Investigation of the (Solid−Liquid and Vapor−Liquid) Equilibrium of the System H2O + CH3OH + Na2SO4

Abstract

New experimental results for the vapor−liquid equilibrium of the system (water + methanol + sodium sulfate) are presented at (salt-free) solvent mixture methanol mole fractions from about 0.036 to 0.23, at sodium sulfate molalities up to about 1.25 mol·kg-1 of solvent mixture (but always below the solubility limit of the salt), at temperatures from about 314 to 394 K, and total pressures from about 10 to 390 kPa. An extension of Pitzer's model for the Gibbs excess energy of aqueous electrolyte solutions to mixed-solvent electrolyte systems is applied to describe the (solid−liquid and vapor−liquid) equilibrium of the ternary system (water + methanol + sodium sulfate). That extended model requires information on the solubility of the salt in pure water, on the vapor−liquid equilibrium of the salt-free binary system and on the (molar) Gibbs energy of transfer of sodium sulfate from pure water to mixtures of water and methanol. Literature data for the solubility of sodium sulfate in mixtures of water and methanol are used to determine that Gibbs energy of transfer. The new model is then able to predict the new experimental results for the vapor−liquid equilibrium of the ternary system (at salt concentrations up to the solubility limit) within experimental uncertainty. Furthermore, the model explains Furter's empirical equation for the salt-effects (“salting-out” and “salting-in”) on the vapor−liquid equilibrium of binary (or multicomponent) solvent mixtures, giving Furter's “salt-effect parameter” a better physical meaning.