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Influence of the depolarizer on polarographic maxima of the second kind
Electroanalytical Chemistry and lnterfacial Electrochemistry, 42 (1973) 429-432 (© ElsevierSequoia S.A., Lausanne- Printed in The Netherlands
429
I N F L U E N C E OF THE D E P O L A R I Z E R ON P O L A R O G R A P H I C M A X I M A OF THE SECOND K I N D SUDARSHAN LAL, T. W. HOLT* and H. H. BAUER Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506 (U.S.A.) (Received 8th August 1972)
INTRODUCTION Maxima of the second kind are attributed to the hydrodynamic flow of mercury inside the drop causing upward streaming of the mercury at the sides of the drop electrode and hence upward streaming of the solution around the dropping electrode x. It is also said that maxima of the second kind, being of hydrodynamic origin, are not affected by electrochemical processes occurring at the electrode z. The streaming velocity, v, of the motion of the drop surface at its equator relative to its centre has been expressed by the following equation, which is due to Frumkin and Levich3: V
~---
1 ( d ' - d) gr 2 3 2r1+ 3q' + a2/~:
--
where d is the density of solution, d' the density of mercury, 9 the gravitational constant, r/viscosity of solution, q' viscosity of mercury, a surface density of the electric charge, K specific conductivity of solution and r the radius of the mercury drop. The above equation implies that the velocity is dependent on specific conductance, and it has indeed been found that maxima of the second kind are enhanced in concentrated supporting electrolytes. Kryukova 1 found a linear dependence between the mercury flow rate (m 7/6) and the speed of the solution. On the other hand, the height of a second kind maximum is believed to be directly proportional to the depolarizer concentration 4, which implicitly suggests that the velocity of streaming does not change with increase in depolarizer concentration. At times, especially at high mercury flow-rates, simultaneous occurrence of maxima of the first and second kind, and their inversion, has been also observed ~ 7. The maximum is of the second kind at low depolarizer concentrations, and becomes of the first kind at high depolarizer concentrations, with (at potentials positive to the electrocapillary maximum) a change in direction of streaming from upward ("second kind") to downward ("first kind"). However, no influence of depolarizer on streaming has been reported for maxima that are essentially of the second kind. * Tobacco Research Institute, Universityof Kentucky,Lexington,Kentucky40506, U,S.A;
430
S. LAL, T. W. HOLT, H. H. BAUER
In the present communication, it is shown, from measurements of the speed of solution streaming that the upward streaming in second kind maxima is accentuated in the presence of depolarizer (here, copper or cadmium ions). In these cases, the presence of depolarizer affects the rate of streaming of the solution but not in the sense of a conversion of the maximum to one of the first kind: the upward streaming characteristic of second kind maxima is enhanced. EXPERIMENTAL Reagent grade chemicals were employed and solutions prepared in doubly distilled water. The systems examined were 8 m M Cu(II) in 0.25 M Na2SO 4 (pH 2) and 1 m M Cd(II) in saturated KC1. Polarograms were recorded on a PAR electrochemistry system model 170 in the three-electrode mode. The cell temperature was maintained at 25°C. Streaming of the solution was observed visually by means of a stereo microscope (Anchor Optical Co.) at 4 × magnification. Speed measurements were carried out as described elsewhere 9. RESULTS A polarogram of 8 m M Cu(II) in purified 0.25 M Na2SO 4 solution, at a mercury flow rate of 5.36 mg s -1, is shown in Fig. 1. The current-voltage plot at such a high flow-rate exhibits a compounded maximum and streaming of the solution around the mercury drop was observed over the entire potential range; however, the rate and direction of streaming varied with potential. On the foot of the wave (region A), an upward m o t i o n was observed, presumably second-kind streaming at this high flow-rate. The velocity of the upward movement increased as the current began to increase at more cathodic potentials. This upward swirling persisted until about - 2 5 mV applied potential and then I 1,6 1,4
<~[ 1,2 ~,ol 0.8 ('~ 0.6
,~
0.4 0.2
o i i'o,
_,1 E/V (vs. S C E )
Fig. l. Polarogram of 8 x 1()-~ M Cu(ll), 0.25 M NaaSO4, pH 2.0, 298°K, m=5.36 mg of the arrow is approx, proportional to soln. speed.
