Aqueous Salt Solutions


The NaNO3-Na2SO4-H2O system

Phase diagram for the NaNO3-Na2SO4-H2O system The figure to the right shows a ternary phase diagram for the NaNO3-Na2SO4-H2O system in the temperature range from -20 to 110°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 -20 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. At temperatures lower than the eutectic, an equilibrium mixture of water and the two salts will consist of the three separate phases: Na2SO4•10H2O (Glauber's salt), NaNO3, and Ice.

The ordinate in the diagram above is "Salt fraction": mol Na2SO4 divided by (mol Na2SO4 + mol NaNO3). The points in the diagram therefore mark compositions on a dry basis. The corresponding water content is not indicated. It could be marked as a third dimension. Alternatively solubility isotherms can be calculated. The temperature is held constant, and all concentrations can be shown in two dimensions.

The 30°C isotherm is marked in the diagram with a vertical brown line with the letters A, B, C, D, and E corresponding to the same points in the solubility isotherm below.

Experimental data from different sources are marked in the diagram. The experimental data come from the following sources:

Solubility isotherm at 30°C

Solubility isotherm for NaNO3 - Na2SO4 - H2O at 30°C The 30°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 solubility isotherm is marked with the letters A, B, C, D, E, corresponding to the points marked with the same letters in the figure above. The concentration unit in the diagram to the left is weight percent. The green lines in the diagram are tie lines, marking the borders of fields yielding a certain solid salt. Experimental data from two of the sources mentioned above are shown in the figure.


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