A speciation-based model for mixed-solvent electrolyte systems

A comprehensive model has been developed for the calculation of speciation, phase equilibria, enthalpies, heat capacities and densities in mixed-solvent electrolyte systems. The model incorporates chemical equilibria to account for chemical speciation in multiphase, multicomponent systems. For this purpose, the model combines standard-state thermochemical properties of solution species with an expression for the excess Gibbs energy. The excess Gibbs energy model incorporates a long-range electrostatic interaction term expressed by a Pitzer–Debye–Hückel equation, a short-range interaction term expressed by the UNIQUAC model and a middle-range, second virial coefficient-type term for the remaining ionic interactions. The standard-state properties are calculated by using the Helgeson–Kirkham–Flowers equation of state for species at infinite dilution in water and by constraining the model to reproduce the Gibbs energy of transfer between various solvents. The model is capable of accurately reproducing various types of experimental data for systems including aqueous electrolyte solutions ranging from infinite dilution to fused salts, electrolytes in organic or mixed, water+organic, solvents up to the solubility limit and acid–water mixtures in the full concentration range.