Expressions of supersaturation in crystallization studies

The driving force for crystallization in a binary system is considered from the thermodynamic viewpoint. Exact relationships, derived for particular cases, are compared with commonly used expressions of supersaturation calculated from solution concentrations. Errors incurred when activity coefficient ratios are ignored are evaluated. The need to incorporate the number of ions, into which a molecule dissociates, in the expressions of supersaturation for electrolyte solutions is emphasized. Examples are given for the crystallization of several inorganic salts from aqueous solution.