Mechanism of dissolution of sparingly soluble electrolytes

Recent constant composition dissolution studies of sparingly soluble calciu m phosphates have revealed an interesting and unusual behavior in that the rates decreased, eventually resulting in effective suppression, even though the solutions remained undersaturated. Contrary to traditional theories of dissolution, these experimental results indicated the importance of not on ly the particle size on the dissolution rate but also the participation of critical phenomena. In these theories, it is assumed that when the dissolut ion reactions are initiated, they continue spontaneously until all solid ph ase has disappeared. in terms of these mechanisms, there are no critical ph enomena in the dissolution mechanism. Although the crystal size decreases d uring dissolution, when the reaction is controlled by polypitting (formatio n and growth of pits), the edge free energy increases at the very first sta ge due to the creation of pits and dissolution steps. The constant composit ion experimental results demonstrate the development of surface roughness a s the dissolution steps are formed, implying an increase of the total edge length during the reactions. In an exactly analogous mechanism to crystal g rowth, the participation of critical conditions involving dissolution steps is a possibility. In contrast to crystal growth, dissolution is a process of size reduction and, when the particle size is sufficiently reduced, crit ical phenomena become important so that the influence of size must be taken into consideration. This paper proposes such a model for dissolution react ions, and although these unusual phenomena probably apply to all mineral ph ases, they are more evident for sparingly soluble electrolytes in which the critical conditions are attained much more readily.