Mechanism of anodic dissolution of copper in aqueous acidified solutions of different anions

The potentials of copper anodes were measured as a function of current density in aqueous acidified solutions of different anions. Tafel lines were obtained, whose slopes depend on the nature of the anion. This denoted the participation of the anion in the anodic dissolution of the metal. Hence, mechanisms were suggested based on the formation of intermediate cuprous compounds, which are further oxidized electrochemically, or undergo chemical disproportionation to cupric salts. The results in phosphate solutions indicated that the second electrochemical oxidation is the rate determining step. In chloride, nitrate and sulphate solutions, both the second electrochemical step and the disproportionation reaction govern the overall reaction rate. Measurements in acidified copper salt solutions showed that Cu2+ ions affect the reaction mechanism. Thus, in chloride and sulphate solutions, the disproportionation reaction becomes predominating, whereas in nitrate solutions the intermediate is mainly oxidized electrochemically. An interpretation is provided, based on the adsorption of Cu2+ ions.