Effects of Thiourea on Anodic Dissolution of Au and Surface Oxidation Behaviour in aq HClO4 Studied by Means of an EQCN

Anodic dissolution of Au is facilitated by the presence of thiourea owing to formation of strongly complexed Au ions. The present paper reports studies of this process using cyclic voltammetry (CV), chronopotentiometry and chronoamperometry, usefully complemented by nanogravimetry employing an electrochemical quartz-crystal nanobalance (EQCN). The molar masses per faraday for Au dissolution were determined from EQCN measurements, coupled with information derived from CV, chronopotentiometry and chronoamperometry, and clearly indicate that Au becomes dissolved over the potential range 0.45—0.65 V vs RHE via a 1e^– oxidation process in 0.5 M HClO₄ solution containing thiourea. The peak potential for Au dissolution in the presence of thiourea is about 600 mV less positive than that in the presence of Br^– or Cl^– (1.20 V vs RHE for Br^– and 1.39 V vs RHE for Cl^–). The linear relationship between anodic peak currents at about 0.630 V vs RHE and square-root of the sweep rate indicates that the Au dissolution process is diffusion-controlled. The anodic current efficiency for Au dissolution is 93%.