Effect of Na2SO3 on electrochemical aspects of gold dissolution in alkaline thiourea solution

Electrochemical methods were applied to study the anodic behavior of gold and its associated elements, e.g., silver, copper, nickel and iron, in alkaline thiourea solution in the absence and presence of Na2SO3. Gold is difficult to dissolve in alkaline thiourea solution because of the passivation of elemental sulfur produced by the irreversible decomposition of thiourea. However, addition of Na2SO3 accelerates the selective dissolution of gold in alkaline thiourea solution. The additive Na2SO3 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. The reaction order of the anodic dissolution of gold in alkaline thiourea solution is 1.65 with respect to Na2SO3, over the Na2SO3 concentration range of 0.01 mol/l to 0.5 mol/l at the potential of 0.34 V vs. NHE, The studies on the kinetics of the dissolution of gold in alkaline thiourea solution show that the Na2SO3 reduces the dissolution potential of gold, and decreases the apparent reaction activation energy of gold dissolution. (C) 1999 Elsevier Science B.V. All rights reserved.