Measurements and Thermodynamic Model on the Solid–Liquid Phase Equilibria of the Ternary System KHCO3–NaHCO3–H2O at Multitemperature (288.15, 308.15, and 323.15 K)

The thermodynamic solid–liquid phase equilibria of the ternary system KHCO₃–NaHCO₃–H₂O at 288.15, 308.15, and 323.15 K were investigated using the isothermal dissolution equilibrium method. The phase diagrams of the ternary system KHCO₃–NaHCO₃–H₂O are simple, containing one invariant point, two solubility curves, and two crystallization fields (corresponding to KHCO₃ and NaHCO₃), with no double salts or solid solutions formed. As the temperature increases, the crystallization fields of both KHCO₃ and NaHCO₃ expand, but the crystallization field of NaHCO₃ always remains larger than that of KHCO₃. This suggests that in this saturated ternary system, NaHCO₃ is more prone to crystallize out. Based on the solid–liquid phase equilibrium experiments, the mixed salt Pitzer parameters (Ψ_K⁺_, Na⁺_, HCO3^–) for the ternary system KHCO₃–NaHCO₃–H₂O at 308.15 and 323.15 K were fitted and calculated. The Pitzer model was further employed to theoretically predict the solid–liquid phase equilibria of this ternary system at 288.15, 308.15, and 323.15 K. The modeling and experimental phase diagrams are in good agreement. This result validates the applicability and accuracy of the Pitzer model and the fitted mixed salt parameters for this ternary system.