- Email: [email protected]
A new type of electrochemical oscillator in acetic acid
335
Chem., 316 (1991) 335-340 Elsevier Sequoia S.A., Lausanne
J. Electroanal.
JEC 01667SC Short communication
A new type of electrochemical
oscillator in acetic acid
Tibor J. Pastor and Milka M. Pastor Department of Chemisty, Faculty of Sciences, YU-I 1001 Belgrade (Yugoslavia)
Belgrade
University,
P.O.
Box 550, Studentski
trg 16,
(Received 4 January 1991; in revised form 21 May 1991) INTRODUCTION
Theoretical and experimental studies of oscillating reactions are now being carried out in many laboratories and the interest in this type of phenomena is increasing very rapidly [l]. The earliest reported examples of chemical oscillations were found in electrochemical systems [2]. Today, there are countless examples of current oscillations under potentiostatic conditions and potential oscillations under galvanostatic and zero current conditions in aqueous media [2,3]. The majority of the observed electrochemical oscillations originate in anodic processes. A few cases of electrochemical oscillations in non-aqueous media have also been reported [4-71. This paper describes a new type of electrochemical oscillator in acetic acid. EXPERIMENTAL
Analytical reagent grade chemicals were used. Anhydrous acetic acid was obtained by distillation of glacial acetic acid, the fraction boiling at 1WC being collected. Cobalt(I1) acetate (Co(CH,COO), - 4 H,O) was dehydrated by treatment with boiling acetic anhydride; after cooling the reaction mixture was filtered through a sintered glass crucible No. 4, and the precipitate was washed several times with acetic acid. The anhydrous salt was kept in a desiccator over phosphorous(V) oxide. Before use, sodium perchlorate was dried at 12O’C. The supporting electrolyte used was 0.2 M NaClO,. The solution of the oscillating system to be investigated was prepared by saturating the supporting electrolyte with cobalt(I1) acetate (0.0194 M). The apparatus used for cobalt(II1) generation and the study of the oscillating behaviour of the system has been described elsewhere [S]. The anode compartment (3.2 cm in diameter) and cathode compartment (1.0 cm in diameter) of the electrolytic cell were separated by a G-4 sintered glass disc (0.7 cm in diameter). The 0022-0728/91/$03.50 0 1991 - EIsevier !kquoia S.A. AII rights reserved
336
height of the vessel was 6.0 cm. The working electrodes of the generating circuit were platinum spirals with a surface area of 2.0 cm*. The voltage between the two spiral platinum electrodes ranged from 30 to 70 V when the current in the circuit was 1.00 mA. Bright platinum electrodes of an area of 0.2 cm* were used for biarnperometric investigations (biamperometry = amperometry with two indicator electrodes in the same solution [9]). They were polarized by a potential difference ranging from 250 to 2000 mV. In the automatic recording of the oscillatory behaviour of the system, the indicator electrodes (electrodes of the biamperometric circuit) were connected directly to a Radiometer P03h polarograph. The behaviour of the system at a desired and constant temperature, within the range 15 to 45’ C, was followed in a vessel with a double wall (3.2 cm in diameter and a height of 12 cm) which was connected to a Medingen NB thermostat. The same vessel was used in experiments in which nitrogen was bubbled through the solution. Oxygen was partly removed from the solution by passing a stream of nitrogen prior to measurement; during the course of the measurements a nitrogen atmosphere was maintained in the vessel. Procedure
The supporting electrolyte was poured into the cathode compartment of the vessel to cover the sintered glass disc. An appropriate volume (about 20 cm3) of the saturated solution of cobalt(I1) acetate was placed in the anode compartment. After immersion of the electrodes, the levels of the solutions were equalized by the addition of more supporting electrolyte to the cathode compartment. Cobalt(II1) was generated by means of a constant current of 1.0 mA, with vigorous stirring of the anolyte. Water was added before or immediately after the generation of cobalt(II1) and the behaviour of the system under the desired conditions was recorded. In investigations of the effect of the air on the behaviour of the system, as in the case of constant temperatures, after the generation of cobalt(III), the anolyte was transferred to a vessel with a double wall and the indicator electrodes were immersed in the solution which was being thermostated. RESULTS AND DISCUSSION
It has been found that a biamperometric system whose electrodes are immersed into a solution of sodium perchlorate, cobalt(I1) and cobalt(II1) in (100 - x)/x acetic acid/water (w/w) solvent, under ideal conditions exhibits oscillatory behaviour for long time. The oscillating system was followed either continuously or with short or prolonged interruptions during 24 h, although the system still exhibited its oscillatory behaviour after this period. On repeated switching on of the system after a longer break, the frequency of the oscillations was smaller (Fig. 1). The general shape of the oscillations was always the same. In the first, upward part
337
I1
10 UNITS
10
20
30
40
50
60
TIME /MIN Fig. 1. Current oscillations in the biamperometric system whose electrodes are immersed into a 98.0/2.0 acetic acid/water (w/w) solvent with [NaClOJ = 0.2 M, [Co(II)] = 0.01863 M, [Co(III)] = 0.00077 M, T= 25°C: (1) immediately after the generation of cobaIt(II1) and (2) at repeated switching on of the system after a break of 13.5 h. The electrodes were polarized by a potential difference of 1000 mV.
