Thermodynamic properties of binary aqueous solutions of orthophosphate salts, sodium, potassium and ammonium at T = 298.15 K

• Thermodynamic properties of the aqueous solutions of orthophosphate salts Na, K, NH4 are investigated.
• We measure water activity of aqueous solutions at molality from dilution to saturation at 298 K.
• We determine the osmotic and activity coefficients, Gibbs energy and excess solubility.
• Solid phases, molality of saturated binary solutions ms(exp) and solubility product K°sp are determined.
• This investigation allows understanding physicochemical nature and structure of phosphate solutions.

In this investigation, the thermodynamic properties of the aqueous solutions of orthophosphate salts M3−nHnPO4 H2O with n = 0, 1, or 2 and M = Na+, K+ or NH4+ at 298.15 K are determined. The water activity measurements for NaH2PO4(aq), Na2HPO4(aq), Na3PO4(aq); KH2PO4(aq), K2HPO4(aq), K3PO4(aq), (NH4)H2PO4(aq), (NH4)2HPO4(aq) and (NH4)3PO4(aq) are effectuated from dilution to saturated solution using the hygrometric method. The activity coefficients are evaluated using ionic-interaction model. The mineral solubility of these phosphate salts is investigated at T = 298.15 K. The prediction of solubility is made for minerals and other solid phases in aqueous solutions of phosphates. Correlating and predicting the phase behavior in such systems is a difficult task, partially caused by chemical reactions in the liquid phase but also due to the possible formation of one or more solid phases. Solid phases, molality of saturated binary solutions ms(exp) and logarithm of solubility product K°sp are also determined at 298.15 K. The standard molar Gibbs energies of dissolution ΔG°diss, formation ΔGf Calc° and the literature ΔGf Ref°, of solid phases crystallizing from saturated phosphate salts solutions are evaluated.This investigation allows understanding physicochemical nature and structure of phosphate solutions, in particular, questions of anion or cation hydration, especially at higher concentrations, ion-pairing formation, and the possibility of phosphate complex formation.