Potential measurements in steady state voltammetry at low electrolyte/analyte concentration ratios. Role of convection on ohmic drop: a simplified model

Abstract

Voltammetric measurements performed at low [electrolyte]/[analyte] ratios are affected by migrational transport, as well as by ohmic drop contributions. The latter depend on the current as well as on the charge of the initial electroactive species because the local electrolysis changes the ionic composition in the vicinity of the electrode. Extraction of thermodynamic or kinetic data from wave shapes and positions is thus impossible without correction of these ohmic drop components. In this work we extend and generalize a previous experimental approach for eliminating such ohmic drop contributions from an experimental voltammogram. The method is tested based on four one-electron reversible redox systems: oxidation and reduction of DPA; reduction of DCN; and oxidation of ferrocene. It is thus confirmed that under conditions where there is no excess of supporting electrolyte, the cell resistance has two origins. One, already recognized by previous theories, depends on the potential because the current flow affects the ionic content within the diffusion layer. The second, which was not considered in previous theories, is independent of the potential since it reflects the resistance of the solution in a range in which the ionic content is not affected by the electrode current. This occurs because convection precludes development of infinite diffusion layers, an important issue not considered in previous theories. At low [electrolyte]/[analyte] ratios, the second term becomes significant and may even become the major one for the portion of experimental interest in voltammetric waves. A simplified model is proposed to describe this phenomenon. Despite its simplicity and the crudeness of some approximations performed in order to achieve an analytical solution, this simple model predicts all the trends as well as the order of magnitude of the experimental deviations vis a vis previous theoretical predictions.