ABSTRACT
This chapter describes the main features of two different dissolution processes of great practical importance: the dissolution of single phases formed by two or more metal oxides, and the dissolution of multiphase solids. The distinguishing feature of MNOn+m systems is the possible operation of diffusion control soon after a layer of insoluble reaction products has formed. Whether this layer forms will crucially depend on the solubility of possible secondary (leached) phases. Phase diagrams for the binary metal oxides systems MOn + NOm are required for a rational description of dissolution. Normally, the different phases accept a certain degree of substitution and the range of compositions for each phase may be wide. The acidity changes brought about in a metal oxide MOn by partial substitution with a second metal oxide NOm is a matter of importance in the design of supported catalysts.
