Solid–liquid equilibrium of quaternary system Na+//H2PO4−, Cl−, SO42−–H2O at 298.15 K

• The solid–liquid equilibrium of the quaternary system Na+//H2PO4−, Cl−, SO42−–H2O at 298.15 K was investigated.
• The quaternary equilibrium data of Na+//H2PO4−, Cl−, SO42−–H2O at 298.15 K were obtained.
• The quaternary phase diagram and water-content diagram were constructed based on the measured solubility data.
• The diagrams of the physic-chemical properties (nD, κ, pH, η, ρ) vs the mass fraction of NaH2PO4 in solution were plotted.

The sodium dihydrogen phosphate is one important chemical product, which has the widespread application in medicine, food, chemical industry, water treatment, agriculture aspects and so on. The solid–liquid phase equilibrium and physic-chemical properties of the quaternary system Na+//H2PO4−, Cl−, SO42−–H2O at 298.15 K were obtained by means of an isothermal solution method. There are four crystallization regions, one invariant point, and four univariant curves in the system. The crystallization region of NaH2PO4·2H2O (II) is the smallest, and its solubility is the biggest, which indicated that NaH2PO4·2H2O has a strong salt-out effect on Na2SO4·10H2O, Na2SO4 and NaCl. The physic-chemical properties (refractive index, conductivity, pH values, density and viscosity) of solution changed regularly with the increasing mass fraction of NaH2PO4. All results obtained in this experiment can be used for the solvent extraction process of NaH2PO4 in the industrial production and further theoretical studies.