Experimental Study of the \({\text{Mo}}{{{\text{O}}}_{{\text{2}}}}{\text{Cl}}_{{2\left( {{\text{aq}}} \right)}}^{^\circ }\) Stability in Hydrothermal Solutions at 100–350°C and Saturated Vapor Pressure

The solubility of crystalline MoO₃ in HCl solutions with variable concentration was investigated at 100, 155, 200, 250, 300, 350°C and saturated vapor pressure. The results showed that the MoO₃ solubility increases with increasing HCl concentration. Using the OptimA program, the Gibbs energies of MoO₂Cl₂ complex have been determined. The stability constants of MoO₂Cl₂ are calculated according to the reaction:

$${\text{Mo}}{{{\text{O}}}_{{{\text{3(c)}}}}} + 2{\text{HC}}{{{\text{l}}}_{{{\text{(aq)}}}}} \to {\text{Mo}}{{{\text{O}}}_{{\text{2}}}}{\text{Cl}}_{{2{\text{(aq)}}}}^{^\circ } + {{{\text{H}}}_{{\text{2}}}}{{{\text{O}}}_{{{\text{(l)}}}}}.$$

The pK values are 1.07 ± 0.29; 1.06 ± 0.49; 1.74 ± 0.71; 1.83 ± 0.47; 1.50 ± 0.28; 0.95 ± 0.57, respectively, at 100, 155, 200, 250, 300, 350°C (saturated vapor pressure).