e in a c c u r a c y o f t h e D a v ie s e q u a t io n , i t a p p e a r s t h a t e0 , a n d t o a

m u c h g r e a t e r e x t e n t e x v a r y w i t h c h a n g in g io n ic s t r e n g th . A ls o i n t h e e v a lu a t io n o f

t h e a s s o c ia t io n c o n s ta n t K t h e p r e s e n t v a lu e s f o r e0 a n d &1 g iv e m u c h m o r e c o n s is te n t

re s u lts t h a n th o s e o b t a in e d b y E r n s t a n d M e n a s h i .4

DETERMI NATI ON OF ASSOCIATION CONSTANT

A n in s p e c t io n o f e q n . ( 3 .1 ) , ( 2 .1 8 ) , ( 2 .1 9 ) , ( 2 .2 6 ) a n d ( 2 .2 7 ) s h o w s t h a t i f D c w e r e

k n o w n t h e v a lu e s o f A /z a n d c, c o r r e s p o n d in g t o a n y g iv e n v a lu e o f h°, c o u ld b e c a l ­

c u la t e d b y s o lv in g e q n . ( 2 .1 8 ) a n d ( 2 .2 6 ) . I t w o u ld t h e n b e p o s s ib le t o e v a lu a t e th e

a s s o c ia t io n c o n s ta n t K b y p l o t t in g y ' = (AIix + T lK J /c a g a in s t x a n d m e a s u r in g th e

s lo p e a o f t h e s t r a ig h t l in e so o b t a in e d .

S in c e o n ly t h e a p p r o x im a t e v a lu e o f Dc ( o b t a in e d f r o m t h e v a r i a t i o n o f AD w i t h

/z ° ) w a s k n o w n b e f o r e h a n d , t h e v a lu e o f K w a s c a lc u la te d b y v a r y in g t h e v a lu e o f Dc i n t h e r a n g e 0 - 2 1 - 0 - 2 4 a n d f in d in g t h e b e s t v a lu e o f t h e s lo p e a . T h e v a lu e s o f h° a n d

x w e r e f i r s t o b t a in e d f r o m e q n . ( 3 . 1 ) ; n e x t , t h e v a lu e s o f Ah a n d c, c o r r e s p o n d in g t o

4 * 0

riOX£ 3-0

2-0

0*212 0*216 0 *2 2 0 0*224 0*228 0*232 0*236 0 *2 4 0

DcF ig . 1.— Variation o f w ith Dc.

a n y g iv e n v a lu e o f Dc, w e r e c o m p u t e d b y m e a n s o f e q n . ( 2 .1 8 ) a n d ( 2 .2 6 ) u s in g th e

m e t h o d o f s u c c e s s iv e a p p r o x im a t io n s . T h e v a lu e s o f y ' w e r e t h e n f o u n d a n d t h e s lo p e

o f t h e p l o t o f y ' a g a in s t x d e t e r m in e d b y a p p ly in g t h e m e t h o d o f le a s t s q u a re s . T h e

g o o d n e s s o f f i t a t t a in e d a t a g iv e n v a lu e o f Dc w a s assessed b y c a lc u la t in g t h e s t a n d a r d

d e v ia t io n 10 aa o f a . T h e v a r i a t i o n o f <ra w i t h Dc is g iv e n i n f ig . 1 ; as Dc in c re a s e s <ra

d e c re a s e s , p a s s es t h r o u g h a m in i m u m a n d t h e n in c re a s e s . T h e v a lu e s o f a a n d Dc w h ic h c o r r e s p o n d t o th is m in i m u m , a n d w h ic h e n a b le t h e e x p e r im e n t a l d a t a t o b e b e s t

f i t t e d in t o e q u a t io n ( 2 .1 9 ) a r e a = 4 - 5 9 7 a n d Dc = 0 - 2 2 6 . T h e re s u lts a r e s u m m a r iz e d

i n t a b le 3 w h e r e t h e p r e c is io n o f t h e e x p e r im e n t a l d a t a is e x p re s s e d in t e r m s o f

t h e r e s id u a ls Ay' a n d Ay", a n d o f t h e s t a n d a r d d e v ia t io n s <ra, a*, a n d aK. I n

c o m p u t in g t h e v a lu e K = ( 1 - 1 8 8 + 0 - 0 0 8 ) x 1 0 2 f o r t h e a s s o c ia t io n c o n s ta n t t h e v a lu e s

34

1058 ASSOCIATION CO NS TANT OF L A N T H A N U M SALICYLATE

a = 4 - 5 9 7 + 0 - 0 2 4 , a n d Dc = 0 - 2 2 6 + 0 - 0 0 1 w e r e u s e d ; f o r t h e a c t i v i t y c o e f f ic ie n ts t h e

v a lu e s / i = 0 - 7 8 5 , f 2 = 0 - 3 8 0 , a n d / 3 = 0 - 1 1 3 , o b t a in e d f r o m t h e D a v ie s e q u a t io n

