Anodic oxidation of halides in anhydrous acetic acid

Electroanalytical Chemistry and Interfacial Electrochemistry, 44 (1973) 3745

37

© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

A N O D I C O X I D A T I O N O F HALIDES IN A N H Y D R O U S

ACETIC ACID

M. MASTRAGOSTINO, G. CASALBORE and S. VALCHER

Laboratorio di Polarografia ed Elettrochimica Preparativa, C.N.R., Padova (Italy) Centro di Studio di Elettrochimica Teorica e Preparativa, lstituto Chimico G. Ciamician, Universitd di Bologna, Bologna (Italy) (Received 17th October 1972)

A number of papers have been published 1-7 on the oxidation of halide ions in non-aqueous solvents. Our interest in this problem is also due to the fact that it is a preliminary step in the study of the mechanism of anodic halogenation. We have chosen anhydrous acetic acid as the solvent since the most interesting results in the chlorination of organic substrates by anodic oxidation in the presence of chloride ions have been obtained in this medium s. The I - oxidation on the platinum electrode has already been studied 1 and we have extended the investigation to the oxidation mechanism of C1- and B r - . Most of the results in the literature about the electrolytic oxidation of halides in non-aqueous solvents were obtained with acetonitrile, acetone, nitromethane and dimethylformamide as the solvent. Two different reaction schemes have been proposed for these electrochemical processes: 3 X- ~X~-+2 e-

(a)

possibly followed by X~- ~ X 2 + e 2X- ~X2+2e-

(a')

(b)

The first scheme has been proposed in a number of cases 1' 2,4.,5 and implies first the oxidation of the halide to trihalide ions followed by a further oxidation to the halogen. The second reaction scheme has also been proposed by a number of authors 3'6'7. .In some cases e.g. the oxidation of C1- in nitromethane 2'3 both reaction schemes have been proposed. EXPERIMENTAL

The controlled potential (C.P.V.), controlled current, (C.C.V.), null current (N.C.V.) voltammetric measurements were made with apparatus designed and built in our laboratory 9'1°. An Amel potentiostat model 557/SU was used as a linear sweep voltage generator for the C.P.V. measurements, while for the C.C.V.-N.C.V. the same potentiostat was used as a galvanostat to obtain a linear current sweep. A Hewlett-Packard model 7000 recorder was used in both cases. The working electrode was a platinum disc of area 0.126 cm2 and contained 5 ~ or iridium. It was mounted in a glass jacket and the renewal of the diffusion

38

M. MASTRAGOSTINO, G. CASALBORE, S. VALCHER

layer was achieved by expelling solution from the electrode compartment with a stream of very pure nitrogen as described in ref. 9. In all of the experiments with C1- the working electrode was kept at the counter electrode potential before the diffusion layer renewal step in order to eliminate the reversible modifications of the electrode surface. It was possible to obtain reproducible measurements with this method only. In order to obtain reproducible measurements in the C1- oxidation, it was necessary first to make an electrolysis at - 2 V in a solution of the same composition as that to be examined according to the method of Durand and Tremillon 1. A Thalamid electrode model 40 manufactured by Schott and Gen was used as the reference electrode and all potentials are referred to this electrode. The counter electrode was a platinum disc with a large surface area. An Amel model 558 electronic integrater was used for the coulometric measurements made during the controlled potential electrolysis experiments. These experiments were performed in a 30-cm 3 cell and the working electrode was a platinum net containing 5 ~ of iridium. Before each electrolysis this electrode was washed in boiling nitric acid, rinsed with water and then ethanol and dried. The counter electrode was a platinum spiral and the cathode compartment was separated from the solution by a sintered glass disc. CHEMICALS AND REAGENTS

Solvent: the anhydrous acetic acid R.P. Carlo Erba was purified by distillation under reduced pressure. Indifferent electrolyte: sodium perchlorate R. P. Carlo Erba. The water of crystallization was eliminated by adding a stoichiometric quantity of acetic anhydride to the solution. THEORETICAL CONSIDERATIONS

