Hydrolysis and Complex Formation of Zr and Hf in Aqueous Solutions of HClO4, HCl, and NaOH in Equilibrium with Baddeleyite (Zr and Hf)O2(cr) at 250°C

Abstract

The equilibrium concentrations of Zr and Hf in aqueous solutions of HClO₄ {0.8–1.08 mol (kg H₂O)⁻¹}, HCl {0.1–1.05 mol (kg H₂O)⁻¹}, and NaOH {0.24–1.02 mol (kg H₂O)⁻¹} equilibrium with baddeleyite (Zr and Hf)O₂(cr) are determined according to solubility at 250°C and the saturated vapor pressure of the solutions. It is found that hydroxo complexes \({\text{Me}}({\text{OH}})_{3}^{ + }\), \({\text{Me}}({\text{OH}})_{4}^{0}\), \({\text{Me}}({\text{OH}})_{5}^{ - }\), and \({\text{Me}}({\text{OH}})_{6}^{{2 - }}\) are the main products of the hydrolysis of Zr and Hf. In solutions of HCl, the growth in the solubility in comparison with solutions of HClO₄ is due to the formation of hydroxo chloride complexes \({\text{Me}}{{({\text{OH}})}_{{\text{2}}}}{\text{Cl}}_{3}^{ - }\), while for Hf, the reaction proceeds up to the formation of \({\text{HfOHCl}}_{4}^{ - }\). The stability of the complexes of Hf is higher when compared to Zr. This is especially true for neutral complexes \({\text{Me}}({\text{OH}})_{4}^{0}\), the stability of which differs by an order of magnitude, which can lead to the fractionation of these elements in the region of pH from weakly acidic to weakly alkaline. The thermodynamic properties of the complexes of Zr and Hf are determined.