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Effect of shielding on polarographic streaming maxima
475
Eleetroanalytical Chemistry and Interfilcial Electrochemistry Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
EFFECT OF SHIELDING ON POLAROGRAPHIC STREAMING MAXIMA
D. F. T A Y L O R * , R. G. B A R R A D A S * * AND M. J. D I G N A M
Department of Chemistry, Universityof Toronto, Toronto 181, Ontario (Canada) (Received 17th October 1971 )
The vigorous mercury/solution interfacial motion associated with streaming maxima of the first kind at both the hanging mercury drop electrode (HMDE) and the dropping mercury electrode (DME) apparently requires both the presence of charged species in the double layer region, and a gradient in the electrode potential (and thus surface tension) across the mercury surface 1"2. Such gradients can be produced by the application of an external electric field3, but spontaneous streaming has been observed only when faradaic processes occur, i.e. when current flows across the electrode/solution interface and within the mercury electrode. The initiation and/ or propagation of streaming might therefore be a result either of a non-uniform current distribution within the mercury drop, or of shading or shielding4 by the capillary, since both will generate non-uniformity in the local current density at the electrode surface. We have conducted several experiments with a HMDE which show that initiation and propagation of streaming maxima of the first kind are dependent on the physical presence of an inert structure, and independent of the current distribution within the electrode. From this we conclude that the streaming phenomenon is related to shielding, with the surface tension theory as proposed by Frumkin and Levich1 von Stackelberg and Doppelfeld 5, de Levie4, and others probably being correct. All experiments were conducted in a cylindrical Pyrex cell which had a centrally located, optically flat window. Four HMDE capillaries were drawn from small-bore Pyrex tubing, two with finely tapered tips, and two with blunt tips. Each was fitted with a precision micrometer syringe, and electrical contact to the mercury was made through a platinum wire sealed into the syringe barrel. One capillary of each type was designed to support a mercury drop from below; i.e., the tip pointed vertically upwards when the capillary was irr position. The cell could accommodate simultaneously two HMDE's, one oriented vertically upwards, the other vertically downwards (see Fig. 1). It was therefore possible to form a mercury drop which made contact with both the upward-facing and downward-facing capillaries (Fig. la and b). Carefully rinsed talcum powder was introduced into the electrolyte for each experiment in order that electrolyte motion be made visible. A stereo zoom microscope was focussed on the capillary tips, and the area of interest was illuminated at right angles to the line of-vision. Experiments were carried out with aqueous solutions of 0.1 M KC1, both with * Present address : General Electric Research and Development Center, Schenectady, New York 12301, U.S,~. ** Present address: Department of Chemistry, Carleton University, Ottawa KIS 5B5, Ontario, Canada.
J. Electroanal. Chem., 36 (1972)
D. F. TAYLOR,R. G. BARRADAS,M. J. DIGNAM
476
!! k.lt..A
a
b
c
d
e
f
Fig. 1. Schematicdrawings of the various configurationsof the capillaries and the mercury drops. and without dissolved oxygen, and with 5 m M solutions of Pb 2 +, Ni 2 +, and Zn 2 + ions respectively in a 0.09 MKC1, 0.01 M HC1 aqueous base electrolyte. Conventional DME d.c. polarograms were obtained for each of the solutions containing a depolarizer to determine the optimum potential for observing streaming. A large area SCE 6 served as both the reference and counter electrode. In the first experiment, a doubly contacted drop was formed (Fig. la and b) and the cell was filled with 0.1 M KC1 solution. After deaeration of the electrolyte with purified nitrogen, d.c. currents of up to 100 mA were passed, first in one direction through the drop, then in the other, by connecting the positive and negative outputs of an electrically floating power supply to the upper and lower capillaries, respectively, and then reversing the leads. The experiment was carried out for both configuration a and b of Fig. 1. The same result