Effect of thiourea on the rate of anodic processes at gold and graphite electrodes in thiosulfate solutions

Dependencies of the anode current on the potential were measured by means of voltammetry on the fresh surface of gold and graphite electrodes in the solutions of 0.01 M sodium thiosulfate with the addition of thiourea (1 x 10(-5)-1 x 10(-3) M). Using EQCM measurements, we studied the partial currents of gold dissolution in the indicated solutions. The results give us ground to state that small amounts of thiourea (Tu) added cause a substantial increase both in the rate of thiosulfate oxidation and in the rate of gold dissolution. On graphite, the addition of Tu causes a decrease in the electrode polarization and is likely to lead to the change of the mechanism of the electrode reaction of thiosulfate oxidation. On the gold electrode, the addition of Tu has a multivariable effect on the anode process in thiosulfate solutions. On one hand, Tu functions as a depassivating agent with respect to the reaction of thiosulfate oxidation. On the other hand, it acts as an accelerating agent for the reaction of gold dissolution. After the addition of a small amount of Tu, the rate of gold dissolution becomes comparable with the rate of thiosulfate oxidation. Results show that no indices of gold passivation by polysulfides are observed with the solutions with Tu addition. At the same time, gold depassivation by Tu additive is seems not to be the reason of an increase in the rate of gold dissolution. (C) 2013 Elsevier Ltd. All rights reserved.