Electrochemical oxidation of gold and thiourea in acidic thiourea solutions

The dissolution of gold in acidic thiourea solutions, together with the oxidation of thiourea on gold, platinum, and glassy carbon (GC) electrodes, has been investigated using voltammetric methods. The cyclic voltammograms on platinum and GC electrodes showed a single pair of peaks for the oxidation of thiourea to formamidine disulfide and its subsequent reduction. The mechanism of the reaction is discussed. When a gold electrode was used, gold dissolution took place at a more negative potential than thiourea oxidation, with the latter reaction being shifted to a potential about 300 mV more positive than that on the platinum electrode. It was found that, at 25 degreesC and without formamidine disulfide, gold did not dissolve when the potential was scanned rapidly. This result shows that the process of gold dissolution might include a slow surface step which can be accelerated by formamidine disulfide. It is possible that the slow surface step results from the strong adsorption of thiourea on the gold surface. The adsorption of thiourea on gold was demonstrated using the electrochemical quartz crystal microbalance. (C) 2001 The Electrochemical Society.