was obtained for all four combinations of configuration and current direction : a slow upward motion of the solution occurred in the vicinity of the drop, increasing with the current. This was simply the result of i2R heating and thermal convection. Applying a potential of - 4 0 0 mV vs. SCE to the drop, in addition to applying the d.c. current, had no effect. Oxygen was then dissolved in the KC1 electrolyte by bubbling ultra pure reagent grade gas through the solution for several minutes, following which the series of experiments described above were repeated. The results were the same when no potential was applied, but on applying - 400 mV Vs. SCE, violent electrolyte streaming, characteristic of the positive oxygen maximum of the first kind, was observed. In every case, the streaming motion was directed from the blunt to the tapered capillary, independent of (i) which type of capillary (blunt or tapered) was positioned on top, (ii) which capillary (top or bottom) was connected to the polarizing potential source, and (iii) the magnitude and direction of the superimposed d.c. current flowing through the drop (until the i R drop across the mercury in the capillaries was sufficient to produce a significant change in the potential of the drop vs. SCE). These experiments were repeated with the other depolarizers, and consistent results were obtained. For positive maxima (O2,Pb2+), streaming was always directed from the blunt to the tapered capillary. For negative maxima (Ni2+,Zn2+), the streaming action was weaker, but always from the tapered to the blunt capillary. In several cases, the J. Electroanal. Chem., 36 (1972)
SHIELDING AND POLAROGRAPHIC MAXIMA
477
tapered capillary had a larger internal diameter than the blunt one. It appears that the external diameter of the capillary is the important factor. In the final sequence of experiments, the upper and lower capillaries were positioned so that a mercury drop formed from either capillary would touch the face of the other just prior to dislodgement (see Fig. lc, d, e and f). Using a small mercury drop on the tapered capillary as the HMDE (Fig. lc and d) streaming was always directed away from the tip for positive maxima. As the drop size was increased, a point was reached where streaming ceased. A further increase in drop size produced a reversal of the streaming direction. Corresponding changes were observed with the depolarizers which exhibit negative maxima; i.e. streaming which was initially directed toward the tapered HMDE capillary ceased, then reversed direction as the drop surface approached the tip of the blunt capillary. The observations did not depend on whether or not the blunt capillary was filled with mercury. No unusual effects were observed when the blunt capillaries were used as the HMDE (Fig. le and f) and the drop approached the tapered capillary : streaming for positive maxima was directed away from the blunt tip, and for negative maxima, toward it. We conclude from these experiments that the physical presence of the capillary plays a vital role in the initiation and propagation of polarographic streaming maxima of the first kind. Any valid theoretical analysis of the HMDE system in relation to the streaming mechanism must therefore take into account the influence of the capillary on mass transport and current flux to the electrode surface. To the best of our knowledge, these experiments represents the only case where polarographic streaming maxima have been totally suppressed in the absence of a surfactant or other surface contaminant. It should be possible to incorporate a suitably designed inert structure into cells used for analyses with a HMDE, thereby eliminating the undesirable effects which result from spontaneous streaming. ACKNOWLEDGEMENTS
The authors are grateful to the National Research Council of Canada for supporting this research, and for a scholarship (D.F.T.). REFERENCES V. G, LEVICH,PhysicochemicalHydrodynamics, Prentice-Hall, Englewood Cliffs, 1962, chap. X. D. F. TAYLOR,M.Sc. Thesis, University of Toronto, 1968. T. J. POPOVAANDT. A. KRYUKOVA,Zh. Fiz. Khim., 25 (1951)282. R. D~ LEVm,J. Electroanal. Chem., 9 (1965) 311. M. YON STACKELBERGAND R. DOPPELFELDin I. S. LONGMUIR(Ed.), Advances in Polarography, Vol. 1, Pergamon, New York, 1960, p. 68. 6 L. MEITES,Polarographic Techniques, Interscience, New York, 1965, p. 63. 1 2 3 4 5
