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+–PO43––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.