Aqueous Salt Solutions

The figure to the right shows a ternary phase diagram for the MgSO_{4}-K_{2}SO_{4}-H_{2}O
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:

- H
_{2}O (s) Ice - K
_{2}SO_{4}(potassium sulfate) - MgSO
_{4}·12H_{2}O (magnesium sulfate dodecahydrate) - MgSO
_{4}·7H_{2}O (epsom salt) - MgSO
_{4}·6H_{2}O (hexahydrite) - MgSO
_{4}·H_{2}O (kieserite) - K
_{2}SO_{4}·MgSO_{4}·6H_{2}O (schoenite) - K
_{2}SO_{4}·MgSO_{4}·4H_{2}O (leonite) - K
_{2}SO_{4}·2MgSO_{4}(langbeinite)

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 MgSO_{4}·12H_{2}O, K_{2}SO_{4},
schoenite, and Ice.

The ordinate in the diagram above is "Salt fraction": mol K_{2}SO_{4}
divided by (mol K_{2}SO_{4} + mol MgSO_{4}). 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:

- K
_{2}SO_{4}(potassium sulfate) - K
_{2}SO_{4}·MgSO_{4}·4H_{2}O (leonite) - K
_{2}SO_{4}·2MgSO_{4}(langbeinite) - MgSO
_{4}·H_{2}O (kieserite)