ABOUT POTENTIAL MEASUREMENTS IN STEADY-STATE VOLTAMMETRY AT LOW ELECTROLYTE ANALYTE CONCENTRATION RATIOS/

At low [electrolyte]/[analyte] ratios, besides being affected by tande m diffusional-migrational transport, voltammograms obtained at ultrami croelectrodes are altered by ohmic drop and junction potentials. This makes impossible the direct extraction of thermodynamic or kinetic dat a from wave shapes and positions. In this work, we propose two experim ental strategies for eliminating distortions of voltammograms due to j unction potential, and test them based on four one-electron reversible redox systems (oxidation and reduction of DPA; reduction of DCN; oxid ation 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 orig ins. One, which has been considered in the presently available theory, relates to the convection-free diffusion layer and is current depende nt because of the current dependent ionic enrichment or depletion of t he diffusion layer. The other, not considered by previous theories, is due to the resistance of the unaffected bulk solution. When the curre nt flow corresponds to the overall creation of ionic charges, the late r component becomes critical at low [electrolyte]/[analyte] ratios and high currents, because the former is considerably reduced by the ioni c enrichment of the convection-free diffusion layer. This duality is p articularly important in describing the shapes of voltammetric waves r ecorded with low supporting electrolyte concentration and sets limits on the usefulness of presently available theories which consider only one component of ohmic drop. (C) 1998 Elsevier Science S.A.