Solubilities of metal carbonates

Solubilities in aqueous media of sparingly-soluble metal carbonates play an important role in chemical processes whether they occur in a laboratory, o n an industrial scale, or in the geologic environment. Solubility phenomena (i.e. dissolution and precipitation reactions) of metal carbonates control procedures for preparing, separating and purifying chemicals. Moreover, th e interactions of the hydrological cycle with the cycle of carbonate rocks, the naturally occurring dissolution of carbonate minerals in water as well as their precipitation on the ocean floor and in the sediments of rivers a nd lakes can be described by the principles of solubility, although giganti c quantities of material may be involved. The experimental methods for the determination of carbonate solubilities ar e reviewed and the results are discussed within the frame-work of equilibri um thermodynamics. It will be shown that Gibbs functions of pure stoichiome tric carbonate phases can be determined by accurately measuring solubilitie s in aqueous electrolyte systems. In cases where solid-solid phase transfor mations and recrystallisations are kinetically inhibited, the methods devel oped were successfully applied to metastable equilibria. Occasionally, solu bility data were even employed to estimate activity coefficients of compone nts forming a series of carbonate solid solutions. Whenever possible the activity coefficients of the reacting species were co ntrolled by use of a constant ionic medium. Solubility constants obtained a t various fixed ionic strengths were fitted to the Fitter equations and ext rapolated to infinite dilution. Provided the standard potentials of the res pective metal ion/metal electrode are known over a range of ionic strengths the values of solubility constants at infinite dilution can be calculated independently. In any case the optimised thermodynamic constants were incor porated in a comprehensive computer model which permits a wide variety of s olubility calculations. These are illustrated by a case study of a multicom ponent system with industrial relevance.