The mathematical equations of the N.C.V. curves which may be expected in the cases studied here may be derived from the equations given in ref. 10. ( a ) Oxidation o f halide anion to trihalide anion

For a process of the type: 3X- ~X~-+2e in the absence of X~- in the bulk of the solution, eqns. (4) of ref. 10 can be written: io = 0 ia = - ½" ~ F q C °_ (Dx- z~/tp)

(la) (lb)

while eqns. (9) become: 2 = i/2VqDx3 0 = i/~VqD x -

(2a) (2b)

These equations, with eqns. (10) give the concentrations of X - and X~- at the electrode surface at the time of potential recording:

OXIDATION OF C1- AND Br IN CH3COOH

39

Cx;¢ o, to) = - (i/Fq)(tp/Dx3 Ir)~(2½- 1) Cx-(o, tp)= CO + (3i/Fq)(tp/Dx- re)½(2~ - 1)

(3a) (3b)

For the process under examination, in reversibility conditions which may be approximately assumed for the N.C.V. technique, the following form of the Nernst equation can be used: (4)

E = Eeo+(RT/2F)ln(Cx3(o, ,p)/C3x-(o, ,,))

Cx~ is given by eqn. (3a). The X - concentration can be obtained from eqns. (3b) and (lb). Cx (o, t,) = [i( 2 ~ - 1) - i,](3/Fq)(tp/Dx- 7r)~

(5)

Combination of eqns. (3a), (4) and (5) gives eqn. (6). RT / D 3 - \ ½ ? Fq \2 rt RT -i E=Efo + ~ F - l n k ~ - x ~ ) ~ ) 2-~p+~ff-lni_ia(2½=l) 3

(6)

For the analysis of the null current voltammetric curves, we can use the parameters AE and E~ro~. defined as follows: (7) (8)

AE -(dE/di)i=ia " ia E{_rev"~ Ei=½ia(2{+l )

Now, we can obtain the parameter AE by differentiating the eqn. (6) with respect to i, by substituting i by ia and by multiplying the result by i~. In such a manner can be obtained: AE = (R T/2F)( 1 + 3/2 {)

(9a)

that is, at 25 ° AE = 0.040 V

(9b)

The parameter E½.... cannot be directly determined from the graphs; it can however be evaluated by relating it to another parameter, directly determinable as the potential E~= ~afor which the value of the current is equal to the value of the limiting current of the C.C.V. curve. Substituting ia for i in eqn. (6) gives: R T , { D ~ ' \ ~ [ " Fq ~2 ~ RT 1 Ei=ia=Efo + ~ - m ~ x ~ ) ~2-T~-l) 2 - ~ p + ~ - l n ( _ i ~ ) 2 . 2

~-

(10)

and substitution of i by ½ia(2++ 1) in eqn. (7), gives: R T l n ( D 3 - ) ½ ( Fq ~ 2 lz E½.... = El° + 2if- kDx;] \ 2 " - 1 : / ~

RT 1 + 2if- In {_½i~(2½ + i)}2

(11)

then: E{rev. = Ei=i,, + (RT/2F) In 1.94

(12a)

that is, at 25 ° E~.... = E~= i~+ 0.009 V

(12b)

4O

M. MASTRAGOSTINO, G. CASALBORE, S. VALCHER

(b) Oxidation of halide ion to halogen For a process of the type: 2X-~X2+2e a similar derivation to that described above for case (a) gives:

io=0

(13a)

i a : - l FqC ° (~D x /tp)

(13b)

corresponding to the eqns. (la) and (lb). One has also the equations:

2 = i/2Fq Dx2 0 = i/Fq Dx

(14a) (14b)

similar to eqns. (2a) and (2b). The equations: Cx2(o, tp) = - (i/Fq)(tp/Dx~ ~z)~(2 1 - 1)

(15a)

Cx (o, t . ) = {(i( 2 1 - l)-ia)/Fq}(tp/rcDx )l

(15b)

are obtained for the concentrations. By substituting the preceding equations in the Nernst equation:

E = Efo+(RT/ZF)ln(Cx2~o ' tp)/C 2 (o, tp))

(16)

the following expression is obtained:

R T ln(D2x-) ½ Vqn ½ RT -i E=Ef° + - ~ \Dx2/ t ~ 4 ( 2 t - 1 ) + ~ - l n [ i - i a ( 2 1 + l ) ] 2

(17)

The parameter AE may be obtained from eqn. (17) in a similar manner to the way it was derived from eqn. (6) in case (a).

