How valid is the electroneutrality approximation in the theory of steady-state voltammetry?

Abstract

We carry out two accurate analyses, one with and one without the electroneutrality approximation, of conditions attending the steady-state voltammetry of the reversible M/M n+ reaction at a small hemispherical electrode in a solution containing only a single n: n electrolyte. In the absence of electroneutrality, the electroactive cation and its counterion are assumed to have equal concentrations only in the bulk. The discrepancy between the shapes of the two voltammograms is significant only for very small electrodes at which the ionic strength is low and is therefore unlikely to be detectable experimentally. Accordingly, this study lends credence to the validity of voltammetric theories based on the electroneutrality assumption. As well as predicting the small effect that the electroneutrality assumption has on the voltammogram, quantitative predictions are made of other properties of the system, including the ionic concentration, potential and charge profiles. A mild space charge, invariably positive, is found to occupy a region, of width comparable to the electrode radius, adjacent to the working electrode. This faradaically created space charge has properties distinctly different from those characteristic of traditional double layers.