J. Electroanal. Chem., 36 (1972)
Eleetroanalytical Chemistry and Interfilcial Electrochemistry Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
EFFECT OF SHIELDING ON POLAROGRAPHIC STREAMING MAXIMA
D. F. T A Y L O R * , R. G. B A R R A D A S * * AND M. J. D I G N A M
Department of Chemistry, Universityof Toronto, Toronto 181, Ontario (Canada) (Received 17th October 1971 )
The vigorous mercury/solution interfacial motion associated with streaming maxima of the first kind at both the hanging mercury drop electrode (HMDE) and the dropping mercury electrode (DME) apparently requires both the presence of charged species in the double layer region, and a gradient in the electrode potential (and thus surface tension) across the mercury surface 1"2. Such gradients can be produced by the application of an external electric field3, but spontaneous streaming has been observed only when faradaic processes occur, i.e. when current flows across the electrode/solution interface and within the mercury electrode. The initiation and/ or propagation of streaming might therefore be a result either of a non-uniform current distribution within the mercury drop, or of shading or shielding4 by the capillary, since both will generate non-uniformity in the local current density at the electrode surface. We have conducted several experiments with a HMDE which show that initiation and propagation of streaming maxima of the first kind are dependent on the physical presence of an inert structure, and independent of the current distribution within the electrode. From this we conclude that the streaming phenomenon is related to shielding, with the surface tension theory as proposed by Frumkin and Levich1 von Stackelberg and Doppelfeld 5, de Levie4, and others probably being correct. All experiments were conducted in a cylindrical Pyrex cell which had a centrally located, optically flat window. Four HMDE capillaries were drawn from small-bore Pyrex tubing, two with finely tapered tips, and two with blunt tips. Each was fitted with a precision micrometer syringe, and electrical contact to the mercury was made through a platinum wire sealed into the syringe barrel. One capillary of each type was designed to support a mercury drop from below; i.e., the tip pointed vertically upwards when the capillary was irr position. The cell could accommodate simultaneously two HMDE's, one oriented vertically upwards, the other vertically downwards (see Fig. 1). It was therefore possible to form a mercury drop which made contact with both the upward-facing and downward-facing capillaries (Fig. la and b). Carefully rinsed talcum powder was introduced into the electrolyte for each experiment in order that electrolyte motion be made visible. A stereo zoom microscope was focussed on the capillary tips, and the area of interest was illuminated at right angles to the line of-vision. Experiments were carried out with aqueous solutions of 0.1 M KC1, both with * Present address : General Electric Research and Development Center, Schenectady, New York 12301, U.S,~. ** Present address: Department of Chemistry, Carleton University, Ottawa KIS 5B5, Ontario, Canada.
J. Electroanal. Chem., 36 (1972)
D. F. TAYLOR,R. G. BARRADAS,M. J. DIGNAM
476
!! k.lt..A
a
b
c
d
e
f
Fig. 1. Schematicdrawings of the various configurationsof the capillaries and the mercury drops. and without dissolved oxygen, and with 5 m M solutions of Pb 2 +, Ni 2 +, and Zn 2 + ions respectively in a 0.09 MKC1, 0.01 M HC1 aqueous base electrolyte. Conventional DME d.c. polarograms were obtained for each of the solutions containing a depolarizer to determine the optimum potential for observing streaming. A large area SCE 6 served as both the reference and counter electrode. In the first experiment, a doubly contacted drop was formed (Fig. la and b) and the cell was filled with 0.1 M KC1 solution. After deaeration of the electrolyte with purified nitrogen, d.c. currents of up to 100 mA were passed, first in one direction through the drop, then in the other, by connecting the positive and negative outputs of an electrically floating power supply to the upper and lower capillaries, respectively, and then reversing the leads. The experiment was carried out for both configuration a and b of Fig. 1. The same result was obtained for all four combinations of configuration