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Electrochemical behaviour of the bromide ion at a platinum electrode in acetonitrile solvent
Electroanalytical Chemistry and Interfacial Electrochemistry, 47 (1973) 461-468
461
¢~DElsevier SequoiaS.A.,Lausanne Printedin The Netherlands
ELECTROCHEMICAL BEHAVIOUR OF THE BROMIDE ION PLATINUM ELECTRODE IN ACETONITRILE SOLVENT
AT A
FRANCO MAGNO,GIAN-ANTONIOMAZZOCCHINand GINO BONTEMPELLI lstituto di Chimica Analitica delrUniversitd di Padova, Via Marzolo, 1-35100 Padua; Laboratorio di Polarografia ed Elettrochimica Preparativa del C.N.R., Padua ( ltaly)
(Received 2nd March 1973)
INTRODUCTION Numerous and exhaustive studies dealing with the electrochemical behaviour of the bromine/bromide system in aqueous 1-9 and non-aqueous 9-22 solvents have been published but only three reports 1°'11' 18 describe the voltammetric behaviour in acetonitrile medium. The first two works are mainly qualitative in character while the third one reports the kinetics of the electrode reactions. These last works state that the bromide ion, in acetonitrile solvent, can be oxidized to Br~ and to Br/ in two subsequent steps. Only Popov and Geske 11 report, in addition, the existence of a third anomalous anodic wave but no interpretation is advanced. Therefore, in the present paper we intend to confirm the existence of this third anodic step and to elucidate the associated electrode reaction. EXPERIMENTAL Chemicals and reagents
All chemicals were reagent grade (RP ACS C.. Erba and Fluka puriss.). Reagent grade acetonitrile was purified by distilling repeatedly from phosphorous pentoxide 23. LiC10~ was dried according to Billon z4. Oxygen was removed with 99.99Vo nitrogen, dried by flowing it through a trap cooled with liquid nitrogen and equilibrated to the vapour pressure of CH3CN. Apparatus and methods
All experiments were carried out at 25_+0.1°C. All potential values are referred to a silver~.l M silver nitrate-acetonitrile electrode (+337 mV vs. aqueous SCE25). In the voltammetric measurements a previously described 26 platinum sphere microelectrode with periodic renewal of the diffusion layer was employed. Under our experimental conditions a whole cycle of operation was complete in 10 s. In the 0-1.2 s interval the solution was stirred by vertical movement of the electrode; in the 6.5-9.9 s interval the electrode was polarized while the current was recorded only in the 8.0-9.8 s interval. The same electronic polarograph in conjunction with a Hewlett-Packard 2D-2M XY recorder was used in the cyclic tests with scan rates up to 200 mV s- 1. For higher scan rates the same operational system was used in conjunction with
462
F. MAGNO. G.-A. MAZZOCCHIN, G. BONTEMPELLI
a Hewlett-Packard 3300-A function generator and a Hewlett-Packard 1201-A oscilloscope. In the a.c. voltammetric experiments the electronic assembly was the same as described in a previous paper 27. In the cyclic and a.c. voltammetric tests a stationary spherical platinum microelectrode with a diameter of 0.2 cm was always employed. In the controlled potential electrolyses the working electrode was a platinum gauze polarized by an Amel 557/SU potentiostat. The associated coulometer was an Amel integrator model 558. A three compartment cell with a mercury pool as counter electrode and Ag/0.1 M Ag+-acetonitrile as reference electrode was always employed. Before each run the platinum electrode was cleaned with hot dichromic acid, rinsed with twice distilled water, dried and polarized in acetonitrile medium up to 2.2 V v s . Ag/0.1 M Ag+-acetonitrile electrode. Gas chromatographic experiments were carried out with a Perkin-Elmer model F l l chromatograph equipped with either a column of Lac-3 R-728 on Chromosorb W or with a Scot OS138 capillary column. For the quantitative gas chromatographic evaluation of the electrochemically generated phenylbromide the internal standard method was followed: with the former column the standard was n-octanol while with the latter it was o-xylene. Standard solutions of (n-But)4NBr and of Br 2 in acetonitrile were both prepared by adding acetonitrile to known amounts of the two pure products. The bromine solution was prepared immediately before use. Known volumes of these solutions were added to the cell by a E457 Metrohm microburette. RESULTS
Figure 1 shows an experimental voltammogram for a 1.69 x 10- 3 M (n-But)4NBr, 0.1 M LiC10 4, acetonitrile solution. Three oxidation steps can be observed:
Eil/ir[irjjjllllllllllllllllll ' lf,i I2JaA
1.50 1.00 0.50 0.00 Fig. 1. Voltammogram for the oxidation of 1.69 x 10 -3 M (n-But)4NBr in 0.1 M LiC10~, acetonitrile soln. Platinum working electrode with periodic renewal of the diffusion layer.
