Concentration and Density Changes at an Electrode Surface and the Principle of Unchanging Total Concentration

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The principle of unchanging total concentration as described by Oldham and Feldberg [J. Phys. Chem. B, 103, 1699 (1999)] is invoked to analyze systems comprising a redox pair (Xz11 and Xz22) plus one or more non-electroactive species () where Xzjj is the jth species with charge zj and concentration; cj. The principle states that if the diffusion coefficients for all species are identical and mass transport is governed by the Nernst-Planck expression, the total concentration does not change during any electrochemical perturbation, i.e.: With this principle we deduce the electrochemically induced difference between the surface and bulk concentrations for each species. Those concentration differences are translated into density differences which are a function of the density of the solvent and of the concentration differences, molecular masses and the standard partial molar volumes of all species. Those density differences in turn can induce convection that will ultimately modify the observed current. However, we did not attempt to quantify details of the natural convection and current modification produced by those density differences. The principle of unchanging total concentration also allows us to suggest experimental ploys that might minimize, if not eliminate, density differences; if there are no density differences there should be no convection save for the possibility of spontaneous convection which Amatore, Szunerits, Thouin and Warkocz [J. Electroanal. Chem., 500 62 (2001)] have identified as a mode of convection that does not depend upon “macroscopic flow or density gradient”. Following the lead of Ngamchuea, Eloul, Tschulik and Compton [Anal. Chem., 87, 7226 (2015)] we did not consider spontaneous convection in the present work.