Thermodynamics of ions precipitation in mixed-solvent mixtures

A framework derived from thermodynamic principles of solid-liquid equilibrium criteria was formulated to correlate and predict the precipitation of salts from aqueous solutions using organic solvents. The activity coefficient of a given salt in a mixed-solvent mixture was related to the activity coefficients of such a salt in each of the pure solvents (water and organic) using the excess Henry's constant approach (H(i)(E)). The Wohl's expansion was then employed to model the excess Gibbs free energy (g(E)) function. Two equations were provided; the two-suffix equation, and the three-suffix equation. A previously acquired precipitation database was used to evaluate the correlative ability of the framework equations. The precipitation measurements were adequately correlated and predicted by the two interaction parameters equation; the two-suffix equation. However, the three-suffix equation, with three interaction parameters, was more accurate. The regressed interaction parameters can be used as predictive tools to estimate the precipitation fractions (P) for the tested systems for which no experimental data are available. Furthermore, such parameters can be employed to predict the solubilities of the tested salts in the organic solvent.