Effect of shielding on polarographic streaming maxima

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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 12. Such gradients can be produced by the application of an external electric field 3, 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 shielding 4 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 Levich 1 von Stackelberg and Doppelfeld 5, de Levie 4, 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