The mechanism of oxidation of chromium to chromate

Abstract

The oxidation of chromium to chromate proceeds via six consecutive charge-transfer reactions. The intermediates cover the chromium surface completely with an adsorbed monolayer. In a seventh step chromate is irreversibly desorbed. Steady-state polarization curves and faradaic impedance spectra were measured. The equivalent circuit of the electrode reaction derived from the impedance spectra contains a series of six pairs of a resistance and a capacitance in parallel. From the steady-state cd and from one impedance spectrum a set of 13 rate constants of the seven reaction steps was obtained. The rate constant were used to calculate the stationary polarization curve, the potential dependence of the mean state of oxidation of the adsorbed layer, and the transient currents after a potential step. A good agreement of the calculations with the respective experiments was observed. From the pH dependence of the stationary polarization curves and from the known set of rate constants, it is concluded that for all or most of the individual steps of the reaction two parallel paths are possible, one being catalysed by acids, the other being catalysed by bases.