Phase equilibria in the Mg(NO3)2–Ln(NO3)3 (Ln = La, Ce, Pr)–HNO3 (∼21%)–H2O systems at 298.15 K and thermodynamic properties of 3Mg(NO3)2·2Ln(NO3)3·24H2O

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The solid-liquid equilibria of the Mg(NO3)2–Ln(NO3)3 (Ln=La, Ce, Pr)–HNO3–H2O systems in the (∼21%) HNO3 regions at T=298.15K were studied by using the method of isothermal solubility, the phase diagrams were obtained based on the measured data. The Schreinemaker’s wet residues method was used to determine the compositions of solid-phases. The three quaternary systems are all composed of three equilibrium solid-phases Mg(NO3)2·6H2O, 3Mg(NO3)2·2Ln(NO3)3·24H2O, Ln(NO3)3·6H2O. The new solid-phase compounds 3Mg(NO3)2·2La(NO3)3·24H2O, 3Mg(NO3)2·2Ce(NO3)3·24H2O and 3Mg(NO3)2·2Pr(NO3)3·24H2O are congruently soluble in an average medium of ∼21mass% HNO3, and they were characterized by chemical analysis, X-ray diffraction (XRD), X-ray diffraction single crystal structure analysis, and thermogravimetric/differential thermogravimetric (TG–DTG) techniques. The XRD graphs of 3Mg(NO3)2·2La(NO3)3·24H2O, 3Mg(NO3)2·2Ce(NO3)3·24H2O and 3Mg(NO3)2·2Pr(NO3)3·24H2O are identical due to their isomorphic structure. The X-ray diffraction single crystal structure analysis show that 3Mg(NO3)2·2Ln(NO3)3·24H2O (Ln=La, Ce, Pr) all belong to hexagonal systems, with space group R3, for 3Mg(NO3)2·2La(NO3)3·24H2O, a=b=1.1082±0.0009nm, c=1.7422±0.0013nm, V=1.853±0.002nm3, Dc=3.976g·cm−3, Z=3, for 3Mg(NO3)2·2Ce(NO3)3·24H2O, a=b=1.1056±0.0003nm, c=1.7438±0.0009nm, V=1.8462±0.0008nm3, Dc=3.997g·cm−3, Z=3, for 3Mg(NO3)2·2Pr(NO3)3·24H2O, a=b=1.0992±0.0009nm, c=1.7211±0.0007nm, V=1.8012±0.0013nm3, Dc=4.101g·cm−3, Z=3. The ΔsolHθm (standard molar enthalpies of solution) of the compounds 3Mg(NO3)2·2La(NO3)3·24H2O, 3Mg(NO3)2·2Ce(NO3)3·24H2O and 3Mg(NO3)2·2Pr(NO3)3·24H2O in water (aq) were measured to be (107.96±0.74)kJ·mol−1, (110.18±0.54)kJ·mol−1 and (102.96±0.55)kJ·mol−1, respectively, and their ΔfHθm (standard molar enthalpies of formation) were calculated to be −(12242.6±2.5)kJ·mol−1, −(12223.1±2.4)kJ·mol−1 and −(12232.6±2.5)kJ·mol−1, respectively.