The solubilities and solubility products of zirconium hydroxide and oxide after aging at 278, 313, and 333 K

The solubilities of zirconium hydroxide and oxide after aging at 278, 313, and 333 K were measured at 278, 298, 313, and 333 K in the pH_c range of 0.3-7 in a 0.5 M ionic strength solution of NaClO₄ and HClO₄. Size distributions of the colloidal species were investigated by ultrafiltration using membranes with different pore sizes, and the solid phases were examined by X-ray diffraction. The apparent solubility of zirconium amorphous hydroxide (Zr(OH)₄(am)), prepared by the oversaturation method, decreased with increasing aging temperature (T_a), and the size distributions obtained after aging at elevated temperatures indicated the growth of the colloidal species. We, therefore, suggested that agglomeration of the colloidal species and dehydration and crystallization of Zr(OH)₄(am) as the solubility-limiting solid phase occurred over the course of aging at elevated temperatures. For sample solutions of the crystalline oxide (ZrO₂(cr)), the aging temperature had no significant effect on the solubility, but the solubility data at lower temperatures were found to be slightly higher than those at higher temperatures, implying a small fraction of the amorphous components. In the analysis of different solid phases (Zr(OH)₄(s,T_a), T_a = 278, 313, and 333 K) depending on the aging temperatures, the solubility products (K_sp, T_a) were determined at different measurement temperatures, and the enthalpy change (Δ_rH ^circle) for Zr⁴⁺ 4OH^- <=> Zr(OH)₄(s,T_a) was calculated using the van't Hoff equation. The solid-phase-transformation process at elevated temperatures was also analyzed based on the obtained K_sp, T_a and Δ_rH ^circle values.