New Nonlinear Phenomenon in Galvanostatic Reduction of S₂O₈^2-

Potential oscillations during S₂O₈^2- reduction under galvanostatic conditions in 1mol/dm³ NaOH solution on a silver electrode are reported here for the first time. The bifurcation boundaries were measured in the parameter plane of applied current vs. S₂O₈^2- concentration. In addition to some interesting damped oscillations occurring on the upper boundaries, a typical oscillation sequence in the boundaries is given which appeared when the applied current was increased monotonously at fixed S₂O₈^2- concentration. This kind of oscillation has its typical voltammetric characteristics. There is a broad limiting current plateau in the current-potential curve and the oscillation amplitudes are within that region, and there is a large crossing cycle in the cyclic voltammogram. A model, i. e., a bistability （S₂O₈^2- reduction and hydrogen evolution at different potentials） coupled with a negative feedback （depletion of the S₂O₈^2- surface concentration by reduction under diffusion control） and a positive feedback（replenishment of the S₂O₈^2- surface concentration by convection induced through the hydrogen evolution）, has been proposed to explain the oscillations, which was supported by the experimental results of current sweep and cydic voltammetry.