Kinetics of arsenite-ferricyanide reaction in an alkaline medium

J. inorg,nucl.Chem.,1970,Vol.32. pp. 1257to 1262. PergamonPress. Printedin Great Britain

KINETICS

OF ARSENITE-FERRICYANIDE IN AN ALKALINE MEDIUM

REACTION

M. C. A G R A W A L , V. K. J I N D A L and S. P. M U S H R A N Department of Chemistry, University of Allahabad, AUahabad, India

(First received 17 March 1969; in revised form 16 July 1969) A b s t r a c t - T h e kinetics of the oxidation of arsenite to arsenate by ferricyanide has been studied in an alkaline medium. At low alkali concentrations the reaction is very slow and the reaction rates have therefore been determined in highly alkaline media (OH- > 10-2M). The overall kinetics are first order in ferricyanide, arsenite and hydroxide ions. Addition of neutral salts viz. KC1 and K2SO4 has a pronounced accelerating effect. Addition of ferrocyanide has no effect while methanol considerably lowers the reaction rate. The temperature coefficient of the reaction has been found to be unusually low. The energy and entropy of activation have been obtained as 8.2 Kcal and --42.8 cal. deg -I mole-I respectively. A mechanism has been proposed which involves the primary equilibria between various arsenite species and the hydroxide ions and is followed by the rate determining bimolecular reaction between a tribasic arsenite ion and ferricyanide.

INTRODUCTION

THE OXIDATIONS of thiourea, thioacetamide and ascorbic acid by ferricyanide have been studied earlier in this laboratory [ 1,2]. While studying the oxidation of several carbonyl compounds Waters[3] has suggested the primary step in all ferricyanide oxidations is the enolisation of the organic compound with alkali, followed by the decomposition of the enol by ferricyanide. Other publications [4, 5] in the field of ferricyanide oxidations also deal with the kinetics of the oxidation of organic compounds in both acidic and alkaline media, but the literature on the mechanism of oxidation reactions of inorganic materials by ferricyanide is scanty. The present paper describes the kinetics observed in the oxidation of sodium arsenite by potassium ferricyanide. It has been observed that the reaction is quite slow and proceeds only in sufficiently high alkali (> 0-05 M) concentrations at 40 °. However in presence of neutral salts like KCI and K2SO4 the reaction is possible in lower alkali concentrations. This not too fast reaction can be catalytically accelerated by osmium (VIII) oxide[6]. It has been postulated that an equilibrium exists between arsenites and the hydroxide ion. EXPERIMENTAL Chemicals. Aqueous solution of sodium arsenite was prepared by dissolving an accurately weighed sample of recrystallised arsenic trioxide of analytical grade in 1N sodium hydroxide and finally neutralising the excess alkali by 1N hydrochloric acid to a pH of 7.0. This neutral solution is stable for several months. M. C. Agrawal and S. P. Mushran, J. phys. Chem. 72, 1497 (1968). U. S. Mehrotra, M. C. Agrawal and S. P. Mushran, J. phys. Chem. 73, 1996 (1969). P.T. Speakman andW. A. Waters, J, chem. Soc. 40 (1955). !. M. Kolthoff, E. J. Meehan, M. S. Tsao and Q. W. Choi, J.phys. Chem. 66, 1233 (1962). E.J. Meehan, I. M.Kolthoff and H. Kakiuchi, J. phys. Chem. 66, 1238 (1962). 6. F. Solymosi,Acta chim. Hung. 16,267 (1958). 1. 2. 3. 4. 5.

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M . C . A G R A W A L , V. K. J I N D A L and S. P. M U S H R A N

All other materials used and procedures were identical with those described earlier[l] with the exception that owing to high N a O H concentration (0.153 M), no buffer was employed. All experiments were carried out at 40 °. RESULTS

Dependence on arsenite and ferricyanide. In mixtures containing excess of arsenite the disappearance of ferricyanide always follows first order kinetics at all concentrations of arsenite (curves 1-5, Fig. 1). The rate of the reduction of

100 ~

_

tr

-o,,.

20 1

IOl

V

I

I

I

30

40

I

IO

20 Time,rain

Fig. 1. Reaction of ferricyanide, [Fe(CN)63-] = 4 × 10-4 M, [NaOH] = 0-153 M; [arsenite. = (I) 1-2, (II) 1-6, (III) 2.0, (IV) 2.8 and (V) 4.0 x 10-3 M.

Table 1. Rate of reduction of ferricyanide with arsenite* [Fe(CN)e 3-] (M × 104)

[Arsenite] (M x 103)

kl x 104 (sec -1)

kl/[Arsenite]t (1. mole -1 sec -1)

4.0 4.0 4.0 4.0 4.0 3.2 3"6 4-4 4.8

1"2 1.6 2.0 2"8 4.0 2-0 2.0 2-0 2.0

1.88 2-53 3.03 4.21 6-07 3.21 3.11 3.00 3-03

0.016 0.018 0-016 0.016 0.016 0.017 0-016 0.016 0-016

*[NaOH] = 0.153 M. ?Second order rate constants, (k2) obtained by using average concentration of arsenite during the run.

