Indian Journal of ChemistryVol. 21A, August 1982. pp. 773-776

Kinetics & Mechanism of Oxidation of Indigo Carmine by Vanadium(V)in Presence of Salicylic & Substituted Salicylic Acids-Applicability of Marcus

Relationship between Catalytic Effectiveness & Redox Potential

K SRINIVAS, (Smt) M VlJAYASREE & P V SUBBA RAO·

Department of Chemistry, Andhra University. Waltair 530003

Received 25 September 1981; revised 31 December 1981; rereuised 26 March 1982; accepted 20 May 1982

The title reaction studied spectrophotometrically in perchloric acid medium is first order each in vanadium(V) and indigocarmine. Plots of Ilk, versus 1![L] (where L represents salicylic or substituted salicylic acids) are linear with positive interceptson Ilk ,-axes. suggesting the formation of a I: I complex between vanadium(V) and the ligand. The complex is believed to be amore potent oxidant than vanadiumtv). The reaction is acid-catalysed and the plot of Ilk, versus Ij[H +] is linear with apositive intercept on Ijk ,-axis. The activation and thermodynamic parameters have been calculated. The applicability of theMarcus relationship between catalytic effectiveness and redox potential of V(V)/V(lV) couple in the presence of differentligands, viz. salicylic and substituted salicylic acids is discussed.

Earlier we reported the oxalic acid t and EDT A 2.3_

catalysed oxidations of organic and Inorganiccompounds by vanadium(V) and observed that thecomplexes of these carboxylic acids with vanadium(V)are more potent oxidants than free vanadiumtV).Recently, Pelizzetti et al." and Kochi et al."; appliedMarcus relationship between catalytic effectivenessand redox potential for the oxidation of some organiccompounds by iron(III) complexes. While extendingthis study to other oxidants we have presentlyinvestigated the kinetics of oxidation of indigo carmineby vanadium(V) in the presence of salicylic andsubstituted salicylic acids. These acids are found tocatalyse the reaction.

Materials and MethodsSodium vanadate and indigo carmine solutions were

prepared and standardised according to the method ofGopala Rao et al.6.

7 The dye solution was alwaysprepared afresh. Salicylic and substituted salicylicacids were either commercial products or prepared inthe laboratory. Purities of these were checked beforeuse and their solutions were prepared in acetic acid. Allother chemicals used were of AR grade. Perchloric acid(E. Merck) was used to maintain the acidity.

The kinetic runs were carried out under pseudoconditions in 50/~acetic acid (vjv) at constant ionicstrength (jl). The course of the reaction was followed bymeasuring the absorbance ofunreacted indigo carminespectrophotornetrically at 610 nm using HilgerUvispek spectrophotometer. The temperature wasmaintained with an accuracy of ± 0.1 "C. The redoxpotentials of V(V)/V(lV) couple in the presence ofsalicylic and substituted salicylic acids were measuredon a digital type Toshniwal CL 46 pH meter against a

saturated calomel electrode using a bright platinumrod as the indicator electrode. The reversibility ofV(V)/V(IV) couple was tested by determining thepotential values at different concentrations ofvanadium(V) and vanadium(IV) and in each caseNernst equation was obeyed showing that the couple. isreversible.

Results and DiscussionThe results of stoichiometric runs revealed that one

mol of the substrate consumed 4 mol of vanadium(V)giving isatin monosulphonic acid as one of the reactionproducts identified by the spot tests. The stoichiomet-ric results and product analysis have shown that therewas no induced oxidation by atmospheric oxygen andfurther, salicylic and substituted salicylic acids werenot consumed. At the low temperatures and theconcentrations employed in this kinetic study, theextent of the uncatalysed reaction was negligibly small.Further, the reaction mixture did not initiate vinylpolymerization. The salient kinetic features can besummarised as follows:(i) The reaction is first order in indigo carmine asrevealed by the linear plots of log (absorbance) versustime under the conditions [Indigo carmine] ~ [V(V)].The pseudo-first order rate constants (k I) areindependent of the initial [Indigo carmine] (Table I).(ii) The reaction is first order in vanadium(V) since thepseudo-first order rate constants k I at various [V(V)]are directly proportional to [V(V)] (Table I).(iii) The plots of Ilkl versus Ij[Iigand] are linear withpositive slopes and intercepts on Ijk I-axes suggestingthe formation of a I: I complex between V(V) and theligand. Fig. I shows the plot in the presence of salicylicacid. Similar plots have been obtained for the

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INDIAN 1. CHEM .. VOL. 21A. AUGUST 1982

Table I-Effect of Varying [Indigo carmine], [V(V)] and Ionic Strength (Il) on Reaction Rate[V(V)] = 1.75 x 10-4 M; [Salicylic acid] = 2.8 x I0 ~3 M; [H +] = 0.2 M; Acetic Acid = 50% (vIv); temp. = 25 ± 0.1'C

