ABSTRACT
As the charge of the metal ion increases and its radius decreases, the electronic band structure of the semiconducting oxides becomes characterized by larger gaps between the valence and conduction bands. Metal oxides eventually become electrical insulators. In parallel, the affinity of oxygen atoms for protons decreases and the oxides become increasingly acidic. In a practical attempt to achieve dissolution, weakly acidic covalent oxides interact better with OH- ions. Alkaline fusion procedures to make them soluble are well known. Hydroxide ions provide both a reasonably good thermodynamic driving force to break the very strong oxo bonds and the best available kinetic aggressiveness. Owing to the interest in the behavior of silicates, one of the major controversies in the literature regarding the dissolution of sparingly soluble silicates has centered on the possible control of the rate either by diffusion through a leached layer or by chemical surface reactions.
