Aqueous uranyl complexes. 3. Potentiometric measurements of the hydrolysis of uranyl(VI) ion at 25°C

Potentiometric titrations of uranyl(VI) solutions were conducted using a standard glasslcalomel electrode combination over the pH range 3 to 12 at 0.1 molkg^–1 ionic strength with tetramethylammonium trifluoromethanesulfonate as the supporting electrolyte. The electrodes were calibrated directly on the hydrogen ion concentration scale during the initial stage of each titration. The species, UO ₂ ²⁺ , (UO₂)₂(OH) ₂ ²⁺ , (UO₂)₃(OH) ₅ ⁺ , (UO₂)₃(OH) ₇ ^– , (UO₂)₃(OH) ₈ ^2– , and (UO₂)₃(OH) ₁₀ ^4– identified in an earlier Raman study were compatible with the analysis of the titration data. Based on this analysis and application of the extended Debye-Hückel treatment, the polynuclear species indicated above were assigned overall formation constants at 25°C and at infinite dilution of –5.51±0.04, –15.3±0.1, –27.77±0.09, –37.65±0.14, and –62.4±0.3, respectively. The results are discussed in reference to hydrolysis quotients reported in the literature for the first three species. Formation quotients for the last two species have not been reported previously.