Br3/Br- ELECTRODE AT Pt IN CH3CN
40
//
463
iL/.UA 30-
o~ °~
20-
//] 0o
1
2 Earl / m~ol]-'
Fig. 2. Plots of the limiting currents v s . bromide concentration for the three anodic waves. ( x ) limiting current for the first wave, (©) limiting current for the second wave, ( 0 ) limiting current for the third wave.
the first wave is about twice the height of the second one for any bromide concentration while the height of the third wave is rather lower and its dependence on bromide concentration is quite different. Figure 2 shows the plots of the limiting currents relative to the three subsequent oxidation steps as a function of the bromide concentration. The first two plots indicate a linear relationship while in the third curve clearly a limiting value is attained. The E½ values of the first and the third wave did not shift by changing the bromide concentration, while the E½ of the second step shifted towards more anodic potentials when the bromide concentration was increased. The logarithmic analysis of the second wave gave a linear plot with a slope of 23 mV instead of 29.6 mV as required by the theoretical equation for a reversible behaviour:
E = K + ( R T / Z F ) In [i3/[(iL)a .--i)2] Cyclic voltammetric tests (Fig. 3) performed on bromide solutions indicate that the products of the first two steps are in their turn reducible while no reduction peak associated with the third one could be observed. This overall picture was observed also at scan rates up to 2.0 V s- 1. The AEpvalues for a scan rate of 0.2 V s- l were larger than 0.4 V for the first system ( B r - / B r ~ ) and 0.1 V for the second system (Br3/Br2). The third oxidation step was detectable with both the above cited techniques, only when bromide or tribromide solutions were tested; on the contrary, bromine solutions did not exhibit the third oxidation wave. Controlled potential coulometric experiments carried out at potential values corresponding to the three oxidation steps (+0.4, +0.80 and + 1.5 V, respectively) gave for n e (referred to the bromide) an average value of 0.66___0.01 for the first step, 1.00 + 0.01 for the second and 1.15 ___0.02 for the third oxidation process.