( a n d , a c c o r d in g t o D a v ie s ,8 s u b je c t t o a p r o b a b le e r r o r o f + 3 % ) w e r e e m p lo y e d . T h e

s t a n d a r d d e v ia t io n o f K is less t h a n 1 % , th u s in d ic a t in g t h a t s a t is fa c to r y p r e c is io n h a s

b e e n a t t a in e d . T h e p r e s e n t v a lu e ( 1 - 1 8 8 + 0 - 0 0 8 ) x 1 0 2 f o r K is a p p r e c ia b ly lo w e r t h a n

t h a t o b t a in e d b y C e f o la , T o m p a , C e l ia n o , a n d G e n t i le a t 3 0 ° C u s in g t h e g la s s

e le c t r o d e .11

T a b l e 3 .— A s s o c ia t io n c o n s t a n t o f l a n t h a n u m s a l ic y l a t e

2 l = 2-509 x 10- 4 m o le /1 .; 7 m = 1 -0 0 2 x 1 0 --2 m ole/1. ; i = 0 -1 ; 2 = 326 m fi; 1— 4 c m

Dc - = 0 -226

ax 103 6x102 D AD Ah X 10? c x 105 y* X 102 A /X 1 0 2 y " X 10« A y "X 106mole/I. mole/l.

0-201 3-937 0-332 0-019 1-050 4-109 0-687 - 0 - 0 1 7 9-444 - 0 - 0 8 21-241 3-846 0 -399 0-027 1-464 3-529 0 -834 + 0 -0 2 3 8-040 0-0032-238 3-754 0-479 0-028 1-506 2-550 1-204 + 0-021 5-761 + 0 -0503-253 3-632 0-516 0-026 1-366 2-046 1-524 - 0 - 0 1 7 4-613 + 0-0494-232 3-540 0-542 0-024 1-288 1-790 1-770 + 0 -0 1 1 4-015 + 0 -0 6 35-748 3-388 0-565 0-021 1-119 1-446 2-218 - 0 0 1 2 3-304 - 0 - 1 0 0

a = 4 -5 9 7 ; ca= ± 0 - 024. D* = 0-228 ; aD%= ± 0 - 0 0 1 ; K (present w o rk ) =1-188 x 102 ; <*K = ± 0 -0 0 8 x 102 ; K (p revious w o rk ) = 4-37rx l 0 2 [ref. Cl 1)1.

A s a f u r t h e r te s t o f t h e r e l i a b i l i t y o f th e s e c a lc u la t io n s t h e v a lu e o f Dc w a s c o m p u t e d

b y m e a n s o f e q n . ( 2 . 2 7 ) ; v a lu e s o f y" w e r e c a lc u la t e d u s in g t h e v a lu e s o f Ah a n d c s h o w n i n t a b le 3 , a n d t h e s lo p e o f t h e p l o t o f y" a g a in s t c w a s o b t a in e d b y a p p ly in g t h e

m e t h o d o f le a s t s q u a re s . T h e v a lu e 0 -2 2 8 o f Dc, s h o w n in t a b le 3 as 2 ) * , is i n g o o d

a g r e e m e n t w i t h t h e v a lu e 0 - 2 2 6 o b t a in e d f r o m t h e p l o t o f cra a g a in s t Dc.

T h e a u t h o r s t h a n k t h e S c ie n c e R e s e a r c h C o u n c i l f o r t h e a w a r d o f a re s e a rc h

s tu d e n ts h ip t o o n e o f u s ( P . J . N . ) .

1 Z. L. Ernst and J. Menashi, Trans Faraday Soc., 1963, 59, 2838.2 H . Irving and H. Rossotti, Acta Chem. Scand., 1956,10, 72.3 J. G. Jones, J. B. Poole, T. C. Tomkinson, and R. J. P. Williams, J. Chem. Soc., 1958,2001,2010.4 Z. L. Ernst and J. Menashi, Trans Faraday Soc., 1963, 59, 230.5 Z. L. Ernst and J. Menashi, Trans Faraday Soc., 1963, 59, 1794.6 J. Korbl and R. Pribil, Chemist Analyst, 1956, 45, 102.7 H. A. Flaschka, E.D.T.A. Titrations (Pergamon Press, London, 1959).8 C. W. Davies, Ion Association (Butterworths, London, 1962).9 H . G. Jerrard and D. B. McNeil, Theoretical and Experimental Physics (Chapman and Hall,

London, 1960).10 W. E. Deming, Statistical Adjustment o f Data (Chapman and Hall, London, 1943).11 M. Cefola, A. S. Tompa, A. V. Celiano and P. S. Gentile, Inorg. Chem., 1962,1, 290.

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