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The NaNO₃-Na₂SO₄-H₂O system

The figure below 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: 

• Ice
• Na₂SO₄•10H₂O (Glauber salt)
• Na₂SO₄,(Sodium sulfate)
• NaNO₃•Na₂SO₄•H₂O (Darapskite)
• NaNO₃ (Sodium nitrate)

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 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 marked. 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.

Solubility isotherm at 30°C

The 30°C solubility isotherm is shown in the figure below. 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 below is weight percent. The green lines in the diagram are tie lines, marking the borders of fields yielding a certain solid salt.

• The solubility line from A to B mark solutions in equilibrium with glauber salt. A mixture with a gross composition in the field marked I, will at equilibrium yield solid glauber salt and a saturated liquid on AB. 
• The solubility line from B to C mark solutions in equilibrium with anhydrous sodium sulfate. A mixture with a gross composition in the field marked II, will at equilibrium yield solid anhydrous sodium sulfate and a saturated liquid on BC
• CD mark saturated solutions in equilbrium with darapskite
• DE mark saturated solutions in equilibrium with sodium nitrate

• A solution with a gross composition in the field marked V will at equilibrium yield a saturated liquid of composition B and the two solid phases: glauber salt and anhydrous sodium sulfate.

• A solution with a gross composition in the field marked VI will at equilibrium yield a saturated liquid of composition C and the two solid phases: anhydrous sodium sulfate and darapskite.

• A solution with a gross composition in the field marked VII will at equilibrium yield a saturated liquid of composition D and the two solid phases: darapskite and sodium nitrate.