and current direction : a slow upward motion of the solution occurred in the vicinity of the drop, increasing with the current. This was simply the result of i2R heating and thermal convection. Applying a potential of - 4 0 0 mV vs. SCE to the drop, in addition to applying the d.c. current, had no effect. Oxygen was then dissolved in the KC1 electrolyte by bubbling ultra pure reagent grade gas through the solution for several minutes, following which the series of experiments described above were repeated. The results were the same when no potential was applied, but on applying - 400 mV Vs. SCE, violent electrolyte streaming, characteristic of the positive oxygen maximum of the first kind, was observed. In every case, the streaming motion was directed from the blunt to the tapered capillary, independent of (i) which type of capillary (blunt or tapered) was positioned on top, (ii) which capillary (top or bottom) was connected to the polarizing potential source, and (iii) the magnitude and direction of the superimposed d.c. current flowing through the drop (until the i R drop across the mercury in the capillaries was sufficient to produce a significant change in the potential of the drop vs. SCE). These experiments were repeated with the other depolarizers, and consistent results were obtained. For positive maxima (O2,Pb2+), streaming was always directed from the blunt to the tapered capillary. For negative maxima (Ni2+,Zn2+), the streaming action was weaker, but always from the tapered to the blunt capillary. In several cases, the J. Electroanal. Chem., 36 (1972)
SHIELDING AND POLAROGRAPHIC MAXIMA
477
tapered capillary had a larger internal diameter than the blunt one. It appears that the external diameter of the capillary is the important factor. In the final sequence of experiments, the upper and lower capillaries were positioned so that a mercury drop formed from either capillary would touch the face of the other just prior to dislodgement (see Fig. lc, d, e and f). Using a small mercury drop on the tapered capillary as the HMDE (Fig. lc and d) streaming was always directed away from the tip for positive maxima. As the drop size was increased, a point was reached where streaming ceased. A further increase in drop size produced a reversal of the streaming direction. Corresponding changes were observed with the depolarizers which exhibit negative maxima; i.e. streaming which was initially directed toward the tapered HMDE capillary ceased, then reversed direction as the drop surface approached the tip of the blunt capillary. The observations did not depend on whether or not the blunt capillary was filled with mercury. No unusual effects were observed when the blunt capillaries were used as the HMDE (Fig. le and f) and the drop approached the tapered capillary : streaming for positive maxima was directed away from the blunt tip, and for negative maxima, toward it. We conclude from these experiments that the physical presence of the capillary plays a vital role in the initiation and propagation of polarographic streaming maxima of the first kind. Any valid theoretical analysis of the HMDE system in relation to the streaming mechanism must therefore take into account the influence of the capillary on mass transport and current flux to the electrode surface. To the best of our knowledge, these experiments represents the only case where polarographic streaming maxima have been totally suppressed in the absence of a surfactant or other surface contaminant. It should be possible to incorporate a suitably designed inert structure into cells used for analyses with a HMDE, thereby eliminating the undesirable effects which result from spontaneous streaming. ACKNOWLEDGEMENTS
The authors are grateful to the National Research Council of Canada for supporting this research, and for a scholarship (D.F.T.). REFERENCES V. G, LEVICH,PhysicochemicalHydrodynamics, Prentice-Hall, Englewood Cliffs, 1962, chap. X. D. F. TAYLOR,M.Sc. Thesis, University of Toronto, 1968. T. J. POPOVAANDT. A. KRYUKOVA,Zh. Fiz. Khim., 25 (1951)282. R. D~ LEVm,J. Electroanal. Chem., 9 (1965) 311. M. YON STACKELBERGAND R. DOPPELFELDin I. S. LONGMUIR(Ed.), Advances in Polarography, Vol. 1, Pergamon, New York, 1960, p. 68. 6 L. MEITES,Polarographic Techniques, Interscience, New York, 1965, p. 63. 1 2 3 4 5
J. Electroanal. Chem., 36 (1972)