464
F. MAGNO, G.-A. MAZZOCCHIN, G. BONTEMPELLI
Fig. 3. Cyclic voltammetric curve at a stationary platinum electrode for a 1.8 x 10- a M (n-But)4NBr, 0.1 M LiC104, acetonitrile soln. Scan rate, 0.2 V s 1 In order to gain some information about the nature of the oxidation product of the third electrodic process experiments were carried out in the presence of large amounts of benzene. Voltammetric tests indicated that the presence of benzene did not shift the E~ values of the three waves but sharply increased the height of the third wave. Experiments carried out at variable benzene and at constant bromide concentration showed that the third wave ,increased non-linearly with increasing benzene content; the rate of increase became less with successive additions. Controlled potential coulometric tests carried out at + 1.5 V in these solutions gave the same n~ value as obtained in the absence of the hydrocarbon. Moreover the presence of phenyl bromide in the electrolyzed solution was detected by gas chromatography. Cyclic voltammetric experiments carried out at the end of these electrolyses revealed, in addition, the presence of hydrogen ions. When the electrolyses in the presence of benzene were performed at potential values of +0.4 and +0.8 V, respectively, neither phenyl bromide nor hydrogen ions were found. Quantitative gas chromatographic analyses indicated that the yield of phenyl bromide is about 7 0 ~ when referred to the difference between the charge consumed in the third and in the second anodic process: no appreciable difference in the yield was noted either by ranging the B r - concentration from 5.10 . 4 to 5.10 -3 M or the C6H 6 concentration from 5 . 1 0 - 2 to 1 M. The highest concentration of bromide was conditioned by three factors: the ohmic resistance of the cell, the maximum voltage supplied by our potentiostatic assembly and mainly the simultaneous occurrence at 1.5 V of the three subsequent anodic processes. A.c. voltammetric experiments indicated that the capacitive component of the third peak is very much higher than the faradaic one (Fig. 4). Furthermore the plot of the a.c. current of this peak against the square root of the frequency increases more than linearly. The same a.c. experiments carried
465
B r a / B r - E L E C T R O D E AT Pt IN CHaCN
0
50 mV E+
Fig. 4. Fundamental harmonic a.c. voltammogram in a 4.94x 10 -3 M (n-But)4NBr, 1.0 M LiC104, acetonitrile soln. 90 Hz, 30 mV peak-to-peak applied a.c. voltage. D.c. scan rate, 20 mV s -1. (a) 0 ° component, (b) 90 ° component.
out in the presence of benzene showed a higher capacitive component but no variation of the faradaic one. DISCUSSION
Our experimental results confirm that in the first and second anodic steps the following overall electrode processes occur: 3 Br- -~ Br~ + 2 e 2 Br~ ~ 3 Br 2 + 2eBoth the waves exhibit a certain degree of irreversibility. In fact the A E p values for the two cathodic-anodic processes are greater than the theoretical ones and, in addition, the logarithmic analyses did not agree with the equations valid for reversible behaviour. Furthermore the a.c. results indicated phase angles appreciably less than 45 ° for the two processes z8. However, it may be noted that all three reversibility criteria employed indicate that the first process is more irreversible than the second one. All these results agree with the data reported by other authors 1°'1x'18 concerning the greater degree ofirreversibility of the first wave 18. The most significant result of the present investigation is the statement of the existence of a, well defined third anodic surface controlled process. This process involves a charge transfer reaction as proved by the coulometric data and by the existence of a non-zero faradaic component in the a.c. voltammogram. In addition, the recovery of phenyl bromide after electrolysis at + 1.5 V in the presence of benzene, besides proving the faradaic character of the process, demonstrates that the oxidation product should be the Br + species. The phenyl bromide originates, therefore, according to the following scheme:
466
F. MAGNO, G.-A. MAZZOCCHIN, G. BONTEMPELLI
Br + q- C6H6--~ C6HsBr + H + which explains also the recovery of hydrogen ions in the electrolyzed solutions. A radical reaction mechanism leading to the formation of phenyl bromide can be excluded on the basis of the above reported results which indicated that phenyl bromide was not produced at working potential values lower than the start of the third wave. The phase angle markedly larger than 45 ° and the dependence of the current on Br- concentration for the third anodic process strongly suggest that an adsorption step is involved in the overall electrochemical reaction. Consequently the third wave must be attributed to the oxidation of an adsorbed species. Finally, the very small charge involved at the third step suggests that a competitive reaction consumes to a great extent the depolarizer of the third process. On the basis of our experimental data two reaction mechanisms can be advanced. The first hypothesis predicts that the depolarizer of the third wave is an intermediate in the Br/formation and that just the formation of this molecule is the competitive reaction. We can suppose that the intermediate species are the bromine atoms adsorbed on the platinum surface because the adsorption of this species on platinum has been found by marly authors both in aqueous v's and nonaqueous media is. Consequently the reaction mechanism for the third process would be: +Br~- - e
