New Speci$c in Spectrophotometric Determination of Copper

G . FREDERICK SJIITH AYD W. H. M C C U R D T , JR., Noyes Laboratory, University of Zllinois, Urbanu, Ill.

A new reagent for replacement of diquinoline as a specific reagent fnr determination of cuprous copper i n the presence of iron is described. The procedure in the application of 2,9-dimethyl-l,lO-phenan- throline involves formation of the cuprous che- late, in which 2 molecules of the substituted phenanthroline coordinate with each cuprous ion not interfered with by large amounts of iron. Its wave length of maximum absorption is at 4.54 mfi and the molecular extinction coefficient is 7950. The neo-cuproine-cnpper complex is formed

HE use of either diethyl dithiocarbamate or dithizone in the T spectrophotometric micro and macrodetermimtion of copper has not proved entirely satisfactory. The former is not suf- ficiently specific, being burdened by the reaction with the com- monly present iron, nickel, and cobalt ions. The latter is like- wise not satisfactorily selective in its action a t a wide range of pH values, and the reagent itself is colored, requiring in its applica- tions a mixed color procedure. For this reason Irving, Andrew, and Risdon (6) give a new but more complicated procedure in the the use of dithizone for the determination of copper.

A new reagent, 2,2'-diquinoline, was described by Breckenridge, Lewis, and Quick (1). This new copper specific was given the trivial name cuproine by Hoste (4), who studied the application of eight new copper specifics of the 2,2'-dipyridine type, of which 2,2'-diquinoline may be considered to be an analog, and their methyl substituted derivatives.

Hostc. (4) designated the functional group

a8 the copper specific group in 6,6'-dimethyL2,2'-dipyridine. This ragent was sufficiently investigated to prove its superior value as a spectrophotometric reagent, but it was not seriously considered, because of difficulties in its preparation. The use of 2,2'-diquinoline serves as a practical duplicate in properties, as the two carbon atoms of the methyl groups a t the two extremi- ties of the copper specific group are incorporated into the second ring of the two quinoline molecules, giving a compound which is copper specific.

Hoste (4) withheld substituted types of 1,lO-phenanthroline from consideration, although the unsubstituted reagent forms both ferrous and cuprous complex ions of high molecular extinc- tion coefficients. The reason given was that these copper com- plexes could not be extracted from aqueous solutions by the use of known water-immiscible extraction solvents. Recent experi- ments in t,his laboratory have proved this statement incorrect.

As a result of a series of studies, such as that reported by Smith and Brandt (6) , the compound 2,9-dimethyl-l,lO-phenanthroline was a t hand and known not to form the familiar red complex in the presence of ferrous iron. It was found to complex readily with cuprous copper, having the previously specified copper spe- cific group.

The present work is a report on the further investmigation of the spectrophotometric applications of this new organic compound, now given the trivial name neo-cuproine, as a superior copper spe- cific chelating compound. Both cuproine and neo-cuproine are now commercially available (G. Frederick Smith Chemical Co.), each for the first time.

over the pH range of 3 to 10 arid is stable for extended periods of storage. The colored complex obeys Beer's law over the range 1 to 10 p.p.m. Neo-cuproine is soluble in alcohol and the cuprous complex may be extracted from aqueous solutions by use of amyl, isoamyl, or n-hexyl alcohols. No interferences from foreign ions were found, The limit of identification is 0.03 microgram of copper or 1 part of copper in 1,660,000 parts of aqueous solution after extraction with n-hexyl alcohol. Quantitative data are given for the determination of copper in steel.

I'REPiRATION OF 2,9-I)IMETHYL1,10-PHENAIYTHROLINE

The preparation of this compound is described by Case (2).

