Theory of steady-state voltammetry without supporting electrolyte

Abstract

This article addresses the reversible steady-state oxidation or reduction of an electroactive ion when no electrolyte, other than the electroreactant and its counterion, are present in solution. The model adopted is that of a steady state due to convergent transport to a hemispherical microelectrode, but the principles apply more generally. By analyzing the diffusion and migration of the reactant, counterion and product, subject to the constraints imposed by electroneutrality and by the principle of uniform total concentration, it is possible to predict the shapes of the voltammogram resulting from an electrode reaction, for any combination of the charge numbers of the three solute species. It is demonstrated that ohmic polarization may be insignificant (a few millivolts only) even in the total absence of supporting electrolyte. When the product is unchanged, or is an ion of the same sign as the reactant, voltammograms of the familiar wave shape are generated; equations and tables are presented giving the heights, positions and steepnesses of these voltammetric waves. However, when the product is an ion of opposite sign to the reactant ion, a voltammogram of novel shape is predicted. Such voltammograms are not waves, but display a linear ramp in place of the usual plateau, the current increasing limitlessly with potential. This behaviour, which is likely to be difficult to observe experimentally, arises from concentration polarization, not of the reactant ion, but of its counterion.