-e Br~
' (Brz)+Br'(S)I -e
~ 2 Br 2
(a)
, ½ Br 2
(b)
'r ,
Br + In the above reported scheme either mechanism (a) or (b), occurring also at the second step, could be the competitive reaction. However, Iwasita and Giordano TM suggest that reaction mechanism (a) should be preferred on the basis of kinetic data. In addition, thermodynamic considerations of our experimental findings, i.e. the anodic unreactivity of Br_ and the quasi-reversible behaviour of the second electrodic reaction, strengthen t~is hypothesis. In fact if mechanism (b) is operative in the bromine formation, the Br'(S) and Br2 species participate in a chemical equilibrium. The attainment of this equilibrium can be fast or slow under our experimental conditions. The first case should agree with the quasi-reversibility of the second electrochemical step but could not explain why the Br 2 species is not oxidizable, as the Br. (S) produced by the electrochemical oxidation are not distinguishable from those formed by the Br I dissociation. On the contrary, the slow attainment of the equilibrium, allowing an accumulation of bromine atoms at the electrode surface, agrees with the unreactivity of Br 2 but disagrees with the observed voltammetric character of the second anodic process. Therefore the mechanism (b) should be rejected. Our experimental results, which indicate that Br + can be produced only in the presence ofBr-, are, on the other hand, thermodynamically explicable if reaction (a) is the competitive one. This mechanism, in fact, predicts that Br 2 and Br. (S) species are in equil-
Br3/Br- ELECTRODE AT Pt IN CH3CN
467
ibrium, at the appropriate potential values, only in the presence of Br- ions, and consequently does not require that the Br2~2 Br'(S) equilibrium is fast. In the second hypothesis the depolarizer of the third process is just the Brspecies adsorbed on the platinum surface and the competitive reaction is again Br 2 formation. Many authors report the adsorption of Br- at a platinum electrode in aqueous medium 3'6. Johnson and Bruckenstein 6, particularly, account for the formation of HOBr in aqueous 1 M H.SO at a platinum electrode in a surface conz 4. trolled anodic wave by the oxidation of bromide adsorbed on the electrode surface. The observed enhancement of the third anodic wave in the presence of benzene must be attributed to an increase in the capacitive contribution to the current; in fact, a.c. measurements revealed that under these experimental conditions the capacitive component notably increased while the faradaic component remained unaffected. In addition, the coulometric data obtained in the presence of benzene strictly agreed with those found in the absence of the hydrocarbon. SUMMARY
The voltammetric behaviour of the bromide ion at a platinum electrode in acetonitrile solvent has been investigated. The occurrence of three subsequent anodic processes has been established. Experimental evidence of the electrochemical formation of the Br + species is gained and suggestions about the reaction mechanism are advanced.
REFERENCES 1 J. Llopis and M. Vazquez, Electrochim. Acta, 6 (1962) 167. 2 J. Llopis and M. Vazquez, Electrochim. Acre, 6 (1962) 177. 3 M. W. Breiter, Electrochim. Acta, 8 (1963) 925. 4 G. Raspi, F. Pergola and D. Cozzi, J. Electroanal. Chem., 15 (1967) 35. 5 G. Faita, G. Fiori and T. Mussini, Electrochim. Acta, 13 (1968) 1765. 6 D. C. Johnson and S. Bruckenstein, J. Electrochem. Soe., 117 (1970) 460. 7 W. D. Cooper and R. Parsons, Trans. Faraday Soc., 66 (1970) 1698. 8 K. D. Allard and R. Parsons, Chem. Ing. Tech., 44 (1972) 201. 9 G. Dryhurst and P. J. Elving, Anal. Chem., 39 (1967) 606. 10 I. M. Kolthoff and J. F. Coetzee, J. Amer. Chem. Soc., 79 (1957) 1852. 11 A. I. Popov and D. H. Geske, J. Amer. Chem. Soc., 80 (1958) 5346. 12 I. V. Nelson and R. I. Iwamoto, J. Electroanal. Chem., 7 (1964) 218. 13 H. E. Zittel and F. J. Miller, Anal. Chim. Acta, 37 (1967) 141. 14 J. B. Headridge, D. Pletcher and M. Callingham, J. Chem. Soc., A (1967) 684. 15 C. Sinicki and M. Breant, Bull. Soc. Chim. Fr., (1967) 3080. 16 R. L. Benoit, M. Guay and J. Desbarres, Can. J. Chem., 46 (1968) 1261. 17 M. Michlmayr and D. T. Sawyer, J. Eleetroanal. Chem., 23 (1969) 387. 18 T. Iwasita and M. C. Giordano, Electrochim. Aeta, 14 (1969) 1045. 19 G. Adhami and M. Herlem, J. Electroanal. Chem., 26 (1970) 363. 20 Y. Matsui and Y. Date, Bull. Chem. Soc. Jap., 43 (1970) 2828. 21 B. Bry and B. Tremillon, J. Electroanal. Chem., 30 (1971) 457. 22 J. J. Minet, M. Herlem and A. Thiebault, J. Electroanal. Chem., 33 (1971) 69. 23 A. W. Weissberger, E. S. Proskauer, J. A. Riddick and E. E. Toops Jr., Organic Solvents, WileyInterscience, New York, 1955.