AE = (RT/F)(1 +2~), that is, at 25 ° AE = 0.031 V

(18a) (18b)

Similarly for E~ .... the following equations are obtained: E t .... = E,=~, + (RT/2F) ln-11(2+2 ~)

(19a)

That is, at 25 ° E t .... = Ei = ia + 0.007 V

(19b)

RESULTS AND DISCUSSION C.P.V., C.C.V. and N.C.V. measurements have been performed on LiC1 and NaBr solutions in anhydrous acetic acid with 0.4 M NaC104 as the supporting electrolyte. In C.P.V. the C1- ion gave only one anodic wave with E l = + 1.18 V, at concentration of 4.6 x 10 -3 M. The limiting current of such a wave is a linear function of the concentration

OXIDATION OF C1- AND Br- IN CH3COOH

41

in the examined concentration range, that is 3 x 10 -3 M - 7 x 10 -3 M. It is also controlled by diffusion*, as it results from the value of the temperature coefficient (1%/°) evaluated for several C1- concentrations in the temperature range 20-40 °. In C.C.V., the C1- ion, at the concentration 4.6 x 10- 3 M, gives an oxidation wave with E~ = 1.20 V, and in the corresponding N.C.V. curve the parameter E i=i. is 1.00 V. Furthermore the value of AE = (dE/di)i= ia" ia is 42 mV at 25 °. This quantity is very reproducible and is independent of the depolarizer concentration and is in accord with the electrodic process (a), for which 2 electrons are exchanged for three C1- ions. In this case the value of E+ .... should be 9 mV more positive than E~=~a. Since the difference between E~ .... and E~(c.c.v.) is not very large, the present results are in the field of applicability of N.C.V. technique and therefore acceptable 11. Similar measurements carried out on B r - ion gave the following results: 4 × 10 -3 M B r - showed only one anodic wave with E~ =0.83 V. The limiting current of this wave is a linear function of the concentration between 1 x 10- 3 and 6 x 10-3 M. It appears to be diffusion controlled in the temperature range 30-50 ° (the temperature coefficient was found to be 1.2%/°), while in the temperature range 20-30 ° (temperature coefficient= 3%/° ) it seems to assume some kinetic character. Further evidence for the kinetic control of the B r - limiting current comes from the linear sweep voltammetry experiments. Here at 20 ° the peak current increases more slowly than a linear function of the square root of the potential sweep rate. In the C.C.V., at 30 °, the B r - ion, at a concentration of 4 × 10- 3 M gave an oxidation wave with E ~ c . c . v . ) - +0.85 V. F o r the corresponding null current curve the value of the parameter E~=i~ is 0.81 V, and the value of parameter AE is 30 mV. This highly reproducible value of parameter AE is independent of the depolarizer concentration and is in accord with the electrodic process (b) requiring 1 electron exchanged for every bromide ion. Therefore parameter E~ .... should be 7 mV more positive than E~=~o. The coincidence of E~ .... with E~ evaluated by C.P.V. and C.C.V. curves, shows that the system is practically reversible. Also the mathematical analyses of the C.P.V. curves, which are described by the equation: E = K + (0.059/2) log i/(ia - i) z demonstrate the reversibility of the process. To confirm the voltammetric data, some coulometric experiments have been performed on the C1- and B r - solutions, at the temperatures of 20 °, 30 ° and 40 °. The electrolysis potential was + 1.4 V for C1- and + 1.2 V for B r - . F r o m the coulometric measurements performed on the C1- solutions, it has been found that the process, if brought to the end, tends to produce one electron for every C1- ion, and the electrolysis time is greatly increased by lowering the temperature. The fact that, in the case of C I - , the C.C.V.-N.C.V. results are not in accord with the final results of the coulometric measurements can be an indication of the * By correlating the variation of the diffusion coefficientswith the temperature to the variation of the viscosity it was possible to estimate the diffusion current temperature coefficient to be about 1%/°.

