Thermodynamic studies of gypsum in water and aqueous solutions.

The object of the work described in this thesis was to investigate some aspects of the thermodynamics of calcium sulphate in aqueous solutions containing mixed electrolytes. A large Portion of the thesis describes the experimental determination of the thermodynamic solubility product of gypsum (CaS04 ·2H20) by an electromotive force method. The procedure used is novel and involved the use of a calcium responsive membrane electrode in conjunction with a lead two phase amalgam -lead sulphate electrode. The standard potential of the latter electrode and some thermodynamic properties of calcium sulphate solutions were obtained in the course of this work. Gypsum solubility measurements were made up to ionic strengths of six molal in binary and ternary electrolyte solutions in the system Ca2+ - Na+ - SO~- - Cl- - H20. A second estimate of the gypsum solubility product was obtained from the results. Activity coefficients in the mixed electrolyte solutions were investigated using some recent approaches in electrolyte thermodynamics. An electromotive force method was used to obtain calcium chloride activity coefficients in mixed solutions with calcium sulphate. The experiment was designed to provide confirmation of a theoretical limiting law for mixtures of electrolytes having asymmetric charge types. It was concluded that such confirmation was not possible for the system investigated. Literature osmotic coefficients for mixtures of 1:1 and 2:1 charge type chlorides were analysed to check if the data conformed to a related limiting law for solvent properties. The theoretical concentration dependence trend was not observed, but the data were concordant with primitive model calculations.