Solid-solute phase equilibria: From thermodynamic basis information to multicomponent systems

Solubility equilibria in aqueous media are a powerful source of information on Gibbs energy functions of pure ionic compounds and solid-solutions. Because solid-solid phase transformations and/or recrystallizations are often much slower than equilibration between solids and solutes, it is sometimes possible to obtain reliable data from measurements on metastable systems.Thermodynamic quantities of geochemically and industrially important sparingly soluble metal oxides, hydroxides, carbonates and sulfides have been determined by solubility techniques. Activity coefficient variations of the reacting species are either minimized by the method of constant ionic media, or else corrected for by using the Pitzer equations. Measurements carried out at various fixed ionic strengthsI may be used to calculate the value of the solubility constantK _s ⁰ at infinite dilution. Given the availability of Pitzer parameters of sufficient quality, values ofK _s ⁰ extrapolated by this method lead to accurate Gibbs energies of formation. Moreover, when Pitzer parameters are known up to saturation, solubilities of highly soluble salts can easily be exploited for the calculation of Gibbs functions.A consistent set of equilibrium constants for the homogeneous and heterogeneous reactions occurring in the systems under investigation is used in the general Gibbs energy minimizing program ChemSage. Master variables for the depiction of the pertinent solid-solute phase diagrams are deduced from generalized Gibbs-Duhem equations. Application of the resulting data to geochemical and industrial problems is discussed.