Study of metastable equilibria in the ternary system NH4H2PO4–(NH4)2HPO4–H2O at −7 and −9 °C

The solid–liquid phase equilibria of the ternary system H2O–NH4H2PO4–(NH4)2HPO4 were studied under air pressure using a synthetic method based on conductivity measurements. Two isotherms were set at −7 and −9 °C. Two branches of solubility, stable and metastable, have been identified. The solid phases which lay out stable equilibria with the liquid are: α-NH4H2PO4, β-NH4H2PO4, (NH4)2HPO4·2H2O and ice. Only (NH4)2HPO4, which is a solid phase, makes up a metastable equilibrium at this temperature.