42

M. M A S T R A G O S T I N O , G. CASALBORE, S. V A L C H E R

///•,A

I

4.O 3.6 3,2

\

-1,6 "<'*. -'1.2

x,, "X, " "Xx,,

-0.8

"X "

-0,4

"",

Q/C 2 i 2/3

1r n / e l e c t r o n s iOn-1

4

6 2j 3

1 n / e l e c t r o n s ion -1

Fig. 1. Plot of electrolysis current against the charge c o n s u m e d and the n u m b e r of electrons per C I ion transformed. X - c o n c e n t r a t i o n = 4 x i0 a M, volume of the s o l u t i o n = 30 ml, temperature 20 °. Time required for the current to decrease to a constant m i n i m u m value: 5 h 30 rain. Fig. 2. Plot of electrolysis current against the charge c o n s u m e d and the n u m b e r of electrons per C1- ion transformed. X - c o n c e n t r a t i o n = 4 x 10 -3 M, volume of the s o l u t i o n = 3 0 ml, temperature 30 °. Time required for the current to decrease to a constant m i n i m u m value: 3 h 30 rain.

existence of some complication in the electrochemical process. A strong indication of the causes of these apparent disagreements may be seen from Figs. 1, 2 and 3 where the electrolysis currents are plotted against the number of coulombs consumed for three coulometric experiments performed at 20 °, 30 ° and 40 °. At 40 ° the plot is linear, with the production of one electron for every C1 - ion throughout the electrolysis time. At 20 ° and 30 ° straight lines are not obtained. At these temperatures, the intercept of the initial part of the line with coulomb axis, indicates a production of 2/3 electron for every C1- ion transformed. Indeed the experiments at the lowest temperatures show that the production of 1 electron for every C1- ion is due to a reaction implying a species which is formed during the electrolysis; at 40 ° this species reacts more rapidly so that, just from the beginning of the electrolysis, the oxidation proceeds completely. Furthermore if only a partial electrolysis is carried out at 20 ° the following C.P.V. recording indicates a new cathodic wave (E~= +0.96 V) which decreases, while the residual anodic wave increases in the time. The rate of this transformation is enhanced if the temperature, after the electrolysis, is increased (40°). The results of the C.C.V.-N.C.V. measurements and those of the initial part of the coulometric measurements carried out at the lowest temperatures favour the hypothesis that the CI~- ion is the primary product of the electrochemical process. On the other hand C.P.V. measurements carried out after non exhaustive electrolysis show that CI~- anion is not a stable product under these experimental conditions

OXIDATION

O F C1- A N D Br

43

IN CH3COOH

i/uA

'i'~

-3.2

•

"',',,

- 2.6

',\

'~',

- 2.4

4 "~,

-2.0

\

"~,

-1.6

%",~

-1,2

~,

- 0.8

,1,, -1

-0.4

" 12

2/3

•

1

n/electrons

?/c

2

ion -1

4

8 o/c 1 n/electrons ion -1

Fig. 3. Plot of electrolysis current against the charge consumed and the number of electrons per C l ion transformed. X - concentration=4 x 10 .3 M, volume of tl~e solution= 30 ml, temperature 40 °. Time required for the current to decrease to a constant minimum value: 3 h. Fig. 4. Plot of electrolysis current against the charge consumed and the number of electrons per B r - ion transformed. X - c o n c e n t r a t i o n = 2.5 x 10-3 M, volume of the solution = 30 ml, temperature 20 °. Time required for the current to decrease to a constant minimum value: 2h 20 min.

