Resistance of solution without supporting electrolyte under the reduction of HCl

The resistance of HCl solution without supporting electrolyte at a microelectrode was evaluated by the ac impedance technique of which the de-potential was in the diffusion-controlled domain for the reduction of the hydrogen ion. The hydrogen ion is reduced to hydrogen gas. The Cl- takes the same concentration profile as the hydrogen ion in order to keep electric neutrality. Then, no ion is present near the electrode. Consequently, the resistance between the working and the counter electrode should be extremely large, from the theoretical viewpoint. However, well-defined voltammograms can be obtained without adding supporting electrolyte. This contradiction is discussed on the basis of the experimental results of the solution resistance during the reduction. The evaluated conductance was linear with the concentration of HCl. The linearity indicates that Cl- is responsible for the conductivity, by collapsing the electric neutrality in the diffuse layer. A model of the concentration distributions is proposed, in which the concentration of the hydrogen ion is proportional to the distance from the electrode, whereas that of Cl- is invariant to the distance in the diffuse layer. The conductivity evaluated from the model is twice the experimental value. Both the conductance and the current are proportional to the concentration, of which the ratio gives the ohmic potential drop, 0.52 V, independent of values of the current. This is a reason why voltammograms at microelectrodes are not so deformed even at high concentrations of electroactive species. (C) 2000 Elsevier Science Ltd. All rights reserved.