New oscillatory phenomena during gold electrodissolution in sulfuric acid containing Br− or in concentrated HCl

In this communication, we report some new oscillatory phenomena on the electrodissolution of gold in sulfuric solution containing Br− or in concentrated HCl. It was found that gas evolution and/or liquid film can play important roles in the oscillatory electrodissolution of gold, in addition to the solid film of gold oxides as previously recognized. Oscillatory electrodissolution of gold in bromide media is given here for the first time with two cases: (i) current oscillations only with lower concentrations of bromide (0.5 M, M = mol dm−3) and sulfuric acid (0.2 M) in the narrow potential range of active–passive transition and (ii) oscillations both in potential and in current with moderate concentrations of bromide (1 M) and sulfuric acid (1 M). The depletion and replenishment of Br− surface concentration by reactions under diffusion control and by convection through periodic oxygen evolution, respectively, account for the potential oscillations. Whereas the variation of available electrode surface area results in the irregular current oscillations due to the formation–removal of liquid bromine film rather than the periodic oxygen evolution. In addition, we first show both potential and current oscillations in concentrated HCl (8 M). The depletion and replenishment of the surface Cl− concentration involving also periodic gas evolution are the main reasons for the potential oscillations like case (ii) in the Br−-containing solution. Two kinds of current oscillations were found relating the active–passive transitions (like case (i)) and the Cl− depletion–replenishment with periodic gas evolution, respectively. The current oscillations can transform from the former to the latter ones with a positive potential scan. Some preliminary explanations have been given on the mechanism of the oscillatory electrodissolution with the help of the crossed cyclic voltammagrams (CVs) and in situ Raman spectra.