SOLID-SOLUTE PHASE-EQUILIBRIA - FROM THERMODYNAMIC BASIS INFORMATION TO MULTICOMPONENT SYSTEMS

Solubility equilibria in aqueous media are a powerful source of inform ation on Gibbs energy functions of pure ionic compounds and solid-solu tions. Because solid-solid phase transformations and/or recrystallizat ions are often much slower than equilibration between solids and solut es, 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, car bonates and sulfides have been determined by solubility techniques. Ac tivity coefficient variations of the reacting species are either minim ized by the method of constant ionic media, or else corrected for by u sing the Fitter equations. Measurements carried out at various fixed i onic strengths I may be used to calculate the value of the solubility constant K-s(0) at infinite dilution. Given the availability of Pitzer parameters of sufficient quality, values of K-s(0) extrapolated by th is method lead to accurate Gibbs energies of formation. Moreover, when Fitter parameters are known up to saturation, solubilities of highly soluble salts can easily be exploited for the calculation of Gibbs fun ctions. A consistent set of equilibrium constants for the homogeneous and heterogeneous reactions occurring in the systems under investigati on is used in the general Gibbs energy minimizing program ChemSage. Ma ster variables for the depiction of the pertinent solid-solute phase d iagrams are deduced from generalized Gibbs-Duhem equations. Applicatio n of the resulting data to geochemical and industrial problems is disc ussed.