Factors Determining the Rate of Dissolution of Metal Oxides in Acidic Aqueous Solutions

Empirical rules governing the dissolution rates of oxides of the transition metals in acidic solutions containing redox systems are exemplified by the dissolution of CoO. Deviations from these rules shown during the dissolution of NiO (and partly of CuO) are discussed. The presence of organic complexing agents in the solution enhances in general the rate of dissolution of metal oxides. In the case of the iron oxides, induction periods of slow dissolution have been observed. An interpretation of these induction periods is based on a particular formation mechanism of metal chelates. The combined action of redox potential and complexing agent in the solution leads to a decrease of the dissolution rates, due possibly to a blocking of crystal surface sites by constituents of the redox couple and the complexing agent.