A double Skraup synthesis is employed. 0-Nitroaniline reacts with crotonaldehyde diacetate in sulfuric acid solution; arsenic

The 2-methyl-8-nitroquino- &ne thus f o r m J i s reduced to the corresponding amino uinoline by reaction with alcoholic stannous chloride. A seconx Skraup reaction, du licating the first described reaction conditions, re- sults in the fhshed roduct in 7.6% yield based on c-nitroaniline. It is anhydrous d e n obtained from benzene extraction. It forms a hemihydrate when recrystallized from water, melting point -159' to 160" C. 2,9-Dimethyl-l,lO-phenanthroline is slightly soluble in cold water, and very soluble in ethyl alcohol, n- amyl alcohol, isoamyl alcohol, n-hexyl alcohol, chloroform, and benzene.

entoxide is em loyed as oxidant.

pH RANGE PROMOTING COMPLEX FORMATION

Neo-cuproine reacts with cuprous copper in buffered acetic acid solution to produce a bright orange colored complex. The cupric ion is conveniently reduced with hydroxylamine sulfate, the excess of which need not be removed. The complex is formed over the pH range of 3 to 10. The color base material was added in the form of an alcohol-water solution, although other solvente such as amyl alcohol may be employed. The color is completely extracted by using n-amyl alcohol, isoamyl alcohol, or n-hexyl alcohol within the designated pH range. Spectrophotometric evaluation of the colored complex is conveniently performed on the extract solution.

REACTIOY RATIO O F NEO-CUPROINE TO CUPROUS COPPER I Y COMPLEX FORMATION

The n~olecular ratio of neo-cuproine to monovalent copper in the orange colored complex was determined spectrophoto- metrically by application of the method of continuous variations and found to be 2 moles of neo-cuproine to 1 mole of copper in both water and isoamyl alrohol solutions. The complex may he form- ulated as:

SPECTROPHOTOMETRIC EXA\lIiYATION OF ISOAMYL ALCO- HOL SOLUTIONS OF NEO-CUPROINE CUPROUS ION

A eample of pure copper was dissolved in nitric acid and con- verted to sulfate by a slight excess of sulfuric acid and the desired amount of conductivity water was added. The concen-

37 1

372

tration of copper waa calculated to be 0.00015756 gram of copper per gram of solution. Weighed portions of the copper solution were transferred from a weight buret directly into a specially de- signed separatory funnel. This design was made from a standard 60-ml. borosilicate glass separatory funnel with the original stop- cock replaced by a stopcock miniature of 0.5-mm. bore to provide a sharper separation of the two immiscible liquids. After the addi- tion of excm hydroxylamine sulfate, a 100% excess of neo- cuproinein water-ethylalcoholsolution wasintroduced. Thissolu- tion was buffered with 1 gram of sodium acetate, giving a pH be- tween 5 and 6. The colored solution obtained was extracted with 8 10-ml. ortion of isoamyl alcohol. A &minute interval was sufficient for the separation of the two immiscible solutions. Two extractions with isoamyl alcohol were found to be adequate. The combined extracts were diluted to 50-ml. total volume with distilled isoamyl alcohol in each case. The optical density of the isoamyl alcohol solution of the complex was measured in a l-cm. quartz cell with a Model 11 Cary recording spectrophotometer. A reagent blank correction was applied throughout.

A N A L Y T I C A L C H E M I S T R Y

COPPER NEO-CUPROINE COMPLEX IN ISOAMYL ALCOHOL

1 I I 4 I

/--lO.S09 PPM COPPER I I

1.6 BEER'S L A W PLOT AT 4 5 4 7 ~ l ' l ' l ' l ' l ' l ' l ~ l ~ l ' l ' ~

W A V E LENGTH (mp) .~ Figure 1. Copper-Neo-cuproine Complex in Isoamyl

Alcohol

The results are shown in Figure 1. The data obtained are given in Table I.

APPLICABILITY OF BEER'S LAW

The data of Table I have been used in the preparation of Figure 2, irhich shows that Beer's law is obeyed over the range of 0.15 to 10.6 p.p.m. copper. However, other copper concentrations as

Table I . Determination of Optical Density, Wave Length of Maximum Absorption, and Molecular Extinction Coeffi-

cient of Neo-Cuproine Cuprous Ion (Wave Length of Maximum Absorption, 454mp)

Copper Taken, R l g .

0.014914 0.02828 0 ,04777 0 ,10267 0.20870 0.31369 0.42780 0.53046

Optical Density 0.019 0 .071 0 . 1 2 0 0 . 2 5 9 0 . 5 1 4 0 , 7 8 2 1 .055 1 .310

100 ml. of isoamyl alcohol total volume. Av.

