Modeling phase equilibria and speciation in mixed-solvent electrolyte systems

A comprehensive mixed-solvent electrolyte model has been applied to calculate phase equilibria and other thermodynamic properties of multicomponent solutions containing salts, acids and bases in wide concentration ranges. The model combines an excess Gibbs energy model with detailed speciation calculations. The excess Gibbs energy model consists of a long-range interaction contribution represented by the Pitzer–Debye–Hückel expression, a short-range term expressed by the UNIQUAC model and a middle-range term of a second-virial-coefficient type for specific ionic interactions. The model accurately represents the thermodynamic behavior of systems ranging from infinite dilution in water to molten salts or pure acids at temperatures from the freezing point to 300 °C. It has been determined that a physically realistic treatment of speciation is important for the simultaneous representation of vapor–liquid equilibria, osmotic coefficients, solid–liquid equilibria, pH, enthalpies of dilution and heat capacities.