Solubility of calcium molybdate in aqueous solutions at 573 K and thermodynamics of monomer hydrolysis of Mo(VI) at elevated temperatures

Abstract

This study presents new experimental results on solubility of synthetic crystalline calcium molybdate, CaMoO4(cr), at 573 K and pressure ~ 10 MPa in aqueous solutions of HCl (10−4–0.02 m), HClO4 (10−4–0.03 m), NaCl (0–3.82 m) and NaClO4 (0.10–1.61 m). In addition, the thermodynamic analysis of literature and own data allowed recommendations of the equilibrium constants, $$\log_{10} {{K}}^{\text {o}}$$ , of the reactions H+ + MoO4 2− = HMoO4 −, MoO3(cr) + OH− = HMoO4 −, MoO3(cr) + H2O(l) = H2MoO4(aq), and 2H+ + MoO4 2− = H2MoO4(aq) at temperatures 273–623 K and the saturated water vapor pressure, P s. Knowledge of the thermodynamic properties of the molybdate ion, MoO4 2− (this work), Ca2+, and of calcium molybdate CaMoO4(cr) allowed calculating the values of the solubility product of this phase at temperatures 273–623 K and P s. The thermodynamic modeling of solubility of CaMoO4(cr) at 573 K and P s in studied aqueous solutions (HCl, HClO4, NaCl, and NaClO4) showed that all our experimental data can be reproduced using the same set of species, which includes only the molybdate ion, MoO4 2−, and products of its hydrolysis, HMoO4 − and H2MoO4(aq). Although the formation of ion pairs involving Na+ and the molybdate and hydrogenmolybdate ions at high temperatures appears likely due to the fall of the dielectric constant of water, the current data do not support the formation of such forms of Mo(VI), at least they are never the dominating species at conditions of presented experiments. The increase of temperature leads to the expansion of the predominance field of the hydrogenmolybdate ion HMoO4 − and of H2MoO4(aq) and the decrease of the relative share of MoO4 2−.