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

Potentiometric titrations of uranyl(VI) solutions were conducted using a standard glass/calomel electrode combination over the pH range 3 to 12 at 0.1 mol-kg(-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, UO22+, (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-Huckel treatment, the polynuclear species indicated above were assigned overall formation constants at 25 degrees 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.