Aqueous Salt Solutions


The trisodium phosphate - water binary system

Phasediagram for the trisodium phosphate - water system

In the phase diagram for the Na3PO4, trisodium phosphate - water system shown to the right, one eutectic point and four peritectic points are found. The eutectic point is the cryohydratic point, where ice and Na3PO4·12H2O both are in equilibrium with the same liquid.

The phase diagram consists of six different branches including ice, four hydrated forms and anhydrous trisodium phosphate.

Four peritectic points mark the transitions

Above this temperature, the anhydrous form Na3PO4 is the stable phase. The solubility of the hemi-hydrate and the anhydrous form decrease with increasing temperature. There is some discrepancy between experimental solubility data from different sources and between model calculations and experimental data, especially in the temperature range from 100 to 120°C.

The disodium phosphate - water binary system

Next, the phase diagram for the disodium phosphate - water system is shown.  The stable phases at the eutectic point are ice and disodium phosphate dodecahydrate, Na2HPO4·12H2O.

This phase diagram consists of four different branches including ice, dodeca-, hepta- and di-hydrate of disodium phosphate.

Two peritectic points mark the following transitions:

The monosodium phosphate - water binary system

In the monosodium phosphate - water system, three solid phases besides ice can be in equilibrium with saturated solutions: a dihydrate, monohydrate and the anhydrous form. The phase digram is shown at last.

The stable phases at the eutectic point are ice and monosodium phosphate, dihydrate,  NaH2PO4·2H2O.

The phase diagram indicates two peritectic points:

In all three figures, the circles mark experimental data from the open literature. Lines mark the solubility calculated with the Extended UNIQUAC model.

Phase diagram for the Disodium phpshate water system Phase diagram for the mono sodium phosphate - water system