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Application of flow electrolysis on porous electrodes for the electroreduction of potassium p-Nitrobenzoate
Ektmchbnica
Acta, 1972. Vol. 17, pp. 1171 to 1174. Rripmon
Presew printed in
Northern Ireland
APPLICATION OF FLOW ELECTROLYSIS ON POROUS ELECTRODES FOR THE ELECTROREDUCTION OF POTASSIUM p-NITROBENZOATE* R. E. SIODA
and W. KEMULA
Institute of Physical Chemistry, Polish Academy of Sciences, Warszawa 42, ul. Kasprzaka 44, Poland Abstract-The elect&y& of potassium p-nitrobenzoate in alkaline aqueous solution in a flow electrolytic cell employing a platinum grid porous electrode is described. The composition of the solution after the electrolysis was controlled by means of voltammetry on a platinum wire microelectrode. According to the electrochemical evidence the reduction of potassium p-nitrobenu>ate requires 5 electrons per mole, and probably Ieads to the formation of the potassium salt of p, p’dicarboxyhydrazobenzene. R&sum&-Description de l’&ectrolyse du pnitrobenzoate de potassium en solution aquo-alcaline, dans une cellule dlectrolytique a circulation munie d’une 6Iectrode & grille de platine poreuse. Apr&s IUlectrolyse, la composition de la solution est contr618e par une voltam6trie sur micro-&&rode B fil de platine. Comme il l’a 6th reconnu &ctrochimiquement, la &duction du p-nitrobenzoate de potassium n&essite 5 Electrons par mole et conduit 1 la formation probable de se1 de potassium du p,p’-dicarboxyhydrozobenzene. Zusamme&assung--Die Elektrolyse von Kalium p-nitrobenzoat in alkalischer, wiisseriger Lbsung in einer elektrochemischen Striimungszelle wird beschrieben, Als Elektrode verwendete man ein por&es Platiunetz. Die Zu sammensetzung der Liisung nach der Elektrolyse wurde durch Voltarnetrie an einer PiatindrahtMikroelektrode kontrolliert. Aufgrund der Resultate der elektrochemischen Versuche wurde festgestellt, dass die Reduktion des Kalium p-nitroheuzoats-Molektils fiinf Elektronen erfordert und miiglicherweise zur Bildung von p,p’-dicarboxyhydrazobenzol fiihrt. THE NEW technique of electrolysis on porous electrodes with flowing solutions (epf)l+
has recently been developed. Applications of epf have been proposed in electroanalysis,8s4 and for the production of radical-ions of aromatic compounds.6*7 It seems that interesting future applications of epf lie in the field of the preparative electrochemistry. Epf offers advantages over traditional batch electrolysis. Being a continuous process it is in principle easier to control automatically, and it offers the possibility of a reduction of the volume of the electrolytic cell due to the high specific areas of porous electrodes. * In small scale laboratory preparations the advantage of epf is most easily seen in an electrolytic production of unstable species. This is due to the fact that in an epf cell the period between the formation of a product on the porous electrode and its removal from the cell for further use can be made very short. In addition, a high degree of conversion of a substrate can be obtained, if a porous electrode of a sufficiently high internal area is used. The conditions of obtaining the high degree of conversion can in principle be calculated according to the developed models of epf. 3*6ss A further advantage of epf is that for a constant flow rate, other things being equal, the electrolysis current does not change with time. Hence in principle an epf cell can be operated with a constant voltage without use of special potentiostats employed in the batch electrolysis to offset the diminishing of the current with time. To check the possibility of an application of epf for an organic preparative process, the electroreduction of a solution of potassium p-nitrohenzoate has heen conducted in a flow electrolytic cell containing a porous electrode built of twelve disks (diameter 1 I-7 mm) of 80-mesh platinum grid squeezed together (wire diameter l Manuscript received 20 July 1971. 1171
R. E. SIODA and W. KIZMULA
1172
