Phase equilibria in aqueous sodium phosphate solutions at elevated temperatures

Abstract

Solubility and phase studies for the system Na₂O–P₂O₅–H₂O have been carried out at 251 and 300 °C and at the vapour pressures of the solutions. The experiments have been largely confined to the region of composition between the mol ratio Na₂O : P₂O₅= 1.8 and 10.0. At both temperatures, the more alkaline saturated solutions are in equilibrium with a solid phase, 11 Na₂O·4P₂O₅·3H₂O, a complex orthophosphate. The less alkaline saturated solutions are in equilibrium with Na₂[HPO₄] at 251 °C, and with Na₄[P₂O₇] at 300 °C. However, at the higher temperature, a region of liquid–liquid immiscibility also exists.