of the wave the current increased rapidly, whereas in the second, downward part it decreased slowly. A general characteristic of the investigated system is that the oscillation frequency was increased on stirring the solution. The cycle frequency and the oscillation amplitude depend on the working conditions of the system. Changing the conditions often has a dramatic effect, particularly on the frequency of the oscillation. The behaviour of the investigated system depends on the amount of water present in the solution as well as on the moment when it was added. In our experiments the water content in the solution was varied from 0.0 to 10.0%. The oscillation amplitudes were highest in the presence of 1.0 to 2.0% of water, and either on decreasing or increasing this range of water content in the solution the oscillation amplitudes were decreased (Fig. 2). In the absence of water or on increasing its content in the solution above 6-78, the oscillations practically disappeared. The cycle frequency was found to increase with increasing water content up to 5%, which, however, depended also on the other experimental conditions applied. On the addition of. a desired amount of water after the generation of cobalt(III), the oscillation frequency was found to be higher than when water had been added before the anodic oxidation of cobalt(I1). The potential difference used for the polarization of the electrodes was varied from 250 to 2000 mV. At small potential differences the current in the biamperometric system was so small that its oscillations could not be followed. At sufficiently high potential differences (> 1000 mV) and also depending on other applied working conditions, as described later, the oscillation frequency was found to increase (Table 1) whereas the oscillation amplitude decreased with increasing potential difference.
338
I
10 UNFS
I
I 1
2
3
4
5
6
TIME/MIN Fig. 2. Current oscillations in the biamperometric system whose electrodes are immersed into acetic acid with [NaClOJ = 0.2 M, [Co(II)] = 0.01863 M, [co(III)] = 0.00077 M, T = 25”C, in the presence of: (1) 0.51, (2) l.O%, and (3) 3.0% water. Electrodes were polarized by a potential difference of 1300 mV.
The cobalt(III)/cobalt(II) ratio in the solution was varied within a relatively wide range. After switching on the biamperometric system in the absence of cobalt(III), only at relatively high potential differences between the electrodes and after prolonged working of the system oscillations with longer periods and smaller amplitudes were. The potential difference at the electrodes necessary to produce well developed oscillations was found to decrease with an increasing ratio of the oxidized and reduced forms of cobalt in the solution. The oscillation frequency increased, whereas the oscillation amplitude decreased with an increasing ratio of
TABLE 1 Frequency of current oscillations in the biamperometric system whose electrodes are immersed into a 99.0/1.0 acetic acid/water (w/w) solvent with [NaClO,] = 0.2 M, [Co(II)] = 0.01878 M, [Co(III)] = 0.00062 M, at various temperatures (electrodes were polarized at various voltages) Potential difference/mV
850
Temperature/C
Number of oscillations in 120 min
1000
15 20 25
5.7 7.7 9.6
13.9 15.8 19.1
1150
16.3 22.8 28.7
1300
29.1 40.4 -
339
I
10 UNITS 1
2
3
4
5
TIME / MIN Fig. 3. Current oscillations in the biamperometric system whose electrodes are immersed into 99.0/1.0 acetic acid/water (w/w) solvent with [NaClO,] = 0.2 M, T= 2S°C, and with: (1) [Co(H)] = 0.01863 M, [Co@)] = 0.00077 M; (2) [Co(II)] = 0.01816 M, [Co(III)] = 0.00124 M, and (3) [Co(II)] = 0.01754 M, [Co(III)] = 0.00186 M. Electrodes were polarized by a potential difference of 1200 mV.
cobalt(II1) and cobalt(I1) in the solution, provided other conditions were ideal and constant (Fig. 3). In the presence of perchloric acid, which reduces the stability of cobalt(III), the frequency of the oscillation decreased. The temperature of the solution also has a profound effect on the oscillating properties of the biamperometric system in acetic acid. Thus, for example, at room temperature (20-25”C), in the presence of 1% water and at a potential difference of 1000 mV for electrode polarization, the oscillations observed were very enhanced, whereas the same system at 10°C failed to oscillate even at a potential difference of 1350 mV between the electrodes. The curves obtained at various temperatures (15, 20, 25 and 30 o C, respectively), in solutions of the same composition, showed that with increasing temperature, the oscillation frequency was increased. These findings were also confirmed by data obtained at different potentials and at different temperatures, all other working conditions being the same (Table 1). With increasing temperature of the solution the oscillation amplitudes decreased. At 40-45’C the oscillating behaviour of the investigated system disappeared. The comparison of current oscillations recorded in solutions of the same composition in the anode compartment of the electrolytic cell and in a vessel having the same diameter but of twice the height showed that the oscillation frequency in the
340
vessel of smaller height, was higher. It was also observed that the frequency of the current oscillation was decreased in the case where a stream of nitrogen was passed through the solution, all other working conditions being the same. These phenomena and the characteristic effect of stirring the solution on the oscillatory behaviour of this biamperometric system indicates that oxygen plays an important role in this oscillator. In the presence of nitrogen the oscillation frequency was also decreased with increasing temperature. The current of the biamperometric circuit ranged between 1.5-8 PA when the indicator electrodes were polarized by a potential difference ranging from 1000 to 1750 mV. By means of a spot galvanometer, or a recording pen we have measured only a part of this current (0.6-0.9 CIA). A shunt was used for adjusting the sensitivity of the galvanometer, or the recording pen. Recent measurements have shown that the oscillations arise at both electrodes of the biamperometric circuit, and occur synchronously i.e. the oscillation frequencies and the direction of oscillations are the same. These data suggest that both the reactions at the electrodes and those in the solution are responsible for the oscillatory behaviour of this biamperometric system. ACKNOWLEDGEMENT
The authors are grateful to the Serbian Research Fund for financial support. REFERENCES A.M. Zhabotinskii, H.G. Othmer, R.J. Field, J.J. Tyson, W.C. Troy, S.D. Furrow, et al., in R.J. Field and M. Burger (Eds.), Kolebaniya i Begushchie Volny v Khimicheskikh Sistemakh Mir, Moskow, 1988. G. Nicolis and J. Portnow, Chem. Rev., 73 (1973) 365. J. Wojtowitz, Modem Aspects of Electrochemistry in J.O’M. Bockris and B.E. Conway (Eds.), Vol. 8 (2), Plenum Press, New York, 1972, p. 47-120. M.L. Bhaskara Rao, J. Electrochem. Sot., 114 (1967) 665. H. Degn, Trans. Faraday Sot., 64 (1968) 1349. D. Posadas, A.J. Arvia, J.J. Podesta, EIectrochim. Acta, 16 (1971) 1041. B.E. Conway and D.M. Novak, J. Phys. Chem., 81 (1977) 1459. T.J. Pastor, V.J. Vajgand and Z. Ki&vi& Mikrochim. Acta (Wien), (1976 II) 525. Report from the Commission on Ehxtroanalytical Chemistry, Pore Appl. Chem., 45 (1976) 81.