The details of the cell have been described sepa.rately.g The cell uses gravitational flow, and the flow rate is regulated by attaching capillaries of different sizes to the outlet of the cell. The potential of the frontal (facing the platinum counter-electrode) surface of the porous electrode and the ohmic potential drop, in the direction of the flow, in the solution filling the porous electrode are measured by means of two see references. The counter- and reference electrodes are separated from the electrolysed solution by means of filter-paper plugs. The solution is deaerated before electrolysis by bubbling nitrogen. The glass tube forming the outlet of the cell is equipped with a small platinum micro-electrode obtained by fusing into the tube of a piece of a platinum wire of diameter O-2 mm. The wire sticks out to the centre of the tube, and near the wall it is insulated by a layer of glass. The microelectrode together with one of the reference electrodes serves for determining voltammetric curves of the solution leaving the cell. The cell has been operated at room temperature of 2SC, with solution flow rate O-0083 ml/s. The electrolysed solution contained 0.01 molar-p-nitrobenzoic acid dissolved in a basis solution of 0.02 M KOH and 3 M KC1 in water. The electrolysis was conducted at -0-94 V (us the frontal see). The potential was controlled manually. At this potential the electrolysis current of potassium p-nitrobenzoate was 36 mA, and the potential drop in the solution filling the working electrode was 0.020 V. The voltage applied to the cell was ca 6 V. The electrolysis current of the basis solution alone at -0.94 V was only 0.1 mA. The effect of the electrolysis was followed by registering voltammetric curves on the platinum wire micro-electrode of the solution flowing out from the porous electrode compartment. Figure 1 shows the measured voltammetric curves: the first is that of the basis solution alone; the second is that of the initial non-electrolysed solution of potassium p-nitrobenzoate in the basis solution, and the third that of the solution of potassium p-nitrobenzoate after the electrolysis at the potential of 0403
in.).
A
40-
9 20
-
0’ , 0.0
:!’
I
I
-0.5
-IQ
Vbce) Fro.
1.
Votammetric curves
-..
00
-05 v(sceJ
-I 0
o-o
-0.5
-I-o
wicd
(platinum wire micro-electrode) of the efaulent from the
flow electrolytic cell. Flow rate: O-0083 ml/s. A, Basis solution, O-02 M KOH and 3 M KC1 in water; B, 0.01 M p-nitrobenzoic acid ia 0.02 M KOH and 3 M KC1 in water; C, same as B. after electrolysis at -0.94 V.
Electroreduction of potassium p-nitrobenzoate on porous electrodes
1173
-0.94 V vs the frontal reference electrode, The potential of the electrolysis was chosen so as to avoid the evolution of hydrogen, which starts at about -1 V (see curve A, where the increase of the current at about -1 V corresponds to the evolution of hydrogen in the basis solution). As can be seen from a comparison of curves B and C the reduction wave of potassium p-nitrobenzoate at E,,, = -0*70 V decreased due to the electrolysis as 1: 10. From this decrease the degree of conversion of potassium p-nitrobenzoate can be calculated as O-90. In curve C a new wave of an oxidation of the product of the electrolysis is seen at El,, = -0.34 V. The height of this oxidation wave, compared with that of the reduction wave of potassium p-nitrobenzoate (curves B and C), is in the ratio 1:4*5. Knowing the degree of conversion one can calculate the number of electrons transferred per molecule of potassium p-nitrobenzoate according to the following general equations, T
FE-i_, nFcov
(1)
where Q) is the degree of conversion, I the electrolytic current, n the number of electrons transferred per molecule of a substrate, F the faraday, c, the initial concentration of the substrate and v the volume flow rate. The present data with (1) leads to n = 5.0. From the value of n one can guess the possible product of the reduction, in accordance with the general scheme of the electrolytic reductions of aromatic nitrocompounds.1o Such a product may be the potassium salt of p,p’-dicarboxyhydrazobenzene, which would be a result of an over-all lo-electron reduction of two molecules of potassium p-nitrobenzoate. The possibility of the formation of this product is further substantiated by the existence of the anodic oxidation wave (curve C), which may well correspond to the oxidation of the formed p,p’-dicarboxyhydrazobenzene salt to the potassium salt of p,p’-dicarboxyazobenzene by a two-electron process. The height of the oxidation wave is in accord with this supposition. This wave is about 2%electron, as calculated from the comparison of its height with the decrease in height of the wave of potassium p-nitrobenzoate due to the electrolysis. In