On Gold(I) Complexes and Anodic Gold Dissolution in Sulfite–Thiourea Solutions

Processes involving gold(I) complexes were studied in sulfite–thiourea (TU) solutions. It is shown that at pH >5 the complex [AuTU2+] undergoes irreversible decomposition followed by deprotonation and formation of a solid phase. From the data of pH in mixed solutions, the equilibrium constants were evaluated: Au(SO3)23-+iTU⇌Au(SO3)2-iTUi2i-3+iSO32-, log₁₀β₁ = −1.2, log₁₀β₂ = −3.6. Some aspects of the anodic dissolution of gold in mixed sulfite–thiourea solutions are considered. With the help of the carbonate buffer system the change of the anodic current density j_a was studied at high pH; j_a (pH) has a maximum at pH 11.6–11.9 for E_a = 0.3–0.6 V (vs. NHE). At pH > 12.0, the j_a values decrease sharply. Possible mechanisms of anodic gold dissolution, as well as the role of sulfite, are discussed.