s
1
Length
431
D E P O L A R I Z E R E F F E C T O N M A X I M A O F T H E 2nd K I N D
reversal occurred, resulting in the inversion of the maximum from second kind to first kind. At this point, the solution moved upward, then became quiescent for an instant and then, with further growth of the drop, motion in the opposite direction began; the forces causing the two kinds of maxima were clearly present simultaneously at the reversal potential. After the onset of the positive maximum of the first kind (region B), the downward streaming increased to a maximal intensity at the peak of the polarogram (the intensity of streaming in this case was significantly greater than with two-electrode circuitry, where the shape of the maximum is also different8; this is presumably due to the increased current density brought about by the ohmic drop compensation in the three-electrode mode). The drop appeared to spin downward causing the electrolyte to move in the same direction in this potential region. On the cathodic side of the rounded maximum (region C), a mixture of first-kind and second-kind streaming was again observed on a single drop, the first-kind decreasing and the second-kind increasing with increasingly cathodic potential. In the vicinity of the shoulder ( - 4 2 0 mV) a second reversal point appears. Cathodic of this potential (region D), purely upward motion was observed, which decreased in intensity at far cathodic potentials on the plateau of the polarographic step. TABLE 1 EFFECT OF DEPOLARIZER SECOND KIND
System
Potential/V
ON
STREAMING
VELOCITIES
WITH
MAXIMA OF
THE
Velocities~ram s- 1
(vs. SCE)
KC1 satd. KC1 satd. and 1 m M Cd 0.25 M NazSO ~ 0.25 M Na2SOa and 8 m M Cu
To bottom of drop
To sides of drop
-0.8
2.9-3.3
1.2-1.9
-0.8 -0.01
3.4-3.7 2.5~.0
2.3-2.7 1.1-1.2
-0.01
5.1-6.2
4.0-6.1
The upward motion in region A was strongly influenced by the amount of depolarizer present. Table 1 presents data on the effect on upward streaming of the depolarizer in this system and also with the second-kind maximum shown with Cd(I1) in saturated KC1. DISCUSSION
It is evident from the results on both systems that the streaming velocity is markedly enhanced in the presence of depolarizer. Both systems--at the potentials, concentrations, etc. used--meet the criteria of second-kind maxima as to the type of streaming and its dependence on mercury flow-rate. Yet, the streaming is not solely governed by the hydrodynamics of the mercury flow--it is enhanced in the presence of depolarizer.
432
S. LAL, T. W. HOLT, H. H. BAUER
The explanation that suggests itself in the case of the copper(II) system is that the upward streaming, due to the mercury flow, carries fresh depolarizer to the bottom of the drop and reduced depolarizer to the top. The bottom of the drop thereby is at a more anodic potential, the surface tension there is lower, and there is a tendency for the drop surface to contract towards the top, to the region of higher surface tension. This force is additional to the upward movement caused by the flow of mercury into the drop, and therefore the rate of streaming is enhanced. This explanation is analogous to that put forward by von Stackelberg and Doppelfeld 1° for the persistence of positive maxima of the first kind, and the rapid cessation of negative maxima of the first kind. The streaming in the case of the Cd(II) system occurs on the negative side of the electrocapillary maximum and in this case presumably the maximum has some first-kind character arising from the non-uniform current density around the drop. At these potentials, first- and second-kind streaming are both upward and enhance one another, while in the Cu(II) system they are opposed and one observes reversal points and the inversion of the maximum. ACKNOWLEDGMENT
This work forms part of a study of electrochemical processes under Themis contract DAAB-07-69-C-0366 with the Department of Defense. REFERENCES 1 T. A. Kryukova, Zavod. Lab., 9 (1940) 691,699. 2 J. Heyrovsky and J. Kuta, Principles of Polarography, Academic Press, New York, 1966, p. 454. 3 A. N. Frumkin and V. G. Levich, Zh. Fiz. Khim., 21 (1947) 953. 4 J. Heyrovsky and J. Kuta, Principles of Polarography, Academic Press, New York, 1966, p. 460. 5 T. A. Kryukova and B. N. Kabanov, Zh. Obshch. Khim., 15 (1945) 294. 6 W. M. MacNevin and S. R. Steele, Anal. Chim. Acta, 24 (1961) 381. 7 T. A. Kryukova, Zh. Fiz. Khim., 21 (1947) 365. 8 T. W. Holt, Ph.D. Thesis, University of Kentucky, 1971. 9 S. Lal, A. Kumar and H. H. Bauer, J. Electroanal. Chem., 43 (1973) 423. 10 M. v. Stackelberg and R. Doppelfeld in I. S. Longmuir (Ed.), Advances in Polarography, Pergamon Press, London, 1960, p. 68.
