Thermodynamic Properties of H2O–Na3PO4 and H2O–Na3PO4–K3PO4 Systems: Experimental Study and Modeling

The thermodynamic properties of aqueous solutions in the H₂O–Na₃PO₄–K₃PO₄ system were investigated, expanding on prior research on the H₂O–K₃PO₄ system. The study encompassed both the ternary system and its subsystem, H₂O–Na₃PO₄, across a wide temperature range. Experimental data on vapor–liquid equilibria in the ternary system were obtained for the first time. Water activity was determined using dew point and static vapor pressure methods. Measurements were taken at 298.15 and 323.15 K and sodium phosphate concentrations range w(Na₃PO₄) = 0.078–0.194 in the H₂O–Na₃PO₄ system and within the range of 288.15–323.15 K in the H₂O–Na₃PO₄–K₃PO₄ system. Solubilities were determined in the H₂O–Na₃PO₄–K₃PO₄ system at 298.15 K. Thermodynamic models for the H₂O–Na₃PO₄–K₃PO₄ system and its subsystems were developed. Modeling of the H₂O–Na₃PO₄–NaOH subsystem at low NaOH concentrations was conducted to account for the influence of Na⁺–PO₄^3––OH^– interactions. The combination of experimental methods and modeling has revealed the presence of two solid solutions in the system. The first is an interstitial solid solution based on sodium phosphate dodecahydrate, and the second is a substitutional solid solution based on the lattice of potassium phosphate heptahydrate.