Aqueous Salt Solutions

 

The P2O5 - Na2O - H2O system

Solid phases in the P2O5 - Na2O - H2O system

A large number of different solid phases can form in the P2O5 - Na2O - H2O system. Many of the solid phases reported in the literature are mapped in the triangular diagram below. The red dots in the diagram correspond to the compositions of the various solid phases. The three corners of the diagram represent P2O5, H2O, and Na2O. All the solids mapped in the diagram can be formed by a combination of two or three of the components P2O5, H2O, and Na2O.

All the solids mapped in the diagram below appear at specific concentration and temperature ranges in the P2O5 - Na2O - H2O system.

The phase diagram for the system at 25°C is shown below. The abscissa in the diagram is the Na2O fraction. The ordinate is the number of moles of water per mole of salt. The dashed blue lines are tie-lines connecting the saturated liquid with the corresponding solid. Only the tie-lines marking the transition to another stable solid phase are shown.

Phosphoric acid is very soluble in water. In the left side of the diagram it can be seen that four to five moles of water are required to dissolve one mole of phosphoric acid at 25°C. The phosphates of sodium become gradually less soluble towards the basic side. The least soluble sodium phosphate compound is the basic salt Trisodium Phosphate Dodecahydrate or Sodium Orthophosphate Dodecahydrate, 4(Na3PO4·12H2O)·NaOH. It requires up to 60 moles of water per mole of salt to dissolve.

Experimental data are marked with circles, while the lines mark the solubility calculated with the Extended UNIQUAC thermodynamic model for electrolyte solutions.

The 60°C diagram for the same system is shown below. The solubility of sodium phosphate salts increase significantly with temperature. At 60°C, less than 20 moles of water are required to dissolve one mole of Sodium Orthophosphate Dodecahydrate.

To the left in the diagram, at zero Na2O content, the solution consists of water and pure phosphoric acid, H3PO4. The melting point of pure H3PO4 is approximately 40°C. The solid-liquid equilibrium line therefore does not start in the 60°C phase diagram until there is enough sodium content for NaH2PO4·H3PO4 to form.

Phase diagram for the P2O5-Na2O-H2O system at 25°C

Phase diagram for the P2O5-Na2O-H2O system at 60°C