these calculations it is assumed that the diffusion coefficients of the compounds are the same. Also, it was taken into account that the concentration of thep,p’-dicarboxyhydrazobenzene salt should be one half that of consumed p-nitrobenzoate salt. Attempts to isolate and to analyse the products of the electrolysis were unsuccessful. The solution after electrolysis, when exposed to air, seemed to undergo fast oxidational changes. This could be seen for example by means of voltammetry with the platinum micro-electrode by following the decrease in time of the anodic oxidation wave of the electrolysis product on exposure of the solution to air. On acidification with 1 M HCI, the solution after electrolysis gradually gave a yellow, colloidal precipitate. The precipitate, when Gltered, was gelatinous. When dried at room temperature, it was found to be insoluble in most of common organic solvents. Despite many washings with hot water and ethanol it still contained about 5 per cent of incombustible material, as found from its elementary analysis. The instability of the electrolysis products on exposure to air seems to be in accord with the known ease of oxidation of hydrazobenzenes.lO*ll
1174
R. E. SIODA and W. KEMUU
To summarize, the electrochemical evidence indicates that the main product of the electrolytic reduction of potassium p-nitrobenzoate in aqueous alkaline solution is the potassium salt of p,p’-dicarboxyhydrazobenzene, formed according to 2 NO,--C,H,CO,K
+ 6 H,O + 10 e- --t (-NH-C,H,-CO&),
+ 10 OH-.
(2)
The main purpose of the present work was to investigate the possibility of an application of the flow electrolysis on a porous electrode for organic preparative electrochemistry. In accordance with the experience gained so far, it seems that the technique can be applied for a broad range of preparative electrochemical reactions. Of special advantage would be the use of organic solvents instead of water, as this would increase significantly the number of compounds that could be electrolysed, and also would influence the mechanism and products of electrolysis. For a proper course of an electrolysis it is important that both the reactant and the products are easily soluble in the solvent, to obtain high productivity of the cell and to avoid clogging of the porous electrode. Also, by the use of special solvents and special electrolytes the potential range for an electrolysis can be significantly increased. For example, with aprotic solvents one can reach, on a platinum electrode, reduction potentials inaccessible in aqueous solutions. The same applies for the right choice of the material of the electrode. Organic solvent solutions have however the drawback of increased resistivity in comparison with aqueous solutions. Accordingly, it would be important to develop special porous electrodes of a high internal area and of a minimum thickness, to reduce the ohmic potential drop in the solution f8ling the porous electrode. The present work shows the possibility of application of epf for the determination of the number of electrons transferred in an electrochemical reaction, according to (1). This way of determining n may be easier than the traditional coulometric method, in which the electrolysis has to be conducted over a certain period of time. Further, the coulometric method is subject to error if the products of the electrolysis are unstable, and react in the cell during the time of the measurement to form new electro-active substances. This source of an error seems to be less important in epf, where the time of contact of the electrolysed solution with the electrode can be made very short. REFERENCES 1. 2. 3. 4, 5. 6. 7. 8. 9. 10. 11.
R. M. PERSICAYA and I. A. -ENMAN, Doki. Akad. Nauk SSSR 115,548 (1957). J. MOLNAR, Magy. k&n. Fuly. 68,504 (1962). I. G. GLJREV~CH and V. S. BAQOTZKY. Elecrrochim. Acta 9, 1151 (1964). W. J. BLABDELand J. H. STROHL, Analyr. Chem. 36,1245 (1964). R. E. S~ODA, Electrochint. Actu 15, 783 (1970). R. E. SIODA, J. phys. Chem. 72,2322 (1968). R. E. SIODA and W. KEMULA, 1. electroanal. Chem.. 31, 113 (1971) F. GOODRIDGE, Chem. Process Engn~. 49,93 (1968). R. E. SIODA, J. electroan& Chem., 31, 411 (1972). M. J. ALLEN, Organic Electrode Processes, p. 49. Chapman & Hall, London (1958). I. T. MILLAR and H. D. SPRINOALL, A Shorter Sidgwick’s Organic Chembtry of Nitrogen, p. 373. Clarendon, Oxford (1969).
Acta, 1972. Vol. 17, pp. 1171 to 1174. Rripmon
Presew printed in
Northern Ireland
APPLICATION OF FLOW ELECTROLYSIS ON POROUS ELECTRODES FOR THE ELECTROREDUCTION OF POTASSIUM p-NITROBENZOATE* R. E. SIODA
and W. KEMULA
Institute of Physical Chemistry, Polish Academy of Sciences, Warszawa 42, ul. Kasprzaka 44, Poland Abstract-The elect&y& of potassium p-nitrobenzoate in alkaline aqueous solution in a flow electrolytic cell employing a platinum grid porous electrode is described. The composition of the solution after the electrolysis was controlled by means of voltammetry on a platinum wire microelectrode. According to the electrochemical evidence the reduction of potassium p-nitrobenu>ate requires 5 electrons per mole, and probably Ieads to the formation of the potassium salt of p, p’dicarboxyhydrazobenzene. R&sum&-Description de l’&ectrolyse du pnitrobenzoate de potassium en solution aquo-alcaline, dans une cellule dlectrolytique a circulation munie d’une 6Iectrode & grille de platine poreuse. Apr&s IUlectrolyse, la composition de la solution est contr618e par une voltam6trie sur micro-&&rode B fil de platine. Comme il l’a 6th reconnu &ctrochimiquement, la &duction du p-nitrobenzoate de potassium n&essite 5 Electrons par mole et conduit 1 la formation probable de se1 de potassium du p,p’-dicarboxyhydrozobenzene. Zusamme&assung--Die Elektrolyse von Kalium p-nitrobenzoat in alkalischer, wiisseriger Lbsung in einer elektrochemischen Striimungszelle wird beschrieben, Als Elektrode verwendete man ein por&es Platiunetz. Die Zu sammensetzung der Liisung nach der Elektrolyse wurde durch Voltarnetrie an einer PiatindrahtMikroelektrode kontrolliert. Aufgrund der Resultate der elektrochemischen Versuche wurde festgestellt, dass die Reduktion des Kalium p-nitroheuzoats-Molektils fiinf Elektronen erfordert und miiglicherweise zur Bildung von p,p’-dicarboxyhydrazobenzol fiihrt. THE NEW technique of electrolysis on porous electrodes with flowing solutions (epf)l+
has recently been developed. Applications of epf have been proposed in electroanalysis,8s4 and for the production of radical-ions of aromatic compounds.6*7 It seems that interesting future applications of epf lie in the field of the preparative electrochemistry. Epf offers advantages over traditional batch electrolysis. Being a continuous process it is in principle easier to control automatically, and it offers the possibility of a reduction of the volume of the electrolytic cell due to the high specific areas of porous electrodes. * In small scale laboratory preparations the advantage of epf is most easily seen in an electrolytic production of unstable species. This is due to the fact that in an epf cell the period between the formation of a product on the porous electrode and its removal from the cell for further use can be made very short. In addition, a high degree of conversion of a substrate can be obtained, if a porous electrode of a sufficiently high internal area is used. The conditions of obtaining the high degree of conversion can in principle be calculated according to the developed models of epf. 3*6ss A further advantage of epf is that for a constant flow rate, other things being equal, the electrolysis current does not change with time. Hence in principle an epf cell can be operated with a constant voltage without use of special potentiostats employed in the batch electrolysis to offset the diminishing of the current with time. To check the possibility of an application of epf for an organic preparative process, the electroreduction of a solution of potassium p-nitrohenzoate has heen conducted in a flow electrolytic cell containing a porous electrode built of twelve disks (diameter 1 I-7 mm) of 80-mesh platinum grid squeezed together (wire diameter l Manuscript received 20 July 1971. 1171
R. E. SIODA and W. KIZMULA
1172
The details of the cell have been described sepa.rately.g The cell uses gravitational flow, and the flow rate is regulated by attaching capillaries of different sizes to the outlet of the cell. The potential of the frontal (facing the platinum counter-electrode) surface of the porous electrode and the ohmic potential drop, in the direction of the flow, in the solution filling the porous electrode are measured by means of two see references. The counter- and reference electrodes are separated from the electrolysed solution by means of filter-paper plugs. The solution is deaerated before electrolysis by bubbling nitrogen. The glass tube forming the outlet of the cell is equipped with a small platinum micro-electrode obtained by fusing into the tube of a piece of a platinum wire of diameter O-2 mm. The wire sticks out to the centre of the tube, and near the wall it is insulated by a layer of glass. The microelectrode together with one of the reference electrodes serves for determining voltammetric curves of the solution leaving the cell. The cell has been operated at room temperature of 2SC, with solution flow rate O-0083 ml/s. The electrolysed solution contained 0.01 molar-p-nitrobenzoic acid dissolved in a basis solution of 0.02 M KOH and 3 M KC1 in water. The electrolysis was conducted at -0-94 V (us the frontal see). The potential was controlled manually. At this potential the electrolysis current of potassium p-nitrobenzoate was 36 mA, and the potential drop in the solution filling the working electrode was 0.020 V. The voltage applied to the cell was ca 6 V. The electrolysis current of the basis solution alone at -0.94 V was only 0.1 mA. The effect of the electrolysis was followed by registering voltammetric curves on the platinum wire micro-electrode of the solution flowing out from the porous electrode compartment. Figure 1 shows the measured voltammetric curves: the first is that of the basis solution alone; the second is that of the initial non-electrolysed solution of potassium p-nitrobenzoate in the basis solution, and the third that of the solution of potassium p-nitrobenzoate after the electrolysis at the potential of 0403
in.).
A
40-
9 20
-
0’ , 0.0
:!’
I
I
-0.5
-IQ
Vbce) Fro.
1.
Votammetric curves
-..
00
-05 v(sceJ
-I 0
o-o
-0.5
-I-o
wicd
(platinum wire micro-electrode) of the efaulent from the
flow electrolytic cell. Flow rate: O-0083 ml/s. A, Basis solution, O-02 M KOH and 3 M KC1 in water; B, 0.01 M p-nitrobenzoic acid ia 0.02 M KOH and 3 M KC1 in water; C, same as B. after electrolysis at -0.94 V.
Electroreduction of potassium p-nitrobenzoate on porous electrodes
1173
-0.94 V vs the frontal reference electrode, The potential of the electrolysis was chosen so as to avoid the evolution of hydrogen, which starts at about -1 V (see curve A, where the increase of the current at about -1 V corresponds to the evolution of hydrogen in the basis solution). As can be seen from a comparison of curves B and C the reduction wave of potassium p-nitrobenzoate at E,,, = -0*70 V decreased due to the electrolysis as 1: 10. From this decrease the degree of conversion of potassium p-nitrobenzoate can be calculated as O-90. In curve C a new wave of an oxidation of the product of the electrolysis is seen at El,, = -0.34 V. The height of this oxidation wave, compared with that of the reduction wave of potassium p-nitrobenzoate (curves B and C), is in the ratio 1:4*5. Knowing the degree of conversion one can calculate the number of electrons transferred per molecule of potassium p-nitrobenzoate according to the following general equations, T
FE-i_, nFcov
(1)
where Q) is the degree of conversion, I the electrolytic current, n the number of electrons transferred per molecule of a substrate, F the faraday, c, the initial concentration of the substrate and v the volume flow rate. The present data with (1) leads to n = 5.0. From the value of n one can guess the possible product of the reduction, in accordance with the general scheme of the electrolytic reductions of aromatic nitrocompounds.1o Such a product may be the potassium salt of p,p’-dicarboxyhydrazobenzene, which would be a result of an over-all lo-electron reduction of two molecules of potassium p-nitrobenzoate. The possibility of the formation of this product is further substantiated by the existence of the anodic oxidation wave (curve C), which may well correspond to the oxidation of the formed p,p’-dicarboxyhydrazobenzene salt to the potassium salt of p,p’-dicarboxyazobenzene by a two-electron process. The height of the oxidation wave is in accord with this supposition. This wave is about 2%electron, as calculated from the comparison of its height with the decrease in height of the wave of potassium p-nitrobenzoate due to the electrolysis. In these calculations it is assumed that the diffusion coefficients