Molecular Extinction Coefficient

8090 7990 8030 8030 7840 7930 7850 7850 7960

high as 20 p.p.m., and as low as 0.05 p.p.m. have been successfully evaluated.

STABILITY OF NEO-CUPROINE CUPROUS COMPLEX WITH TIME

A series of solutions of the type whose absorption spectra is shown in Figure 1 was prepared and measured. These solutions were stored for 17 days in clear glass in ordinary light without change in color intensity, as shown by a redetermination of the optical density. During subsequent work, low results were ob- tained in the quantitative determination of copper while using some undistilled isoamyl alcohol. The error was traced to oxi- dizing impurities in the alcohol and was removed by employing carefully redistilled alcohol in all determinations.

SPECTROPHOTOMETRIC CHARACTERISTICS OF NEO-CUPROINE CUPROUS ION

KO absorption was found over the region 600 to 800 mp, but minimum absorption exists a t 354 mp. The wave length of maxi- mum absorption a t 454 mp is almost identical with the 455 mp rc- ported by Hoste ( 4 ) for the cuprous complex of 6,6'-dimethyl- 2,2'-dipyridine. For the cuprous complex l,l0-phenanthroline the wave length of maximum absorption is 435 mp; this indicates an increase of approximately 20 mp due to the presence of two methyl groups in the 2,9 positions.

The 2,9 positioned methyl groups in 1,lO-phenanthroline and the corresponding 6,6' positioned methyl groups in 2,2 '-dipyri- dine increase in the wave length of maximum absorption of the cuprous complex to exactly the same extent when compared with the unsubstituted l,l0-phenanthroline and 2,2'-dipyridine cu- prous complexes. These data are in conformity with observations of Smith and Brandt (6), that the methyl substitution in the 1,10- phenanthroline series of compounds has an exact numerical effect on both the wave length of maximum absorption and the oxida- tion potential of their ferrous complex ions.

I I * I . 1 1 1 l 1 1 1 1 1 1 1 1 . I I

0 1 2 3 4 5 6 7 8 9 1 0 1 1 PPM COPPER

Figure 2. Plot of Beer's Law at 454 mp

The important significance of the influence of the two methyl groups of neo-cuproine is the fact that their presence renders the neo-cuproine cuprous ion completely extractable by water-immis- cible alcohols a t pH's below 7. -4nother important influence of the methyl group substitutions in neo-cuproine concerns the in- crease in magnitude of the molecular extinction coefficient. The value 7950 compares very favorably with that of 5490 for the 2,2'- diquinoline, 6570 for the 6,6'-diniethyl-Z,Z'-dipyridine, and 7040 for the 1,lO-phenanthroline cuprous complexes,

SPECIFICITY

Keo-cuproine is as specific as cuproine in the presence of cations. KO cation other than cuprous copper was found to form a colored

V O L U M E 24, NO. 2, F E B R U A R Y 1 9 5 2 373

Table 11. Determination of Copper i n Iron with Citrate Complexing Agent

Sample Added, Found, Deviation, Grams' hIg. Mg. hlg.

Iron Copper Copper

Spectrographic determination.

complex which was extractable under the conditions employed. The common anions chloride, sulfate, nitrate, perchlorate, tar- trate, citrate, and acetate do not interfere. An orange-yellow pre- cipitate occurs with large amounts of nitrate, perchlorate, and the halides (particularly iodide) upon the addition of neo-cuproine to an aqueous copper solution but causes no interference, as the col- ored complex is quantitatively extracted in all cases. Anions such as periodate, nitrite, thiocyanate, and ferricyanide, which either react with hydroxylamine or give a yellow colored solution, may be suitably eliminated to adapt conditions to the test, Py- rophosphate and phosphate ions do not interfere, although for ex- cessive amounts two or three extractions may be required to col- lect all the copper. Neo-cuproine may thus be said to be com- pletely specific.

LIMIT OF QUALITATIVE IDENTIFICATION

Distribution coefficient measurements on the complex ob- tained using n-amyl alcohol, isoamyl alcohol, and n-hexyl alcohol show that n-hexyl alcohol is definitely the best extraction solvent. Employing visual examination of the aqueous droplet of colored solution in a spot plate without extraction, the limit of identifica-

tion is 0.04 microgram of copper. Using 1 microdrop of n-hexyl alcohol as extractant, the limit of identification is 0.03 micrograni of copper. The limit of concentration in the former case is I to 1,250,000 and in the latter case 1 to 1,660,000. Therefore neo- cuproine is found to be as sensitive as dithisone (3) .