Kinetics of arsenite-ferricyanide reaction

1259

ferricyanide (initially-4 × 10-4 M) was determined in 0.153 M N a O H and at different concentrations of arsenite from 1.2 × 10 -3 to 4.0 × 10-3 M (Table 1) where from the average value of k~/[arsenite] in five experiments was obtained as 0.017 +__0.001 1. mole -1 sec-L showing that the reaction is also first order to arsenite. Effect of five different concentrations of ferricyanide (from 3-2 to 4-8 x 10-4 M) showed no effect on the first order rate constants in ferricyanide (Table 1). It is, therefore, concluded that the reduction of ferricyanide with arsenite corresponds to an overall second-order reaction whose rate may be expressed as d dt [Fe(CN)s3-] = k2[Fe(CN)G3-] [Arsenite]. Owing to high alkali concentration (0.153 M) it is assumed that ionic strength remains constant in these observations. Alkali dependence. As the reduction of ferricyanide proceeds only in highly alkaline solutions (pH > 13, in absence of salts), it was not possible to utilise buffers. At constant ionic strength (K2SO4 = 0.2 M) the effect of [OH-] was studied on the rate of the reaction and it was checked by adding varying concentrations of KC1 to runs containing fixed concentrations of K2SO4 and N a O H that a fairly constant ionic strength was maintained (Table 2). Table 2. Alkali dependence* [KCI] (M)

kl x 104 (sec -1)

NaOH added (M)

kl'X 104 (sec-q

0"0 0"02 0"04 0"06 0"08

3"2 3-3 3'3 3"4 3"5

0.0 0.0186 0.0373 0.0559 0-0745

3.2 4.7 6.1 7.3 9.2

* [ F e ( C N ) s 3-] = 4 × 10 -4 M, [ A r s e n i t e ] = 2 ×

10-3 M, [ K 2 5 0 4 ]

=

0"2M, [NaOH] = 3.72 x 10-2 M.

In presence of 0.2 M K 2 5 0 4 , log k2 is linear with log [ O H - ] with a slope of 1.0 (Fig. 2) i.e., k2 is directly proportional to [OH-]. Average value of k2/[NaOH] in five experiments was obtained as 4-13 12mole -2 sec -1 in presence of 0.2 M K2SO4. Influence of neutral salts. Influence of addition of potassium chloride and potassium sulphate was studied. The results (Table 3) clearly reveal that in the reduction of ferricyanide by arsenite a positive salt effect is observed. Effect of dielectric constant. T h e effect of dielectric constant was studied by investigating the reaction in different amounts of m e t h a n o l - w a t e r mixtures. T h e results (Table 4) show that an increase in methanol concentration has a considerable retarding influence. Other effects. Influence of several other factors was studied. Platinum (IV) seems to be inactive as catalyst but osmium (VIII) exerts a strong catalytic influence. In presence of 0.037 M N a O H , 2 × 10-~ M osmium (VIII) was sufficient to complete the reaction within 15 min. Addition of ferrocyanide has no effect.

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M . C . A G R A W A L , V. K. J I N D A L and S. P. M U S H R A N

1"5

1.4

m

1.3

--

1.2

I.I

1.0

I 0"6

I 0"7

I 0"8

I 0-9

1 I-0

Io0EOH" x l 0 0 ] Fig. 2. log k2 vs. log[OH-].

U n d e r the conditions of Table 3, experiments were repeated at 35, 45 and 50 °. T h e same kinetics were observed and from the values of the rate constants the activation energy and the activation entropy were found to be 8.2 Kcal mole -1 and - 4 2 . 8 cal deg -1 mole -1 respectively. T h e reaction was also studied in nitrogen atmosphere and it was concluded that oxygen, in no way, is directly involved in the oxidation of arsenite. DISCUSSION

T h e first order dependence in O H - and arsenite predicts that the primary step in the reduction of ferricyanide by arsenite would be the interaction between Table 3. Salt effects* [KCI] (M)

kl x 104 (sec-0

[KzSO4] (M)

kl × 104 (sec -1)

0.00 0.05 0-10 0.15 0.20 0.30

3.03 4-55 5-92

0.00 0-02 0-04

3.03 3-97 5.08

7.07

0.06

6.00

7.90 9.92

0.08 0-10

6-77 7.78

*[Fe(CN)63-] = 4 x 10-4 M, [Arsenite] = 2 × 10-3 M and [NaOH] = 0.153 M.