468 24 25 26 27 28
F. MAGNO, G.-A. MAZZOCCHIN, G. BONTEMPELLI
J. P. Billon, J. Electroanal. Chem., 1 (1959/60) 486. C. K. Mann in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 3, Marcel Dekker, New York, 1969. G. Schiavon, G. A. Mazzocchin and G. G. Bombi, J. Electroanal. Chem., 29 (1971) 401. G. Bontempelli, F. Magno, G. A. Mazzocchin and S. Zecchin, J. Electroanal. Chem., 43 (1973) 377. D. E. Smith in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. L Marcel Dekker, New York, 1966.
461
¢~DElsevier SequoiaS.A.,Lausanne Printedin The Netherlands
ELECTROCHEMICAL BEHAVIOUR OF THE BROMIDE ION PLATINUM ELECTRODE IN ACETONITRILE SOLVENT
AT A
FRANCO MAGNO,GIAN-ANTONIOMAZZOCCHINand GINO BONTEMPELLI lstituto di Chimica Analitica delrUniversitd di Padova, Via Marzolo, 1-35100 Padua; Laboratorio di Polarografia ed Elettrochimica Preparativa del C.N.R., Padua ( ltaly)
(Received 2nd March 1973)
INTRODUCTION Numerous and exhaustive studies dealing with the electrochemical behaviour of the bromine/bromide system in aqueous 1-9 and non-aqueous 9-22 solvents have been published but only three reports 1°'11' 18 describe the voltammetric behaviour in acetonitrile medium. The first two works are mainly qualitative in character while the third one reports the kinetics of the electrode reactions. These last works state that the bromide ion, in acetonitrile solvent, can be oxidized to Br~ and to Br/ in two subsequent steps. Only Popov and Geske 11 report, in addition, the existence of a third anomalous anodic wave but no interpretation is advanced. Therefore, in the present paper we intend to confirm the existence of this third anodic step and to elucidate the associated electrode reaction. EXPERIMENTAL Chemicals and reagents
All chemicals were reagent grade (RP ACS C.. Erba and Fluka puriss.). Reagent grade acetonitrile was purified by distilling repeatedly from phosphorous pentoxide 23. LiC10~ was dried according to Billon z4. Oxygen was removed with 99.99Vo nitrogen, dried by flowing it through a trap cooled with liquid nitrogen and equilibrated to the vapour pressure of CH3CN. Apparatus and methods
All experiments were carried out at 25_+0.1°C. All potential values are referred to a silver~.l M silver nitrate-acetonitrile electrode (+337 mV vs. aqueous SCE25). In the voltammetric measurements a previously described 26 platinum sphere microelectrode with periodic renewal of the diffusion layer was employed. Under our experimental conditions a whole cycle of operation was complete in 10 s. In the 0-1.2 s interval the solution was stirred by vertical movement of the electrode; in the 6.5-9.9 s interval the electrode was polarized while the current was recorded only in the 8.0-9.8 s interval. The same electronic polarograph in conjunction with a Hewlett-Packard 2D-2M XY recorder was used in the cyclic tests with scan rates up to 200 mV s- 1. For higher scan rates the same operational system was used in conjunction with
462
F. MAGNO. G.-A. MAZZOCCHIN, G. BONTEMPELLI
a Hewlett-Packard 3300-A function generator and a Hewlett-Packard 1201-A oscilloscope. In the a.c. voltammetric experiments the electronic assembly was the same as described in a previous paper 27. In the cyclic and a.c. voltammetric tests a stationary spherical platinum microelectrode with a diameter of 0.2 cm was always employed. In the controlled potential electrolyses the working electrode was a platinum gauze polarized by an Amel 557/SU potentiostat. The associated coulometer was an Amel integrator model 558. A three compartment cell with a mercury pool as counter electrode