Chemical dissolution of oxides: Experiments with sintered ZnO pellets and ZnO single crystals

The rate of dissolution of zinc oxide has been studied with a technique developed by Engell, in which the rate can continuously be measured as an electric current. The rate decreases with the pH in the acidic range but depends also on the type of anion present in the solution. It is enhanced by weak acids as proton donors. In the alkaline pH range, the presence of NH3 leads to a pronounced increase of the rate which is followed by a decrease between pH 10-13. For the interpretation, a model of the kink sites at a non-polar face of the crystal is constructed and the possible steps in the dissolution process are discussed. The conclusion is that the rate determining step is the hydrolytic splitting of bonds between kink site atoms to the lattice which is catalyzed by proton donors and by molecules or ions which form ligands to the zinc atoms. The electric forces at the interface play a minor role