On the kinetics of solid-liquid-solid phase transformation

Solid-solid phase transformation of different hydrates or polymorphs in suspension proceeds in such a way that the metastable phase dissolves and the stable phase precipitates from the solution. If the starting point lies within the metastable zone of the stable phase, the transformation starts only after elapse of the corresponding induction period. In the case when the starting point is situated outside the metastable zone of the stable solute, the transformation proceeds without any delay. These theoretical considerations are illustrated by an example with recrystallization of calcium sulphate in presence of phosphoric acid; a small concentration change of the acid changes the kinetics significantly.