and Ag/0.1 M Ag+-acetonitrile as reference electrode was always employed. Before each run the platinum electrode was cleaned with hot dichromic acid, rinsed with twice distilled water, dried and polarized in acetonitrile medium up to 2.2 V v s . Ag/0.1 M Ag+-acetonitrile electrode. Gas chromatographic experiments were carried out with a Perkin-Elmer model F l l chromatograph equipped with either a column of Lac-3 R-728 on Chromosorb W or with a Scot OS138 capillary column. For the quantitative gas chromatographic evaluation of the electrochemically generated phenylbromide the internal standard method was followed: with the former column the standard was n-octanol while with the latter it was o-xylene. Standard solutions of (n-But)4NBr and of Br 2 in acetonitrile were both prepared by adding acetonitrile to known amounts of the two pure products. The bromine solution was prepared immediately before use. Known volumes of these solutions were added to the cell by a E457 Metrohm microburette. RESULTS
Figure 1 shows an experimental voltammogram for a 1.69 x 10- 3 M (n-But)4NBr, 0.1 M LiC10 4, acetonitrile solution. Three oxidation steps can be observed:
Eil/ir[irjjjllllllllllllllllll ' lf,i I2JaA
1.50 1.00 0.50 0.00 Fig. 1. Voltammogram for the oxidation of 1.69 x 10 -3 M (n-But)4NBr in 0.1 M LiC10~, acetonitrile soln. Platinum working electrode with periodic renewal of the diffusion layer.
Br3/Br- ELECTRODE AT Pt IN CH3CN
40
//
463
iL/.UA 30-
o~ °~
20-
//] 0o
1
2 Earl / m~ol]-'
Fig. 2. Plots of the limiting currents v s . bromide concentration for the three anodic waves. ( x ) limiting current for the first wave, (©) limiting current for the second wave, ( 0 ) limiting current for the third wave.
the first wave is about twice the height of the second one for any bromide concentration while the height of the third wave is rather lower and its dependence on bromide concentration is quite different. Figure 2 shows the plots of the limiting currents relative to the three subsequent oxidation steps as a function of the bromide concentration. The first two plots indicate a linear relationship while in the third curve clearly a limiting value is attained. The E½ values of the first and the third wave did not shift by changing the bromide concentration, while the E½ of the second step shifted towards more anodic potentials when the bromide concentration was increased. The logarithmic analysis of the second wave gave a linear plot with a slope of 23 mV instead of 29.6 mV as required by the theoretical equation for a reversible behaviour:
E = K + ( R T / Z F ) In [i3/[(iL)a .--i)2] Cyclic voltammetric tests (Fig. 3) performed on bromide solutions indicate that the products of the first two steps are in their turn reducible while no reduction peak associated with the third one could be observed. This overall picture was observed also at scan rates up to 2.0 V s- 1. The AEpvalues for a scan rate of 0.2 V s- l were larger than 0.4 V for the first system ( B r - / B r ~ ) and 0.1 V for the second system (Br3/Br2). The third oxidation step was detectable with both the above cited techniques, only when bromide or tribromide solutions were tested; on the contrary, bromine solutions did not exhibit the third oxidation wave. Controlled potential coulometric experiments carried out at potential values corresponding to the three oxidation steps (+0.4, +0.80 and + 1.5 V, respectively) gave for n e (referred to the bromide) an average value of 0.66___0.01 for the first step, 1.00 + 0.01 for the second and 1.15 ___0.02 for the third oxidation process.