105 x [Indigo carmine] k, x 103 104 X [V(V)] k , X 103* Ionic k , x 103•

(M) (5 ~') M (5 ~') strengthtu) (s ~')

1.75 2.72 1.75 2.72 0.40 2.652.25 2.66 2.50 3.96 0.60 2.752.50 2.75 3.50 5.16 0.80 2.80

4.25 6.54 \.00 2.725.00 7.91 1.20 2.675.50 8.52

* At [Indigo carmine] = 1.75 x 10 ~5 M.

substituted salicylic acids. Satyanarayana 9 has earliershown the formation of a 1:1 complex between V(V)and sulphosalicylic acid.(iv) The reaction is acid-catalysed. The plot of Ijk I

versus Ij[H+] at constant J1. is linear with a positiveintercept on Ijkl-axis (Fig. 2).(v) The change in ionic strength of the reactionmedium, i.e. varying [sodium perchlorate] has notmuch effect on the rate of the reaction (Table I).

Rossotti and Rossotti I 0 have shown that V(V) existsmostly as va; at pH < I. The acceleration of thereaction with increase in [H +] in the case of manyvanadium(V) oxidations has been explained by Waterset al. II as due to the formation of V(OH)~ + accordingto the equilibrium,

va; +H30+ ~V(OH)~+

Based on the observations recorded herein webelieve that the mechanism involves a rate-determining oxidation of substrate by Vanadium(V)complex forming vanadium(IV) complex and a freeradical formed from indigo. The complex is believed tobe a more potent oxidant than uncomplexedvanadium(V) probably due to the presence of

0.2$ 0-35 0·4510-3 ,

~:-:-=:':7-:-:~=. y-[SALICYLIC ACID]

Fig. I-Plots of Ilk, versus Ij[salicylic acid] {[V(V)] = 1.75x 10 -4 M; [indigo carmine}= 1.75 x lO~SM; [H +] =0.2M; solvent;aq. acetic acid (50%, v/v); 1i=1.0(NaCIO .•); temp.=25±0.1°C}

774

electrophilic carboxylic groups in the complex. Thefree radical is oxidised in fast steps to the final product,isatin monosulphonic acid, in fast steps. However thereaction mixture did not initiate vinyl polymerizationin the absence of atmospheric oxygen. hut accordingto Waters and Jones'? the noninitiation of vinylpolymerization should not be construed as evidenceagainst the free radical formation because the freeradical may have too short life to initiate vinylpolymerization.

The attainment of Marcus plot using V(V)jV(IV)potentials with slope agreeing with theoretical slope(Fig. 3) further supports the view that V(IV) is formedin the rate-determining step. Since the products of

40·0

36·0

32·0

28·0

uJl 24·0

'Q>< 20·0

-I...;

16·0

12·0

8·0

4'00~--~1.~0--~2~.0~--~3.~0~~4~.0~--5~.0r-Sr. t.1-1

[H+J

Fig. 2-Plots of Ilk, versus I/[H+] {[V(V)]=1.75xlO-4M;[indigo carmine] = 1.75 x 10~5 M; [salicylic acid] = 2.80 x 10~3 M;Ii = 1.0 (NaCI04); temp. = 25 ±O.I DC;solvent: aq. acetic acid (50%,

v/v)}

SRINIVAS et at. VANADIUM(V) OXIDATION OF INDIGO CARMINE

oxidation of indigo carmine are two molecules of isatinmonosulphonic acid, the site of oxidation is believedto be the -C =C- double bond of the substrate. Hence,we suggest the following mechanism for the reaction(Scheme I).

+ + KI 2+V02 + H ~ V (OH)3

2+ K,V(OH)3+L ~v(v)L

V (V)L + >C = C< ~ >t-c< + vuv) l(INDIGO SLOWCARMINE)

>t-c< + H20 ~ >c-c< + H+IOH

V(V)L + >C-C< FAST. >C-C< + v(Iv) + H+I H20 I I

OH OH OH

>C-C<+V(V)L FAST. >C-CO + v(IV) I + H+I I I 1\

OH OH OH

>C-CO + V(V)L ~2 >C=O+V(IV)I+ H+I 1\ ISATIN

OH "'ONO-SULPHONlc

ACID

SCHEME'

Scheme I leads to the rate-law,

d[Indigo carmine]dt

kKIK2[V(V)] [L] [H+] [Indigo carmine]I +KI[H+] +KIK2[H+] [L]

Under the conditions, [Indigo] ~ [V(V)], thepseudo-first order rate constant, kl is given by,

k _ kKIK2[L] [H+] [V(V)]I -1+KI[H+]+KIK2[H+][L]

Hence,

1 1kl = kKIK2[L] [H+][V(V)] +

1 1+kK2[L] [V(V)] + k[V(V)]

'.6

'·4 70

'·2.6

S.'·0

""o 40g 0·8

+...•3

0·6.2,

0·4

0·2 /!

