Aqueous Salt Solutions

 

The NaCl-NaNO3-H2O system

Phase diagram for the aqueous sodium chloride - sodium nitrate systemThe figure to the right shows a ternary phase diagram for the NaCl-NaNO3-H2O system in the temperature range from -30 to 110°C. The equilibrium compositions were calculated using the Extended UNIQUAC model. Experimental points are marked in the 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 -30 to 110°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 (-25.5°C). At temperatures lower than the eutectic, an equilibrium mixture of water and the two salts will consist of separate phases of hydrohalite, sodium nitrate, and Ice.

Solubility isotherm at -10°C

Solubility isotherm in the aqueous sodium chloride - sodium nitrate system at -10°C The -10°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 used in the figure is weight percent. The green lines in the diagram are tie lines, marking the borders of fields yielding a certain solid salt.

The phase diagram shows that ice will form in dilute solutions (above the upper equilibrium line). The lower equilibrium line consists of two branches. The left branch represents solutions saturated with hydrohalite (NaCl·2H2O). The right branch represents solutions saturated with sodium nitrate (NaNO3).