Chem., 316 (1991) 335-340 Elsevier Sequoia S.A., Lausanne
J. Electroanal.
JEC 01667SC Short communication
A new type of electrochemical
oscillator in acetic acid
Tibor J. Pastor and Milka M. Pastor Department of Chemisty, Faculty of Sciences, YU-I 1001 Belgrade (Yugoslavia)
Belgrade
University,
P.O.
Box 550, Studentski
trg 16,
(Received 4 January 1991; in revised form 21 May 1991) INTRODUCTION
Theoretical and experimental studies of oscillating reactions are now being carried out in many laboratories and the interest in this type of phenomena is increasing very rapidly [l]. The earliest reported examples of chemical oscillations were found in electrochemical systems [2]. Today, there are countless examples of current oscillations under potentiostatic conditions and potential oscillations under galvanostatic and zero current conditions in aqueous media [2,3]. The majority of the observed electrochemical oscillations originate in anodic processes. A few cases of electrochemical oscillations in non-aqueous media have also been reported [4-71. This paper describes a new type of electrochemical oscillator in acetic acid. EXPERIMENTAL
Analytical reagent grade chemicals were used. Anhydrous acetic acid was obtained by distillation of glacial acetic acid, the fraction boiling at 1WC being collected. Cobalt(I1) acetate (Co(CH,COO), - 4 H,O) was dehydrated by treatment with boiling acetic anhydride; after cooling the reaction mixture was filtered through a sintered glass crucible No. 4, and the precipitate was washed several times with acetic acid. The anhydrous salt was kept in a desiccator over phosphorous(V) oxide. Before use, sodium perchlorate was dried at 12O’C. The supporting electrolyte used was 0.2 M NaClO,. The solution of the oscillating system to be investigated was prepared by saturating the supporting electrolyte with cobalt(I1) acetate (0.0194 M). The apparatus used for cobalt(II1) generation and the study of the oscillating behaviour of the system has been described elsewhere [S]. The anode compartment (3.2 cm in diameter) and cathode compartment (1.0 cm in diameter) of the electrolytic cell were separated by a G-4 sintered glass disc (0.7 cm in diameter). The 0022-0728/91/$03.50 0 1991 - EIsevier !kquoia S.A. AII rights reserved
336
height of the vessel was 6.0 cm. The working electrodes of the generating circuit were platinum spirals with a surface area of 2.0 cm*. The voltage between the two spiral platinum electrodes ranged from 30 to 70 V when the current in the circuit was 1.00 mA. Bright platinum electrodes of an area of 0.2 cm* were used for biarnperometric investigations (biamperometry = amperometry with two indicator electrodes in the same solution [9]). They were polarized by a potential difference ranging from 250 to 2000 mV. In the automatic recording of the oscillatory behaviour of the system, the indicator electrodes (electrodes of the biamperometric circuit) were connected directly to a Radiometer P03h polarograph. The behaviour of the system at a desired and constant temperature, within the range 15 to 45’ C, was followed in a vessel with a double wall (3.2 cm in diameter and a height of 12 cm) which was connected to a Medingen NB thermostat. The same vessel was used in experiments in which nitrogen was bubbled through the solution. Oxygen was partly removed from the solution by passing a stream of nitrogen prior to measurement; during the course of the measurements a nitrogen atmosphere was maintained in the vessel. Procedure
The supporting electrolyte was poured into the cathode compartment of the vessel to cover the sintered glass disc. An appropriate volume (about 20 cm3) of the saturated solution of cobalt(I1) acetate was placed in the anode compartment. After immersion of the electrodes, the levels of the solutions were equalized by the addition of more supporting electrolyte to the cathode compartment. Cobalt(II1) was generated by means of a constant current of 1.0 mA, with vigorous stirring of the anolyte. Water was added before or immediately after the generation of cobalt(II1) and the behaviour of the system under the desired conditions was recorded. In investigations of the effect of the air on the behaviour of the system, as in the case of constant temperatures, after the generation of cobalt(III), the anolyte was transferred to a vessel with a double wall and the indicator electrodes were immersed in the solution which was being thermostated. RESULTS AND DISCUSSION
It has been found that a biamperometric system whose electrodes are immersed into a solution of sodium perchlorate, cobalt(I1) and cobalt(II1) in (100 - x)/x acetic acid/water (w/w) solvent, under ideal conditions exhibits oscillatory behaviour for long time. The oscillating system was followed either continuously or with short or prolonged interruptions during 24 h, although the system still exhibited its oscillatory behaviour after this period. On repeated switching on of the system after a longer break, the frequency of the oscillations was smaller (Fig. 1). The general shape of the oscillations was always the same. In the first, upward part
337
I1
10 UNITS
10
20
30
40
50
60
TIME /MIN Fig. 1. Current oscillations in the biamperometric system whose electrodes are immersed into a 98.0/2.0 acetic acid/water (w/w) solvent with [NaClOJ = 0.2 M, [Co(II)] = 0.01863 M, [Co(III)] = 0.00077 M, T= 25°C: (1) immediately after the generation of cobaIt(II1) and (2) at repeated switching on of the system after a break of 13.5 h. The electrodes were polarized by a potential difference of 1000 mV.
