About potential measurements in steady state voltammetry at low electrolyte/analyte concentration ratios

Abstract

At low [electrolyte]/[analyte] ratios, besides being affected by tandem diffusional–migrational transport, voltammograms obtained at ultramicroelectrodes are altered by ohmic drop and junction potentials. This makes impossible the direct extraction of thermodynamic or kinetic data from wave shapes and positions. In this work, we propose two experimental strategies for eliminating distortions of voltammograms due to junction potential, and test them based on four one-electron reversible redox systems (oxidation and reduction of DPA; reduction of DCN; oxidation of ferrocene). Taking advantage of the availability of junction potential-free reconstructed voltammograms, an analysis of ohmic drop distortions is performed. It is concluded that ohmic drop has two origins. One, which has been considered in the presently available theory, relates to the convection-free diffusion layer and is current dependent because of the current dependent ionic enrichment or depletion of the diffusion layer. The other, not considered by previous theories, is due to the resistance of the unaffected bulk solution. When the current flow corresponds to the overall creation of ionic charges, the later component becomes critical at low [electrolyte]/[analyte] ratios and high currents, because the former is considerably reduced by the ionic enrichment of the convection-free diffusion layer. This duality is particularly important in describing the shapes of voltammetric waves recorded with low supporting electrolyte concentration and sets limits on the usefulness of presently available theories which consider only one component of ohmic drop.