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

We carry out two accurate analyses, one with and one without the electroneu trality approximation, of conditions attending the steady-state voltammetry of the reversible M/Mn+ reaction at a small hemispherical electrode in a s olution containing only a single n:n electrolyte. In the absence of electro neutrality, 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 w hich the ionic strength is low and is therefore unlikely to be detectable e xperimentally. Accordingly, this study lends credence to the validity of vo ltammetric theories based on the electroneutrality assumption. As well as p redicting 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 w idth 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. (C) 2001 Elsevier Science BN. All rights reserved.