Studies on the Hydrolytic Behavior of Zirconium

Journal article, 2004

The stability constants of zirconium(IV) hydrolysis species have been measured at 15, 25, and 35°C [in 1.0 mol-dm-3 (H,Na)ClO4] using both potentiometry and solvent extraction. In addition, the solubility of [Zr(OH)4(am)] has been investigated in a 1 moldm -3 (Na,H)(ClO4,OH) medium at 25°C over a wide range of -log [H+] (0-15). The results indicate the presence of the monomeric species Zr(OH)3+, Zr(OH)2+2, Zr(OH)+3, and Zr(OH)04(aq) as well as the polymeric species Zr4(OH)8+8 and Zr2(OH)2+6. The solvent extraction measurements required the use of acetylacetone. As such, the stability constants of zirconium(IV) with acetylacetone were also measured using solvent extraction. All stability constants were found to be linear functions of the reciprocal of temperature (in kelvin) indicating that ΔHo and ΔSo are both independent of temperature (over the temperature range examined in the study). The results of the solubility experiments have shown four distinctly different solubility regions. In strongly acidic solutions, the solubility is controlled by the formation of polynuclear hydrolysis species in solution whereas in less acidic solution the formation of mononuclear hydrolysis species becomes dominant. The largest portion of the solubility curve is controlled by equilibrium with aqueous Zr(OH)04(aq) where the solubility is independent of the proton concentration. In alkaline solutions, the solubility increases due to formation of the zirconate ion. The middle region was used to determine the solubility constant (log ∗Ks10) of Zr(OH)4(s). From the data in the alkaline region, a value of the stability of the zirconate ion has been determined. This is the first time that the possible evidence for the zirconate ion has been identified in aqueous solution that has previously been found only in the solid phase.