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

The hydrolysis of uranium(VI) in tetraethylammonium perchlorate (0.10 mol dm-3 at 25 °C) was\nstudied at variable temperatures (10−85 °C). The hydrolysis constants (*βn,m) and enthalpy of hydrolysis\n(ΔHn,m) for the reaction mUO22+ + nH2O = (UO2)m(OH)n(2m-n)+ + nH+ were determined by titration\npotentiometry and calorimetry. The hydrolysis constants, *β1,1, *β2,2, and *β5,3, increased by 2−5 orders of\nmagnitude as the temperature was increased from 10 to 85 °C. The enthalpies of hydrolysis, ΔH2,2 and\nΔH5,3, also varied: ΔH2,2 became more endothermic while ΔH5,3 became less endothermic as the\ntemperature was increased. The heat capacities of hydrolysis, ΔCp(2,2) and ΔCp(5,3), were calculated to be\n(152 ± 43) J K-1 mol-1 and −(229 ± 34) J K-1 mol-1, respectively. UV/Vis absorption spectra supported\nthe trend that hydrolysis of U(VI) was enhanced at elevated temperatures. Time-resolved laser-induced\nfluorescence 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\nstudy.