Modeling solvent extraction systems with SXFIT

SXFIT is the latest in a series of programs designed to model solvent extraction systems of increasing complexity. Unlike its predecessors, SXFIT permits the user complete freedom to define the constituents from which the composition of each phase is to be specified in the data and from which all species of a model for a system are to be formed. This paper describes the manner in which the physical chemistry of the nonaqueous and aqueous solutions involved has come to be treated in this modelling effort. Included are: (1) solute activity coefficients and solvent activity in the nonaqueous phase, estimated by the Scatchard-Hildebrand-Scott treatment; (2) the effect of ionization on these quantities; (3) solute activity coefficients and water activity in the aqueous phase, estimated by the Fitter treatment; (4) the effect of non ideal behavior on the total concentration of nonaqueous solute species as rendered by a vapor pressure osmometer; (5) a similar treatment of heats of mixing of nonaqueous solutions; (6) the estimation of apparent molar volumes of aqueous species and, from these, the solution density, needed for the conversion of concentrations from the molarity to the molality scale; (7) the conversion of equilibrium constants from one concentration scale to another.