Hydroxo and Chloro Complexes/Ion Interactions of Hf4+ and the Solubility Product of HfO2(am)

The solubility of HfO₂(am) was determined at different equilibration periods from the over- and undersaturation directions, in very acidic to basic solutions (0.1 m HCl to 3.2 m NaOH), and in NaCl solutions ranging in concentrations from very dilute to as high as 5.59 m and in a ${\text{p}}C_{{\text{H}} + }$ range from 1 to 4 to obtain reliable thermodynamic data for the Hf⁴⁺–Cl^?–Na⁺–H⁺–OH^?–H₂O system. The studies indicate that equilibrium is reached rapidly (<5 days) and that HfO₂(am) solubility shows amphoteric behavior. The solubility data obtained in this study, along with the data reported in the literature, at NaOH molalities as high as 21.7 m were interpreted using the ion-interaction model of Pitzer. The log K ⁰ for the solubility of HfO₂(am) HfO₂(am) + 2H₂O ? Hf⁴⁺ + 4OH^?] was determined to be ?55.1 ± 0.7. The log K ⁰ values for the formation of HfOH³⁺, Hf(OH)⁰ ₄, Hf(OH)₅ ^?, and Hf(OH)₆ ^2? according to the reaction (Hf⁴⁺ + xOH^? ? Hf(OH)^4?x _x) were determined to be 13.8, <44.8, 49.7 ± 0.2, and 51.2 ± 0.2, respectively. The thermodynamic model developed in this study is valid for a wide range of conditions (as high as 0.1 m HCl, 21.7 m NaOH, and 5.59 m NaCl). The binary ion-interaction parameters for Hf⁴⁺–Cl^?, HfOH³⁺–Cl^?, and Hf(OH)^2? ₆–Na⁺ were determined in this study to accurately define the observed solubility behavior of hafnium in various systems.