and that it disappears to yield depolarizer C1- again. The results of the coul0metric measurements in the case of exhaustive electrolysis show that the final product yields one electron for each C1- ion and that it can be reasonably identified with C12. As a consequence the following reaction mechanism can be proposed: 3 C1- - ~ C13 + 2 e

(at the electrode)

slo w

CI~

, Clz + C1-

(in the bulk of the solution)

The coulometric measurements carried out in Br- solutions give results which agree with those from the voltammetric measurements. Also at 20 ° the electrolysis does not yield detectable amounts of Br~. Figure 4 shows the plot of the electrolysis current against the charge consumed for an experiment using 30 ml of solution 3 x 10 -3 M with respect t o Br-. It can be seen that the process takes place from the beginning with the production of 1 electron per Br- ion. For bromide ion the following simpler mechanism can be proposed: 2 Br- ~ Br 2 + 2 e

(at the electrode)

44

M. M A S T R A G O S T I N O , G. CASALBORE, S. V A L C H E R

As the nature of the solvent does not allow experiments to be made at lower temperatures, the coulometric measurements cannot confirm the existence of Br~ as a reaction intermediate as indicated by voltammetric measurements. CONCLUSIONS

It appears that one of the most interesting results of this study is that C12 is not an intermediate product in the electrochemical formation of the trichloride anion but it is given by the dissociation of the latter compound which is then the primary product of the electrochemical reaction. As a consequence the mechanism: C1- ~ C l ' + e 2 CI" --* C12 Clz +C1- ~ C13 is not admissible in this case and more probability must be ascribed to the mechanism: C1- ~ C l ' + e CI'+C1- ~ CI~ C I ~ + C I ' ~ C I ~ or 2C1 z---,CI~+C1The same mechanism is probably involved in the oxidation of Br- ion, with the assumption that the rate of dissociation of Br3 -is much greater than that of CI~. Comparison of the study of Durand and Tremillon 1 on iodide with our results shows that the 13 ion is more stable with respect to the equilibrium: X- + X 2 - ~ X ~ than the halogens we have examined. The very slow dissociation of the C13 ion means that its stability constant cannot be determined by the techniques employed in this work. However it is reasonable to assume that: gj3- > K B r s > K c l 3 . ACKNOWLEDGEMENTS

The authors wish to express their gratitude to the late Prof. L. Riccoboni who guided the work of the C.N.R. Polarografia Elettrochimica Preparativa Laboratory with extraordinary qualities of competence, humanity and self-denial. The authors are indebted to Professor G. Semerano for his interest in this work and for his helpful criticism. We thank Mrs. A. F. Randi of the C.N.R., Dr. M. T. Sponza and Dr. G. Vannini for valuable help during this work. SUMMARY

The electrochemical behaviour of chloride and bromide ion in anhydrous acetic acid on a platinum electrode has been studied. The most probable reaction path for the oxidation of the halides seems to be: 3X-~X~+2e X3 ~ X2+X-

OXIDATION OF C1- AND Br- IN CH3COOH REFERENCES 1 2 3 4 5 6 7 8 9 10 11

G. Durand and B. Tremillon, Anal. Chim. Acta, 49 (1970) 135. I. V. Nelson and R. T. Iwamato, J. Electroanal. Chem., 7 (1964) 218. J. C, Marchon and J. Badoz-Lambling, Bull. Soc. Chim. Fr., 12 (1967) 4660. V. A. Macagno and M. C. Giordano, Electrochim. Acta, 14 (1969) 335. T. Iwasita and M. C. Giordano, Electrochim. Acta, 14 (1969) 1045. L. Sereno, V. A. Macagno and M. C. Giordano, Elecrrochim. Acta, 17 (1972) 561. C. Sinicki, P. Desportes, M. Breant and R. Rosset, Bull. Soc. Chim. Fr., 2 (1968) 829. F. Fictey and L. Glantzstein, Ber., 49 (1916) 2473. S. Valcher, J. Electroanal. Chem., 29 (1971) 391. S. Valcher, J. Electroanal. Chem., 31 (1971) 349. S. Valcher, J. Electroanal. Chem., 33 (1971) 208.

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