LIMITING CURRENTS FOR STEADY-STATE ELECTROLYSIS OF AN EQUILIBRIUM MIXTURE, WITH AND WITHOUT SUPPORTING INERT ELECTROLYTE

An exact treatment derives the steady-state limiting current of a one- electron reduction for the N reversible arrow O+ + A(-) mixture at a h emispherical microelectrode. Either or both of the neutral N and catio nic O+ species may be electroactive, A supporting salt is present at a ny concentration, including zero or excess; its ions are electropassiv e acid do not interact with the other solutes or each other. The vario us species are treated as having distinct diffusivities, linked to the ir mobilities through the Nernst-Einstein relationship, Universal elec troneutrality is assumed, The predictions of the model are compared wi th published experimental data on the reduction of aqueous weak acids; agreement is excellent at intermediate, but poor at low, support rati os. Analysis of the unsupported case shows that the neutral N species dissociates in a narrow zone close to the electrode, and the injection of ions there serves to increase the electric field in the outer regi on of the transport zone, This enhances cationic migration enormously, leading to an unsupported limiting current that is much more than dou ble the supported value, However, the limiting current is drastically diminished by traces of foreign electrolyte. Curiously, the limiting c urrent with full support adopts the same value when equilibration is f ast as when it:is very slow, although the mechanisms are totally diffe rent.