Solid–Liquid Phase Equilibria of (Mg(H₂PO₂)₂ + NaH₂PO₂ + H₂O) and (Mg(H₂PO₂)₂ + MgCl₂ + H₂O) Systems at 323.15 K

Abstract

The thermodynamics phase equilibria of the aqueous ternary systems (Mg(H2PO2)2 + NaH2PO2 + H2O) and (Mg(H2PO2)2 + MgCl2 + H2O) at 323.15K were investigated via the classical isothermal solubility equilibrium method. The solubilities and densities of the equilibrated solution were determined. The species of solid phase were confirmed by using the Scherinemakers' wet residue method and with the aid of X-ray powder diffraction testing. In accordance with the experimental data, the phase diagrams and density versus composition diagrams were constructed. Both ternary systems are of complex type with three-species solid-salts of [Na(H2PO2)2·H2O, Mg(H2PO2)2·6H2O, and double-salt NaMg(H2PO2)3], and four-species solid-salts of [hydrate Mg(H2PO2)2·6H2O, MgCl2·6H2O, dehydrate Mg(H2PO2)2, and double-salt species Mg2(H2PO2)2Cl2·6H2O] formed respectively in the corresponding system. The solubility of Mg(H2PO2)2 is very low in Na(H2PO2)2 aqueous solution but large in MgCl2 aqueous solution. Mg(H2PO2)2·6H2O will dehydrate and transform into Mg(H2PO2)2 in the concentrated MgCl2 aqueous solution.