Solid–Liquid Phase Equilibria of (Ca(H₂PO₂)₂ + H₂O), (Ca(H₂PO₂)₂ + CaCl₂ + H₂O), and (Ca(H₂PO₂)₂ + NaH₂PO₂ + H₂O) Systems

Abstract

Fine calcium hypophosphite can be synthesized by sodium hypophosphite and calcium chloride via the replacement reaction, and sodium hypophosphite can be synthesized from phosphorus via the intermediate of Ca(H2PO2)2. Both processes involve the interactive quaternary system of (Ca2+ + Na+)//(Cl– + (H2PO2)− + H2O) and its subsystems. In this study, the (solid + liquid) phase diagrams of the (Ca(H2PO2)2 + H2O) binary system from freezing to boiling point at normal pressure, and (Ca(H2PO2)2 + CaCl2 + H2O), (Ca(H2PO2)2 + NaH2PO2 + H2O) ternary systems at 273.15 K were experimentally determined via the classical isothermal solubility equilibrium method. It was found that (1) one anhydrous salt of Ca(H2PO2)2 occurs in (Ca(H2PO2)2 + H2O) binary systems with temperatures ranging from freezing to boiling points, (2) two solid salts of Ca(H2PO2)2 and CaCl2·6H2O exist in the (Ca(H2PO2)2 + CaCl2 + H2O) system, and (3) Ca(H2PO2)2, NaH2PO2·H2O and one incompatible double salt of NaCa(H2PO2)3 exist in the (Ca(H2PO2)2 + NaH2PO2 + H2O) system, and NaCa(H2PO2)3 has very low solubility and occupies a large solid field in the ternary system.