Potentiostatic determination of kinetic parameters of electrode reactions with generation of a reactant in situ

It is shown that kinetic parameters of simple, fast electrode reactions of the type O + ne = R, where R is soluble in the solution or in the mercury electrode, can be determined by the potentiostatic method with a solution initially containing only the substance O, the substance R being generated in situ during electrolysis, provided that the electrode reaction involves only a single rate-determining step. Current-time curves are recorded with a fast response, electronic potentiostat and an oscilloscope upon applying a potential step from the zero-current potential to various potentials on the ascending part of the current-potential curve. The forward rate constant k_f at a given potential is calculated from the current at zero time found by extrapolation of the linear portion of the plot of current against square root of time, while the backward rate constant k_b is calculated indirectly from the slope of the same straight line. Plotting log k_f and log k_b against potential allows a simultaneous determination of the formal standard rate constant k_s, both cathodic and anodic transfer coefficients, the number of electrons involved in the rate-determining step and the formal standard potential of the system being studied. This method is considerably simpler than the well-known Gerischer-Vielstich method, and it should be particularly advantageous when R is highly reactive or forms an amalgam which is unstable in air. The upper limit of k_a that can be determined by this method is the same as that determinable by the Gerischer-Vielstich method. The kinetic parameters found by the present method for the electrode reactions zinc ion-zinc amalgam in 1M potassium nitrate, copper^II ion-copper amalgam in 1M potassium nitrate and cadmium ion-cadmium amalgam in 0.5M sodium sulphate were in fair agreement with the values reported in the literature.