Study on the phase equilibrium of the quaternary systems Na+ (K+)//H2PO4−, Cl−, SO42−–H2O at 313.15 K

• Equilibria data of NaCl + NaH2PO4 + Na2SO4 + H2O system at 313.15 K was obtained.
• Equilibria data of KH2PO4 + KCl + K2SO4 + H2O system at 313.15 K was obtained.
• Dry-salt phase diagram and relevant water-phase diagram were plotted.
• Diagrams of physico-chemical properties (nD, η, ρ, pH) vs composition were plotted.

The solid–liquid equilibrium of quaternary systems (NaCl + NaH2PO4 + Na2SO4 + H2O) and (KH2PO4 + KCl + K2SO4 + H2O) at 313.15 K was studied by an isothermal solution saturation method and the solubilities of salts and equilibrium solid phases in the two systems were determined. Based on the measured data, dry-salt phase diagrams and the relevant water-phase diagrams were plotted in the two systems. Experimental results indicate that there are one invariant point, three univariant curves and three fields of crystallization in the two quaternary systems. For the quaternary system (NaCl + NaH2PO4 + Na2SO4 + H2O) at 313.15 K, the composition of the solution at equilibrium corresponding to invariant point is NaH2PO4 45.23 wt%, Na2SO4 4.15 wt%, NaCl 6.54 wt%. Three isotherm curves represent the cosaturated process of two salts (NaCl + Na2SO4), (NaH2PO4·2H2O + NaCl) and (NaH2PO4·2H2O + Na2SO4). For the quaternary system (KH2PO4 + KCl + K2SO4 + H2O) at 313.15 K, invariant point is saturated with the three salts (KCl + K2SO4 + KH2PO4) and the composition of the corresponding liquid phase is w (KCl) = 25.39 wt%, w (K2SO4) = 1.42 wt%, w (KH2PO4) = 4.38 wt%. Three isotherm curves represent the cosaturated process of two salts (KCl + K2SO4), (KCl + KH2PO4) and (K2SO4 + KH2PO4). Neither complex salt nor solid solution is found in the two systems at 313.15 K. The physicochemical properties (nD, η, ρ, pH) of equilibrium quaternary system changed regularly with the concentration change of the liquid phase. All results obtained in this experiment can be used for the solvent extraction process of KH2PO4 in the industrial production and further theoretical studies.

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