QUANTITATIVE DETERMINATION OF COPPER IN PRESENCE OF IRON

Using citric acid as masking agent for ferric and ferrous ii nii a t pH 4 to 6, a twofold excess of hydroxylamine hydrochloriilo its reductant, a 100% excess of neo-cuproine as copper reactant, m i l

10 ml. of n-hexyl alcohol as extractant, results were obtaine(1 as shown in Table 11. Six grams of sodium citrate were added for each gram of iron present, but this amount is not critical.

Fluoride, pyrophosphate, oxalate, malonate, and tartrate were studied as potential masking reagents for iron in the determina- tion, but citrate proved to be far superior within the pH range de- sired. PROPOSED EXTENSION IN APPLICATION O F NEO-CUPROINE

FUNCTIONAL GROUP

It has been shown that substitution of methyl groups in thc 4,7- positions of the 1,lO-phenanthroline molecule exerts only a sinall shift in the wave length of maximum absorption but a pronounced increase in the molecular extinction coefficient of the ferrous rani-

plex (6). Therefore it is proposed to investigate the 2,4,7,!)-t~- tramethyl-1 ,lO-phenanthroline derivative as a possibly improved copper specific.

LITERATURE CITED

(1) Breckenridge, Lewis, and Quick, Can. J . Research, B17, 258

(2) Case, F. H., J. Am. Chem. Soc., 70,3994 (1945). (3) Fisher, Mikrochemie, 8,319 (1930). (4) Hoste, Anal. Chim. Acta, 4, 23 (1950). (5 ) Irving, Andrew, and Risdon, Nature, 161, 805 (1948). (6) Smith and Brandt, ANAL. CHEY., 21, 1313 (1949).

RECEIVED May 9, 1951

(1 939).

Colorimetric Determination of the Sodium Salts of Ethylenediaminetetraacetic Acid

ALBERT DARBEY Alrose Chemical Co., Providence, R. I .

THYLENEDIAMINETETRAACETIC acid (Sequestrene) E is readily determined as its alkali metal salts in the absence of alkaline earth metals and heavy metals by titration with standardized calcium acetate solution; sodium oxalate is used as an internal indicator. The end point is the appearance of permanent turbidity. In the presence of metals this method would in general determine essentially the free compound.

The alkali metal salts of ethylenediaminetetraacetic acid are widely used as alkaline earth and heavy metal sequestrants. In many cases, therefore, it is desirable to have methods of deter- mining the total amount of the compound in very low concentra- tions in the presenceof certain metulswhich may have been seques- tered by it.

Nickel may be employed to displace from their ethylenediamine- tetraacetic acid complexes such metals as calcium, magnesium, and many other common metals, the nickel itself being then preferentially sequestered (6). In agravimetricmethodemploying this principle ( 1 ) a known amount of nickel is added, and excess, unsequestered nickel is precipitated as the hydroxide and sub-

sequently converted to the dimethylglyoxime salt €or gravimctric determination. The difference, representing sequestered nickel, may be calculated to ethylenediaminetetraacetic acid, ab the reaction is stoichiometric.

This gravimetric method, however, may not be suEcic.iitlv accurate when very small amounts of ethylenediaminetetraawtic acid salts are to be determined.

The method described here offers a colorimetric procedui P by means of which total ethylenediaminetetraacetic acid in very low concentrations may be quantitatively determined in the pre-ence of certain metals. Nickel is employed to displace from their Sequestrene chelates such metals as calcium and magnehrn. The unsequestered nickel is removed from the sequestered nickel by a modification of the method of precipitation with dimethyl- glyoxime (3 ) ; then, as the nickel chelate is highly dissociated in strongly acid solution, it becomes possible to liberate the se- questered nickel and determine it colorimetrically in acid .solu- tion. The amount of nickel sequestered is related to the aniiiunt of ethylenediaminetetraacetic acid present. Potassium di thb-