of the wave the current increased rapidly, whereas in the second, downward part it decreased slowly. A general characteristic of the investigated system is that the oscillation frequency was increased on stirring the solution. The cycle frequency and the oscillation amplitude depend on the working conditions of the system. Changing the conditions often has a dramatic effect, particularly on the frequency of the oscillation. The behaviour of the investigated system depends on the amount of water present in the solution as well as on the moment when it was added. In our experiments the water content in the solution was varied from 0.0 to 10.0%. The oscillation amplitudes were highest in the presence of 1.0 to 2.0% of water, and either on decreasing or increasing this range of water content in the solution the oscillation amplitudes were decreased (Fig. 2). In the absence of water or on increasing its content in the solution above 6-78, the oscillations practically disappeared. The cycle frequency was found to increase with increasing water content up to 5%, which, however, depended also on the other experimental conditions applied. On the addition of. a desired amount of water after the generation of cobalt(III), the oscillation frequency was found to be higher than when water had been added before the anodic oxidation of cobalt(I1). The potential difference used for the polarization of the electrodes was varied from 250 to 2000 mV. At small potential differences the current in the biamperometric system was so small that its oscillations could not be followed. At sufficiently high potential differences (> 1000 mV) and also depending on other applied working conditions, as described later, the oscillation frequency was found to increase (Table 1) whereas the oscillation amplitude decreased with increasing potential difference.
338
I
10 UNFS
I
I 1
2
3
4
5
6
TIME/MIN Fig. 2. Current oscillations in the biamperometric system whose electrodes are immersed into acetic acid with [NaClOJ = 0.2 M, [Co(II)] = 0.01863 M, [co(III)] = 0.00077 M, T = 25”C, in the presence of: (1) 0.51, (2) l.O%, and (3) 3.0% water. Electrodes were polarized by a potential difference of 1300 mV.
The cobalt(III)/cobalt(II) ratio in the solution was varied within a relatively wide range. After switching on the biamperometric system in the absence of cobalt(III), only at relatively high potential differences between the electrodes and after prolonged working of the system oscillations with longer periods and smaller amplitudes were. The potential difference at the electrodes necessary to produce well developed oscillations was found to decrease with an increasing ratio of the oxidized and reduced forms of cobalt in the solution. The oscillation frequency increased, whereas the oscillation amplitude decreased with an increasing ratio of
TABLE 1 Frequency of current oscillations in the biamperometric system whose electrodes are immersed into a 99.0/1.0 acetic acid/water (w/w) solvent with [NaClO,] = 0.2 M, [Co(II)] = 0.01878 M, [Co(III)] = 0.00062 M, at various temperatures (electrodes were polarized at various voltages) Potential difference/mV
850
Temperature/C
Number of oscillations in 120 min
1000
15 20 25
5.7 7.7 9.6
13.9 15.8 19.1
1150
16.3 22.8 28.7
1300
29.1 40.4 -
339
I
10 UNITS 1
2
3
4
5
TIME / MIN Fig. 3. Current oscillations in the biamperometric system whose electrodes are immersed into 99.0/1.0 acetic acid/water (w/w) solvent with [NaClO,] = 0.2 M, T= 2S°C, and with: (1) [Co(H)] = 0.01863 M, [Co@)] = 0.00077 M; (2) [Co(II)] = 0.01816 M, [Co(III)] = 0.00124 M, and (3) [Co(II)] = 0.01754 M, [Co(III)] = 0.00186 M. Electrodes were polarized by a potential difference of 1200 mV.