429
I N F L U E N C E OF THE D E P O L A R I Z E R ON P O L A R O G R A P H I C M A X I M A OF THE SECOND K I N D SUDARSHAN LAL, T. W. HOLT* and H. H. BAUER Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506 (U.S.A.) (Received 8th August 1972)
INTRODUCTION Maxima of the second kind are attributed to the hydrodynamic flow of mercury inside the drop causing upward streaming of the mercury at the sides of the drop electrode and hence upward streaming of the solution around the dropping electrode x. It is also said that maxima of the second kind, being of hydrodynamic origin, are not affected by electrochemical processes occurring at the electrode z. The streaming velocity, v, of the motion of the drop surface at its equator relative to its centre has been expressed by the following equation, which is due to Frumkin and Levich3: V
~---
1 ( d ' - d) gr 2 3 2r1+ 3q' + a2/~:
--
where d is the density of solution, d' the density of mercury, 9 the gravitational constant, r/viscosity of solution, q' viscosity of mercury, a surface density of the electric charge, K specific conductivity of solution and r the radius of the mercury drop. The above equation implies that the velocity is dependent on specific conductance, and it has indeed been found that maxima of the second kind are enhanced in concentrated supporting electrolytes. Kryukova 1 found a linear dependence between the mercury flow rate (m 7/6) and the speed of the solution. On the other hand, the height of a second kind maximum is believed to be directly proportional to the depolarizer concentration 4, which implicitly suggests that the velocity of streaming does not change with increase in depolarizer concentration. At times, especially at high mercury flow-rates, simultaneous occurrence of maxima of the first and second kind, and their inversion, has been also observed ~ 7. The maximum is of the second kind at low depolarizer concentrations, and becomes of the first kind at high depolarizer concentrations, with (at potentials positive to the electrocapillary maximum) a change in direction of streaming from upward ("second kind") to downward ("first kind"). However, no influence of depolarizer on streaming has been reported for maxima that are essentially of the second kind. * Tobacco Research Institute, Universityof Kentucky,Lexington,Kentucky40506, U,S.A;
430
S. LAL, T. W. HOLT, H. H. BAUER
In the present communication, it is shown, from measurements of the speed of solution streaming that the upward streaming in second kind maxima is accentuated in the presence of depolarizer (here, copper or cadmium ions). In these cases, the presence of depolarizer affects the rate of streaming of the solution but not in the sense of a conversion of the maximum to one of the first kind: the upward streaming characteristic of second kind maxima is enhanced. EXPERIMENTAL Reagent grade chemicals were employed and solutions prepared in doubly distilled water. The systems examined were 8 m M Cu(II) in 0.25 M Na2SO 4 (pH 2) and 1 m M Cd(II) in saturated KC1. Polarograms were recorded on a PAR electrochemistry system model 170 in the three-electrode mode. The cell temperature was maintained at 25°C. Streaming of the solution was observed visually by means of a stereo microscope (Anchor Optical Co.) at 4 × magnification. Speed measurements were carried out as described elsewhere 9. RESULTS A polarogram of 8 m M Cu(II) in purified 0.25 M Na2SO 4 solution, at a mercury flow rate of 5.36 mg s -1, is shown in Fig. 1. The current-voltage plot at such a high flow-rate exhibits a compounded maximum and streaming of the solution around the mercury drop was observed over the entire potential range; however, the rate and direction of streaming varied with potential. On the foot of the wave (region A), an upward m o t i o n was observed, presumably second-kind streaming at this high flow-rate. The velocity of the upward movement increased as the current began to increase at more cathodic potentials. This upward swirling persisted until about - 2 5 mV applied potential and then I 1,6 1,4
<~[ 1,2 ~,ol 0.8 ('~ 0.6
,~
0.4 0.2
o i i'o,
_,1 E/V (vs. S C E )
Fig. l. Polarogram of 8 x 1()-~ M Cu(ll), 0.25 M NaaSO4, pH 2.0, 298°K, m=5.36 mg of the arrow is approx, proportional to soln. speed.
