Zirconium(IV) Tetramer/Octamer Hydrolysis Equilibrium in Aqueous Hydrochloric Acid Solution

Small-angle X-ray scattering measurements were performed on zirconium acidic aqueous solutions to investigate the structure and the size of polynuclear species larger than the previously identified tetrameric species. Solutions with [H+]added and [OH-]added ranging from 0.0 to 1.0 M and 0.0 to 0.02 M, respectively, were analyzed. This study demonstrates that an octameric species, Zr8(OH)20(H2O)24Cl12, exists in equilibrium with the tetrameric species, Zr4(OH)8(H2O)16Cl62+, such that, for 0.05 M Zr(IV) in highly acidic solutions ([H+]added ≥ 0.6 M), the tetramer is the dominant species; and at conditions with [H+]added ≤ 0.05 M, the octameric species becomes predominant. The estimated value of the equilibrium quotient obtained for the tetramer/octamer equilibrium is 0.20 ± 0.05 M3. The tetramer and octamer have radii of gyration of 3.8 and 5.1 ± 0.2 Å and a hydrolysis ratio of 2.0 and 2.5, respectively. The octamer forms by stacking two tetramers on top of each other. At conditions where small amounts of NaOH are added, unidentified polymeric Zr(IV) species larger in size than the octameric species appear.