Kinetics of arsenite-ferricyanide reaction

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Table 4. M e t h a n o l d e p e n d e n c e * Methanol (%)

D'~

kl x 104 sec '

0 5 10 15 20

73.15 71-07 69.00 66-94 64.88

3-03 2.05 1-44 1.11 0.850

k2 × 102 1. m o l e - ' sec 15.8 10.6 7.28 5.63 4-30

*Conditions s a m e as in Table 2. tDielectric constant of the medium.

arsenite and hydroxide ion. Arsenite in alkaline media is known to be present in equilibrium with metaarsenite ion [7] while a less alkaline solution (almost neutral) contains almost exclusively molecules of metaarsenious acid [8]. However Loehr and Plane [9] have ruled out the existence of HAsOz and in an alkaline medium solutions of arsenite have been shown to contain the four species viz., As(OH)3, AsO(OH)2-, AsO2(OH) 2- and AsO3 3-. Further, it has been observed that addition of the neutral solution of sodium arsenite (pH = 7) causes a sufficient decrease in pH when added to an alkali solution. An increase in arsenite concentration enhances the decrease in pH while the effect is less pronounced with dilute alkali solutions (Table 5). Evidently, an equilibrium exists between arsenites and hydroxide ions which depends on the concentration of the reactants. In view of the observations following mechanism has been proposed for the reduction of ferricyanide by arsenite in an alkaline medium. As(OH)3 + O H - . " AsO(OH)2--k- H20 AsO(OH)2-A- O H - . " AsO2(OH) 2- + H20 AsO2(OH) 2-+ OH- . " AsO33- q- H20 AsO33- + Fe(CN)6 a) A s O 3 2- q- Fe(CN)64- slow AsO32- + Fe(CN)6a- + O H ) H AsO42- + Fe(CN)s4- + H20 fast

(la) (lb) (IC) (2) (3)

or

AsOa 2- + Fe(CN)6 a- + 2OH-

) A s O 4 3-

-4- Fe(CN)64- + H20 fast.

(4)

The tribasic arsenite present in the reaction mixture is oxidised to an arsenic (IV) intermediate by ferricyanide in a slow step (2) which is the rate determining one. The formation of arsenic (IV) during the oxidation of arsenic (III) has been demonstrated by Kolthoff e t al.[10, 11]. Arsenic (IV) immediately reacts with a fresh molecule of ferricyanide and alkali to give arsenate as the final product (step 3,4). 7. 8. 9. 10. 11.

V. N. K o c h e g a r o v and T. P. Lomakina, Electrokhimiya 2 , 2 4 0 (1966). D. Y. E v d o k i m o v , Meded. Prom. S. S. S. R. 11 (4), 39 (1957). T. M. L o e h r and R. A. Plane, lnorg. Chem. 7, 1708 (1968). R. W o o d s , I. M. K o l t h o f f a n d E. J. M e e h a n , inorg. Chem. 4, 697 (1965). R. W o o d s , I. M. K o l t h o f f a n d E. J. M e e h a n , J. Am. chem. Soc. 85, 2385 (1963).

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M.C. AGRAWAL, V. K. JINDAL and S, P. MUSHRAN Table 5. Effect of arsenite on an alkali solution [Arsenite] (M × 103) 1"2 1.6 2.0 2.0 2.0

Initial pH pH on addition Fallin [OH-] of alkali of arsenite (M x 104) 10.8 10"8 10"8 10.4 9.9

9"65 9.40 9"20 9"70 8"20

5"66 6"05 6-15 2"58 0"78

Step (2), being slow and rate determining, suggests that the oxidation of arsenite by alkaline ferricyanide would follow second order kinetics being first order in both oxidising and the reducing agent and the rate law expression would be __ d [Fe(CN)63_ ] = k[Fe(CN)63_][AsO ~_] dt

(5)

at constant concentration of alkali, k corresponds to the second order velocity constant and is therefore the same as experimentally obtained k2 = 0.016 I. mole -a sec -1 in 0.153 M N a O H . Also as the reaction has been studied in highly alkaline media ( N a O H > 0-153 M) the more favoured species initially present are AsO2 (OH) 2- and AsO33- while As(OH)3 and A s O ( O H ) 2 - have insignificant existence. Undoubtedly, therefore the contribution towards the kinetics of the reaction would mainly be due to the latter two species which evidently predicts first order dependence in hydroxide ion. Our experimental results also lead to similar conclusions. T h e negative entropy change and positive salt effect indicate a rate determining step involving similarly charged ions and thus support step (2). T h e retarding influence of methanol on the reaction rate (Table 4) also establishes the above view. It may be concluded that the significant effect of hydroxide ions on the arsenite-ferricyanide reaction suggests that various arsenite species and O H ions are in equilibrium with a second arsenic(Ill) intermediate, which ultimately reacts with ferricyanide. due to the Council of Scientific & Industrial Research, New Delhi, India for a Senior Research Fellowship to MCA.

Acknowledgement-Thanks are