cobalt(II1) and cobalt(I1) in the solution, provided other conditions were ideal and constant (Fig. 3). In the presence of perchloric acid, which reduces the stability of cobalt(III), the frequency of the oscillation decreased. The temperature of the solution also has a profound effect on the oscillating properties of the biamperometric system in acetic acid. Thus, for example, at room temperature (20-25”C), in the presence of 1% water and at a potential difference of 1000 mV for electrode polarization, the oscillations observed were very enhanced, whereas the same system at 10°C failed to oscillate even at a potential difference of 1350 mV between the electrodes. The curves obtained at various temperatures (15, 20, 25 and 30 o C, respectively), in solutions of the same composition, showed that with increasing temperature, the oscillation frequency was increased. These findings were also confirmed by data obtained at different potentials and at different temperatures, all other working conditions being the same (Table 1). With increasing temperature of the solution the oscillation amplitudes decreased. At 40-45’C the oscillating behaviour of the investigated system disappeared. The comparison of current oscillations recorded in solutions of the same composition in the anode compartment of the electrolytic cell and in a vessel having the same diameter but of twice the height showed that the oscillation frequency in the
340
vessel of smaller height, was higher. It was also observed that the frequency of the current oscillation was decreased in the case where a stream of nitrogen was passed through the solution, all other working conditions being the same. These phenomena and the characteristic effect of stirring the solution on the oscillatory behaviour of this biamperometric system indicates that oxygen plays an important role in this oscillator. In the presence of nitrogen the oscillation frequency was also decreased with increasing temperature. The current of the biamperometric circuit ranged between 1.5-8 PA when the indicator electrodes were polarized by a potential difference ranging from 1000 to 1750 mV. By means of a spot galvanometer, or a recording pen we have measured only a part of this current (0.6-0.9 CIA). A shunt was used for adjusting the sensitivity of the galvanometer, or the recording pen. Recent measurements have shown that the oscillations arise at both electrodes of the biamperometric circuit, and occur synchronously i.e. the oscillation frequencies and the direction of oscillations are the same. These data suggest that both the reactions at the electrodes and those in the solution are responsible for the oscillatory behaviour of this biamperometric system. ACKNOWLEDGEMENT
The authors are grateful to the Serbian Research Fund for financial support. REFERENCES A.M. Zhabotinskii, H.G. Othmer, R.J. Field, J.J. Tyson, W.C. Troy, S.D. Furrow, et al., in R.J. Field and M. Burger (Eds.), Kolebaniya i Begushchie Volny v Khimicheskikh Sistemakh Mir, Moskow, 1988. G. Nicolis and J. Portnow, Chem. Rev., 73 (1973) 365. J. Wojtowitz, Modem Aspects of Electrochemistry in J.O’M. Bockris and B.E. Conway (Eds.), Vol. 8 (2), Plenum Press, New York, 1972, p. 47-120. M.L. Bhaskara Rao, J. Electrochem. Sot., 114 (1967) 665. H. Degn, Trans. Faraday Sot., 64 (1968) 1349. D. Posadas, A.J. Arvia, J.J. Podesta, EIectrochim. Acta, 16 (1971) 1041. B.E. Conway and D.M. Novak, J. Phys. Chem., 81 (1977) 1459. T.J. Pastor, V.J. Vajgand and Z. Ki&vi& Mikrochim. Acta (Wien), (1976 II) 525. Report from the Commission on Ehxtroanalytical Chemistry, Pore Appl. Chem., 45 (1976) 81.