0'·0 r.t '·2

EO, VOL TS

Fig. 3-Relationship between logarithm of specific rate constantsfor oxidation of indigo carmine by different V(V) ligand complexesat 25° and /1 = 1.0 and formal redox potential ofV(V)jV(IV) couple inpresence of Different Ligands [(I) 5-Nitrosalicylic; (2) 5-Sulphosalicylic; (3) Salicylic; (4) 5-Chlorosalicylic; (5) 5-Bromosalicylic; (6) 5-Iodosalicylic; and (7) 3, 5-Dibromosalicylic

acids]

Thus the plots of llkl versus I/[L] and llkl versus1I[H +] should be linear with positive slopes andintercepts. This has actually been observed(Figs 1&2)providing kinetic evidence for the complex formationbetween vanadium(V) and salicylic or substitutedsalicylic acids. This type of acid dependence in V(V)oxidations has been observed earlier by us! and by SenGupta et a113.

From the slopes and intercepts of the plots in Figs 1and 2 the values of k and K2 were calculated and thevalues are presented in Table 2. From the rate data theactivation parameters have been determined and theseresults are also listed in Table 2.

Table 2-Specific Rate Constants (k), Stability Constants (K2) of the Complex and Activation and ThermodynamicParameters

Salicylic acid 10 -2 x k(dm3 mol -I s -I) at Ea Kz(dm3 mol : ') at I!Ht--_ .._---------- (kcalmol-I) (kcaljmol)

15' 20' 25° 15° 20° 25°

Salicylic acid 2.70 3.37 4.17 7.40 53.00 67.74 85.86 8.225-Sulpho- 2.25 2.86 3.63 8.20 50.10 71.64 101.14 11.975-Nitro- 1.83 2.42 3.16 9.30 46.70 55.37 65.26 5.703, 5-Dibromo- 20.83 22.90 25.12 3.20 5.20 6.44 7.91 7.145-Chloro- 4.76 5.62 6.61 5.60 51.65 73.60 91.20 9.705-Bromo- 9.27 10.57 12.02 4.45 40.93 63.70 97.50 14.805-Iodo- 12.47 14.08 15.85 4.10 71.22 85.73 102.52 6.20

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INDIAN J. CHEM., VOL. 21A, AUGUST 1982

According to Marcus 14 the free energy of activation~Gt for an outersphere electron transfer reaction isrelated to the overall free energy change, ~G, of thereaction by the relation (I).

A( ~G)2~Gt = 4 I +T ... (I)

where A is a parameter related to the reorganisation ofthe inner and outer coordination spheres of reactioncomplex.

Since the overall free energy ~G is proportional tothe difference in the redox potentials of oxidizing andreducing couples (E2 and E; respectively), ~G can bewritten as,

... (2)

The free energy of activation ~Gt is related to therate constant of the reaction (k) by the equation (3).

~Gt = -RTlnk ... (3)

Substituting the terms in Eq. (I) by Eqs (2) & (3) at ).~ .1.G, Eq. (I) reduces to Eq. (4).

logk = constant+8.5(E2 -E1) ... (4)

For a series of substituted ligands and for the samereductant (A and E, are constant), Eq. (4) can be writtenin the form of Eq. (5),

logk = 8.5E2 +constant ... (5)

where E2 is the redox potential of the oxidizingcouple.

Subsequently Marcus reported that this correlationwas also valid for redox reactions proceeding via inner-sphere mechanisms 15. Thus for closely related ligandslog (rate constant) must be linearly related to redoxpotentials (E2) of the oxidizing couple with theoreticalslope equal to 8.5.

To test the validity of this correlation, wedetermined the rate constant k (as defined in themechanism) and also the redox potentials of theoxidizing couple V(V)/V(lV) in the presence of salicylicand substituted salicylic acids which form complexes

776

with vanadium(V) and found that log k is linearlyrelated to the redox potentials ofV(V)/V{lV) couple inthe presence of different ligands with slope equal to9.16 which is in fair agreement with the theoreticalslope of 8. 5 (Fig. 3). It is interesting to note that in caseswhere the Marcus theory has been found to be tenable,slopes between 7.8 and 10.0 have been re-ported4,5,16,17. A slope of9.16 for the present reactionindicates the validity of equation (4). In cases whereequation (4) does not hold good, slopes between 3.9and 4.9 have been reported!".

AcknowledgementTwo of us (K.S. and M.V.S.) are thankful to the

UGC, New Delhi for the award of researchfellowships.

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