464
F. MAGNO, G.-A. MAZZOCCHIN, G. BONTEMPELLI
Fig. 3. Cyclic voltammetric curve at a stationary platinum electrode for a 1.8 x 10- a M (n-But)4NBr, 0.1 M LiC104, acetonitrile soln. Scan rate, 0.2 V s 1 In order to gain some information about the nature of the oxidation product of the third electrodic process experiments were carried out in the presence of large amounts of benzene. Voltammetric tests indicated that the presence of benzene did not shift the E~ values of the three waves but sharply increased the height of the third wave. Experiments carried out at variable benzene and at constant bromide concentration showed that the third wave ,increased non-linearly with increasing benzene content; the rate of increase became less with successive additions. Controlled potential coulometric tests carried out at + 1.5 V in these solutions gave the same n~ value as obtained in the absence of the hydrocarbon. Moreover the presence of phenyl bromide in the electrolyzed solution was detected by gas chromatography. Cyclic voltammetric experiments carried out at the end of these electrolyses revealed, in addition, the presence of hydrogen ions. When the electrolyses in the presence of benzene were performed at potential values of +0.4 and +0.8 V, respectively, neither phenyl bromide nor hydrogen ions were found. Quantitative gas chromatographic analyses indicated that the yield of phenyl bromide is about 7 0 ~ when referred to the difference between the charge consumed in the third and in the second anodic process: no appreciable difference in the yield was noted either by ranging the B r - concentration from 5.10 . 4 to 5.10 -3 M or the C6H 6 concentration from 5 . 1 0 - 2 to 1 M. The highest concentration of bromide was conditioned by three factors: the ohmic resistance of the cell, the maximum voltage supplied by our potentiostatic assembly and mainly the simultaneous occurrence at 1.5 V of the three subsequent anodic processes. A.c. voltammetric experiments indicated that the capacitive component of the third peak is very much higher than the faradaic one (Fig. 4). Furthermore the plot of the a.c. current of this peak against the square root of the frequency increases more than linearly. The same a.c. experiments carried
465
B r a / B r - E L E C T R O D E AT Pt IN CHaCN
0
50 mV E+
Fig. 4. Fundamental harmonic a.c. voltammogram in a 4.94x 10 -3 M (n-But)4NBr, 1.0 M LiC104, acetonitrile soln. 90 Hz, 30 mV peak-to-peak applied a.c. voltage. D.c. scan rate, 20 mV s -1. (a) 0 ° component, (b) 90 ° component.
out in the presence of benzene showed a higher capacitive component but no variation of the faradaic one. DISCUSSION
Our experimental results confirm that in the first and second anodic steps the following overall electrode processes occur: 3 Br- -~ Br~ + 2 e 2 Br~ ~ 3 Br 2 + 2eBoth the waves exhibit a certain degree of irreversibility. In fact the A E p values for the two cathodic-anodic processes are greater than the theoretical ones and, in addition, the logarithmic analyses did not agree with the equations valid for reversible behaviour. Furthermore the a.c. results indicated phase angles appreciably less than 45 ° for the two processes z8. However, it may be noted that all three reversibility criteria employed indicate that the first process is more irreversible than the second one. All these results agree with the data reported by other authors 1°'1x'18 concerning the greater degree ofirreversibility of the first wave 18. The most significant result of the present investigation is the statement of the existence of a, well defined third anodic surface controlled process. This process involves a charge transfer reaction as proved by the coulometric data and by the existence of a non-zero faradaic component in the a.c. voltammogram. In addition, the recovery of phenyl bromide after electrolysis at + 1.5 V in the presence of benzene, besides proving the faradaic character of the process, demonstrates that the oxidation product should be the Br + species. The phenyl bromide originates, therefore, according to the following scheme:
466
F. MAGNO, G.-A. MAZZOCCHIN, G. BONTEMPELLI