of the compounds are the same. Also, it was taken into account that the concentration of thep,p’-dicarboxyhydrazobenzene salt should be one half that of consumed p-nitrobenzoate salt. Attempts to isolate and to analyse the products of the electrolysis were unsuccessful. The solution after electrolysis, when exposed to air, seemed to undergo fast oxidational changes. This could be seen for example by means of voltammetry with the platinum micro-electrode by following the decrease in time of the anodic oxidation wave of the electrolysis product on exposure of the solution to air. On acidification with 1 M HCI, the solution after electrolysis gradually gave a yellow, colloidal precipitate. The precipitate, when Gltered, was gelatinous. When dried at room temperature, it was found to be insoluble in most of common organic solvents. Despite many washings with hot water and ethanol it still contained about 5 per cent of incombustible material, as found from its elementary analysis. The instability of the electrolysis products on exposure to air seems to be in accord with the known ease of oxidation of hydrazobenzenes.lO*ll
1174
R. E. SIODA and W. KEMUU
To summarize, the electrochemical evidence indicates that the main product of the electrolytic reduction of potassium p-nitrobenzoate in aqueous alkaline solution is the potassium salt of p,p’-dicarboxyhydrazobenzene, formed according to 2 NO,--C,H,CO,K
+ 6 H,O + 10 e- --t (-NH-C,H,-CO&),
+ 10 OH-.
(2)
The main purpose of the present work was to investigate the possibility of an application of the flow electrolysis on a porous electrode for organic preparative electrochemistry. In accordance with the experience gained so far, it seems that the technique can be applied for a broad range of preparative electrochemical reactions. Of special advantage would be the use of organic solvents instead of water, as this would increase significantly the number of compounds that could be electrolysed, and also would influence the mechanism and products of electrolysis. For a proper course of an electrolysis it is important that both the reactant and the products are easily soluble in the solvent, to obtain high productivity of the cell and to avoid clogging of the porous electrode. Also, by the use of special solvents and special electrolytes the potential range for an electrolysis can be significantly increased. For example, with aprotic solvents one can reach, on a platinum electrode, reduction potentials inaccessible in aqueous solutions. The same applies for the right choice of the material of the electrode. Organic solvent solutions have however the drawback of increased resistivity in comparison with aqueous solutions. Accordingly, it would be important to develop special porous electrodes of a high internal area and of a minimum thickness, to reduce the ohmic potential drop in the solution f8ling the porous electrode. The present work shows the possibility of application of epf for the determination of the number of electrons transferred in an electrochemical reaction, according to (1). This way of determining n may be easier than the traditional coulometric method, in which the electrolysis has to be conducted over a certain period of time. Further, the coulometric method is subject to error if the products of the electrolysis are unstable, and react in the cell during the time of the measurement to form new electro-active substances. This source of an error seems to be less important in epf, where the time of contact of the electrolysed solution with the electrode can be made very short. REFERENCES 1. 2. 3. 4, 5. 6. 7. 8. 9. 10. 11.
R. M. PERSICAYA and I. A. -ENMAN, Doki. Akad. Nauk SSSR 115,548 (1957). J. MOLNAR, Magy. k&n. Fuly. 68,504 (1962). I. G. GLJREV~CH and V. S. BAQOTZKY. Elecrrochim. Acta 9, 1151 (1964). W. J. BLABDELand J. H. STROHL, Analyr. Chem. 36,1245 (1964). R. E. S~ODA, Electrochint. Actu 15, 783 (1970). R. E. SIODA, J. phys. Chem. 72,2322 (1968). R. E. SIODA and W. KEMULA, 1. electroanal. Chem.. 31, 113 (1971) F. GOODRIDGE, Chem. Process Engn~. 49,93 (1968). R. E. SIODA, J. electroan& Chem., 31, 411 (1972). M. J. ALLEN, Organic Electrode Processes, p. 49. Chapman & Hall, London (1958). I. T. MILLAR and H. D. SPRINOALL, A Shorter Sidgwick’s Organic Chembtry of Nitrogen, p. 373. Clarendon, Oxford (1969).