s
1
Length
431
D E P O L A R I Z E R E F F E C T O N M A X I M A O F T H E 2nd K I N D
reversal occurred, resulting in the inversion of the maximum from second kind to first kind. At this point, the solution moved upward, then became quiescent for an instant and then, with further growth of the drop, motion in the opposite direction began; the forces causing the two kinds of maxima were clearly present simultaneously at the reversal potential. After the onset of the positive maximum of the first kind (region B), the downward streaming increased to a maximal intensity at the peak of the polarogram (the intensity of streaming in this case was significantly greater than with two-electrode circuitry, where the shape of the maximum is also different8; this is presumably due to the increased current density brought about by the ohmic drop compensation in the three-electrode mode). The drop appeared to spin downward causing the electrolyte to move in the same direction in this potential region. On the cathodic side of the rounded maximum (region C), a mixture of first-kind and second-kind streaming was again observed on a single drop, the first-kind decreasing and the second-kind increasing with increasingly cathodic potential. In the vicinity of the shoulder ( - 4 2 0 mV) a second reversal point appears. Cathodic of this potential (region D), purely upward motion was observed, which decreased in intensity at far cathodic potentials on the plateau of the polarographic step. TABLE 1 EFFECT OF DEPOLARIZER SECOND KIND
System
Potential/V
ON
STREAMING
VELOCITIES
WITH
MAXIMA OF
THE
Velocities~ram s- 1
(vs. SCE)
KC1 satd. KC1 satd. and 1 m M Cd 0.25 M NazSO ~ 0.25 M Na2SOa and 8 m M Cu
To bottom of drop
To sides of drop
-0.8
2.9-3.3
1.2-1.9
-0.8 -0.01
3.4-3.7 2.5~.0
2.3-2.7 1.1-1.2
-0.01
5.1-6.2
4.0-6.1
The upward motion in region A was strongly influenced by the amount of depolarizer present. Table 1 presents data on the effect on upward streaming of the depolarizer in this system and also with the second-kind maximum shown with Cd(I1) in saturated KC1. DISCUSSION
It is evident from the results on both systems that the streaming velocity is markedly enhanced in the presence of depolarizer. Both systems--at the potentials, concentrations, etc. used--meet the criteria of second-kind maxima as to the type of streaming and its dependence on mercury flow-rate. Yet, the streaming is not solely governed by the hydrodynamics of the mercury flow--it is enhanced in the presence of depolarizer.
432
S. LAL, T. W. HOLT, H. H. BAUER
The explanation that suggests itself in the case of the copper(II) system is that the upward streaming, due to the mercury flow, carries fresh depolarizer to the bottom of the drop and reduced depolarizer to the top. The bottom of the drop thereby is at a more anodic potential, the surface tension there is lower, and there is a tendency for the drop surface to contract towards the top, to the region of higher surface tension. This force is additional to the upward movement caused by the flow of mercury into the drop, and therefore the rate of streaming is enhanced. This explanation is analogous to that put forward by von Stackelberg and Doppelfeld 1° for the persistence of positive maxima of the first kind, and the rapid cessation of negative maxima of the first kind. The streaming in the case of the Cd(II) system occurs on the negative side of the electrocapillary maximum and in this case presumably the maximum has some first-kind character arising from the non-uniform current density around the drop. At these potentials, first- and second-kind streaming are both upward and enhance one another, while in the Cu(II) system they are opposed and one observes reversal points and the inversion of the maximum. ACKNOWLEDGMENT
This work forms part of a study of electrochemical processes under Themis contract DAAB-07-69-C-0366 with the Department of Defense. REFERENCES 1 T. A. Kryukova, Zavod. Lab., 9 (1940) 691,699. 2 J. Heyrovsky and J. Kuta, Principles of Polarography, Academic Press, New York, 1966, p. 454. 3 A. N. Frumkin and V. G. Levich, Zh. Fiz. Khim., 21 (1947) 953. 4 J. Heyrovsky and J. Kuta, Principles of Polarography, Academic Press, New York, 1966, p. 460. 5 T. A. Kryukova and B. N. Kabanov, Zh. Obshch. Khim., 15 (1945) 294. 6 W. M. MacNevin and S. R. Steele, Anal. Chim. Acta, 24 (1961) 381. 7 T. A. Kryukova, Zh. Fiz. Khim., 21 (1947) 365. 8 T. W. Holt, Ph.D. Thesis, University of Kentucky, 1971. 9 S. Lal, A. Kumar and H. H. Bauer, J. Electroanal. Chem., 43 (1973) 423. 10 M. v. Stackelberg and R. Doppelfeld in I. S. Longmuir (Ed.), Advances in Polarography, Pergamon Press, London, 1960, p. 68.