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The Trisodium Phosphate - Water binary system
In the trisodium phosphate - water system, one eutectic point and threeperitectic points are found. These points are shown in the figure below.
The eutectic point is the cryohydratic point, where ice and Na3PO4·12H2O both are in equilibrium with the same liquid. According to the calculated phase diagram shown above, at temperatures between 54 and 100°C, the octahydrate, Na3PO4·8H2O is the stable solid phase in equilibrium with saturated aqueous solutions. From 100 to 105°, the hexahydrate, Na3PO4·6H2O appears. The hemihydrate Na3PO4·½H2O is the stable phase from 105°C to 165°C. Above this temperature, the anhydrous form Na3PO4 is the stable phase. The solubility of the hemihydrate 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 solubility of trisodium phosphate should be examined more carefully in this temperature range.
The Disodium Phosphate - Water binary system
The phase diagram for the disodium phosphate - water system is shown below:
At temperatures below 35°C, the dodecahydrate, Na2HPO4·12H2O is the stable solid, at higher temperatures a heptahydrate and a dihydrate appear.
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 below:
The circles mark experimental data from the open literature. The line is the solubility line calculated with the Extended UNIQUAC model.
The modeling of phosphate solubility reported at this site was sponsored by the project "Working up phosphate from ashes", ForskEL project nr. 2008-1-0111