Solutions containing electrolytes and more than one solvent are often called "mixed solvent systems". Mixed solvent systems containing one salt have been considered pseudo binary solutions consisting of "mixed solvent" and salt. This view makes it difficult to model "mixed solvent systems" because the standard chemical potentials of ions are functions of the solvent composition. It is necessary to know the numerical values of the standard chemical potentials of ions at the current solvent composition in order to perform solid-liquid equilibrium and liquid-liquid equilibrium calculations in such systems. With this approach, two models are actually required: one for the variation of the standard state properties with composition and another for the excess Gibbs function. An example of this type of modeling is the model by Pérez-Salado Kamps, "Model for the Gibbs Excess Energy of Mixed-Solvent (Chemical-Reacting and Gas-Containing) Electrolyte Systems, Ind. & Eng. Chem. Res., 44(2005)201-225, DOI:10.1021/ie049543y.
A more straightforward method for modeling mixed solvent systems is presented here. It is based on two basic thermodynamic concepts:
This view makes it possible to calculate solid-liquid-equilibria, vapor-liquid equilibria, and liquid-liquid equilibria with great accuracy using the same set of parameters using the relatively simple thermodynamic model, the Extended UNIQUAC model. Some results of this type of modeling are shown below.
This method for modeling mixed solvent electrolyte solutions was described in the following publications, which also contains the required model parameters:
The following five systems are considered on this page:
The phase diagrams below show solubility isotherms for these five systems. The solubility isotherms describe compositions of each system in equilibrium with solid potassium sulfate. In all these systems, K2SO4 is being salted out by the addition of an extra solvent. 'Salted out' means that the solubility decreases and an excess of salt precipitates.
In the triangular diagrams shown below, lines of constant mass % potassium sulfate are lines parallel to the right hand side of the triangle. This side is opposite to the corner representing pure K2SO4 and represents potassium sulfate free solutions. A decreasing solubility of K2SO4 is characterized by solubility lines getting closer to the right side of the diagram.
Of the five solvents added one by one in each of the five diagrams, ammonia has a stronger salting out effect on K2SO4 than the four alcohols. In all the depicted phase diagrams, the red circles represent experimental data while the solubility equilibrium lines were calculated with the Extended UNIQUAC model. Notice that the temperatures vary between the diagrams.
In all the above graphs, the focus is on the H2O corner of the phase diagram. The lower part of the diagrams are therefore not shown, even though in some of the systems experimental data are available for the solubility of potassium sulfate in pure or almost pure non-electrolyte.