Hydrolysis of Uranium(VI) at Variable Temperatures (10−85 °C)

Abstract

The hydrolysis of uranium(VI) in tetraethylammonium perchlorate (0.10 mol dm^-3 at 25 °C) was studied at variable temperatures (10−85 °C). The hydrolysis constants (*β_n,m) and enthalpy of hydrolysis (ΔH_n,m) for the reaction mUO₂²⁺ + nH₂O = (UO₂)_m(OH)_n^(2m-n)+ + nH⁺ were determined by titration potentiometry and calorimetry. The hydrolysis constants, *β_1,1, *β_2,2, and *β_5,3, increased by 2−5 orders of magnitude as the temperature was increased from 10 to 85 °C. The enthalpies of hydrolysis, ΔH_2,2 and ΔH_5,3, also varied: ΔH_2,2 became more endothermic while ΔH_5,3 became less endothermic as the temperature was increased. The heat capacities of hydrolysis, ΔC_p(2,2) and ΔC_p(5,3), were calculated to be (152 ± 43) J K^-1 mol^-1 and −(229 ± 34) J K^-1 mol^-1, respectively. UV/Vis absorption spectra supported the trend that hydrolysis of U(VI) was enhanced at elevated temperatures. Time-resolved laser-induced fluorescence spectroscopy provided additional information on the hydrolyzed species at different temperatures. Approximation approaches to predict the effect of temperature were tested with the data from this study.