Influence of sulphite ions on kinetics of gold anode dissolution in alkaline and acid thiourea solutions

Electrochemical methods were applied to study the anodic behavior of gold in alkaline thiourea solution in the absence and presence of Nа2SO3. Gold is highly soluble in acid thiourea in the presence of Fe (III )oxidant. But, gold is difficult to dissolve in alkaline thiourea solution because of the elemental sulfur passivation produced by the irreversible decomposition of thiourea. However, addition of Nа2SO3 accelerates the selective dissolution of gold in alkaline thiourea solution. Nа2SO3 additive improves the stability of thiourea in alkaline media, enhances the anodic polarization current of gold greatly, and does not change the passivation states of the associated elements of gold in alkaline thiourea solution. During dissolution of gold in alkaline thiourea solution the coelements of gold do not show any hindrance. Identification of the optimal condition of gold dissolution process in alkaline thiourea was carried out. It is the solution of 0.5 mol/l of Nа2SO3; 0.1 mol/l of thiourea in temperature of 323 К, and pH value of 12.5. In kinetic study, gold dissolved in alkaline thiourea solution with Nа2SO3 and the activation energy was 26.79 kJ/mol and it’s respective potential was 0.24 V.