Br + q- C6H6--~ C6HsBr + H + which explains also the recovery of hydrogen ions in the electrolyzed solutions. A radical reaction mechanism leading to the formation of phenyl bromide can be excluded on the basis of the above reported results which indicated that phenyl bromide was not produced at working potential values lower than the start of the third wave. The phase angle markedly larger than 45 ° and the dependence of the current on Br- concentration for the third anodic process strongly suggest that an adsorption step is involved in the overall electrochemical reaction. Consequently the third wave must be attributed to the oxidation of an adsorbed species. Finally, the very small charge involved at the third step suggests that a competitive reaction consumes to a great extent the depolarizer of the third process. On the basis of our experimental data two reaction mechanisms can be advanced. The first hypothesis predicts that the depolarizer of the third wave is an intermediate in the Br/formation and that just the formation of this molecule is the competitive reaction. We can suppose that the intermediate species are the bromine atoms adsorbed on the platinum surface because the adsorption of this species on platinum has been found by marly authors both in aqueous v's and nonaqueous media is. Consequently the reaction mechanism for the third process would be: +Br~- - e
-e Br~
' (Brz)+Br'(S)I -e
~ 2 Br 2
(a)
, ½ Br 2
(b)
'r ,
Br + In the above reported scheme either mechanism (a) or (b), occurring also at the second step, could be the competitive reaction. However, Iwasita and Giordano TM suggest that reaction mechanism (a) should be preferred on the basis of kinetic data. In addition, thermodynamic considerations of our experimental findings, i.e. the anodic unreactivity of Br_ and the quasi-reversible behaviour of the second electrodic reaction, strengthen t~is hypothesis. In fact if mechanism (b) is operative in the bromine formation, the Br'(S) and Br2 species participate in a chemical equilibrium. The attainment of this equilibrium can be fast or slow under our experimental conditions. The first case should agree with the quasi-reversibility of the second electrochemical step but could not explain why the Br 2 species is not oxidizable, as the Br. (S) produced by the electrochemical oxidation are not distinguishable from those formed by the Br I dissociation. On the contrary, the slow attainment of the equilibrium, allowing an accumulation of bromine atoms at the electrode surface, agrees with the unreactivity of Br 2 but disagrees with the observed voltammetric character of the second anodic process. Therefore the mechanism (b) should be rejected. Our experimental results, which indicate that Br + can be produced only in the presence ofBr-, are, on the other hand, thermodynamically explicable if reaction (a) is the competitive one. This mechanism, in fact, predicts that Br 2 and Br. (S) species are in equil-
Br3/Br- ELECTRODE AT Pt IN CH3CN
467
ibrium, at the appropriate potential values, only in the presence of Br- ions, and consequently does not require that the Br2~2 Br'(S) equilibrium is fast. In the second hypothesis the depolarizer of the third process is just the Brspecies adsorbed on the platinum surface and the competitive reaction is again Br 2 formation. Many authors report the adsorption of Br- at a platinum electrode in aqueous medium 3'6. Johnson and Bruckenstein 6, particularly, account for the formation of HOBr in aqueous 1 M H.SO at a platinum electrode in a surface conz 4. trolled anodic wave by the oxidation of bromide adsorbed on the electrode surface. The observed enhancement of the third anodic wave in the presence of benzene must be attributed to an increase in the capacitive contribution to the current; in fact, a.c. measurements revealed that under these experimental conditions the capacitive component notably increased while the faradaic component remained unaffected. In addition, the coulometric data obtained in the presence of benzene strictly agreed with those found in the absence of the hydrocarbon. SUMMARY
The voltammetric behaviour of the bromide ion at a platinum electrode in acetonitrile solvent has been investigated. The occurrence of three subsequent anodic processes has been established. Experimental evidence of the electrochemical formation of the Br + species is gained and suggestions about the reaction mechanism are advanced.
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