Shinichi Takasaki

A kinetic model of the dissolution of copper(II) oxide in EDTA solutions considering the coupling of metal and oxide ion transfer

Abstract The dissolution of metal oxides in solutions is related to the durability of protective oxide films on metals and the removal of corrosion scales on steels, and is important in corrosion science and corrosion protection engineering. In the study here, copper(II) oxide was sintered in a disk shape to maintain a constant surface area throughout dissolution, and the concentration of Cu(II) dissolved in EDTA solutions was measured as a function of time for different pH and EDTA concentrations at 80°C. Generally, only initial dissolution rates have been the object of study, but here the dissolution rate throughout the run could be examined. Without EDTA CuO did not dissolve, but with EDTA the dissolved Cu(II) concentration increased with time linearly at pH⩽7 and in a parabolic manner at pH⩾8.5. The dissolution rate increased with increasing pH at pH⩽7, but it decreased with pH at pH⩾8. As a result the concentration of dissolved Cu(II) at a specific time showed a peak at pH 7–8. Assessment and prediction of the extent of dissolution for given times, pH, EDTA concentrations, etc. with a model would be valuable for engineering purposes. A kinetic model is proposed by assuming the following successive elementary steps: (1) the transfer of Cu(II) ions as EDTA chelates CuY 2− to the solution leaving reactive and unstable “lone oxide ions” –O 2− on CuO with a backward reaction, and this is coupled with (2) the reaction of the “lone oxide ions” with protons to form water. The derived rate equation reproduced the linear and parabolic time changes in the dissolved Cu(II) concentration and the dissolution rate peak at pH 7–8. The deviation from linearity in the alkaline range is due to the increasing backward reaction in step (1). From the pH dependence of the model parameters, the H 2 Y 2− and HY 3− were estimated to be the dissolving EDTA species in solution.