The figure to the right shows a ternary phase diagram for the MgSO4-K2SO4-H2O system in the temperature range from -10 to 120°C. The equilibrium compositions were calculated using the Extended UNIQUAC model. Experimental points from several different investigators are marked in the same diagram in order to illustrate the quality of the fit.
All points in the diagram represent saturated solutions. The lines in the diagram represent compositions, at which two salts are in equilibrium with the same liquid (and with a gas phase). The intersection between lines are compositions at which three solid salts are in equilibrium with the same liquid (and with a gas phase). These points are invariant points.
The solid phases encountered in this system in the temperature range from -10 to 120°C are:
The vertical blue line marks the eutectic temperature i.e. the lowest temperature at which an aqueous solution can be in equilibrium with ice and salts in this ternary system. At temperatures lower than the eutectic, an equilibrium mixture of water and the two salts will consist of separate phases of MgSO4·12H2O, K2SO4, schoenite, and Ice.
The ordinate in the diagram above is "Salt fraction": mol K2SO4 divided by (mol K2SO4 + mol MgSO4). The points in the diagram therefore mark compositions on a dry basis. The corresponding water content is not marked. It could be marked as a third dimension. Alternatively solubility isotherms can be calculated.
The 75°C solubility isotherm is shown in the figure to the right. The solubility isotherm calculated with the Extended UNIQUAC model is shown together with experimental data. The concentration unit in this triagonal diagram is weight percent. The green lines in the diagram are tie lines, marking the borders of fields yielding a certain solid salt.
The 75°C isotherm for this system consists of four branches, appearing from left to right: