Solubility and Ion Association of Calcium Sulfate. An Electrical Conductivity Experiment

This manuscript describes a simple electrical conductance experiment to determine the solubility of calcium sulfate dihydrate (gypsum), which, in addition, provides information on the conductivity of the salt. The experiment is suitable for an introductory general chemistry laboratory course, but it can be extended to a physical chemistry course by including the analysis of the concentration dependence of the electrical conductivity. In that case, it is possible to determine the association constant and limiting conductivity of aqueous calcium sulfate and, through thermodynamic relationships, calculate the solubility product constant of gypsum. In order to justify the assumptions made in the data treatment, several issues related to the equilibrium and transport properties of associated aqueous electrolytes are discussed. Thus, the concept of ion pairs in aqueous solutions in relation to the concentration dependence of the electrical conductivity is reviewed and the experimental association constant (ion-pair formation constant) values are compared with those calculated using theoretical models. The effect of temperature on the equilibrium and transport properties is analyzed, and the solid phase transition between anhydrous calcium sulfate (anhydrate) and gypsum is considered in relation to the design of the experiment.