The behaviour of copper anodes in aqueous thiourea-containing sulphuric acid solutions. Open circuit potentials and electrochemical kinetics

The electrochemical behaviour of copper anodes in aqueous thiourea (TU)-containing sulphuric acid was followed from open circuit potential measurements, voltammetry and rotating disk and ring-disk electrode data. The open circuit potential of copper is controlled by equilibria involving different Cu(I)–TU complex species. The thiourea electro-oxidation which yields formamidine disulphide and Cu(I)–TU complex species can be described as a process under intermediate kinetics taking place on a partially blocked copper anode. The degree of copper surface coverage and the type of predominant adsorbate depend on the concentration of thiourea and applied electric potential. Depending on the potential range adsorbates can be formed directly from either a strong TU–copper interaction or a relatively weaker formamidine disulphide interaction. The passivating layer can be related to the formation of polymer-like Cu(I)–TU layers and residues resulting both from thiourea and formamidine disulphide electro-oxidation. A previously proposed complex reaction pathway can explain the general behaviour of these systems, including adsorbate formation and the influence of the electrolyte solution constituents.