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 degrees C) was studied at variable temperatures (10-85 degrees C). The hydrolysis constants (*beta(n,m)) and enthalpy of hydrolysis (Delta H(n,m)) for the reaction mUO(2)(2+) + nH(2)O = (UO(2))(m)(OH)(n)((2m-n))+) + nH(+) were determined by titration potentiometry and calorimetry. The hydrolysis constants, *beta(1,1), *beta(2,2), and *beta(5,3), increased by 2-5 orders of magnitude as the temperature was increased from 10 to 85 degrees C. The enthalpies of hydrolysis, Delta H(2,2) and Delta H(5,3), also varied: Delta H(2,2) became more endothermic while Delta H(5,3) became less endothermic as the temperature was increased. The heat capacities of hydrolysis, Delta C(p(2,2)) and Delta C(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.

Thermodynamics, Optical Properties, and Coordination Modes of Np(V) with Dipicolinic Acid

“…Procedures and results of the calibration of the calorimeter were provided elsewhere. 34 Three titrations with different concentrations of NpO 2 + (1.0 -2.5 mM) were performed to reduce the uncertainty of the results. In all titrations, 0.9 mL of NpO 2 + solution was put in the reaction cell and n additions of 0.005 mL titrant were made (n = 40 -50) through a 0.250 mL syringe, resulting in n experimental values of total heat (Q ex,j , j = 1 -n).…”

Complexation of Uranium(VI) by Gluconate in Acidic Solutions: a Thermodynamic Study with Structural Analysis

“…Details of the titration setup have been provided elsewhere. 19,20 Electromotive force (EMF, in millivolts) was measured with a Metrohm pH meter (model 713) equipped with a Ross combination pH electrode (Orion model 8102).…”

“…The details of the calorimeter were provided previously. 19 The performance of the calorimeter was tested by measuring the enthalpy of protonation of tris(hydroxymethyl)-aminomethane (THAM). The measured value, -47.7 ± 0.3 kJ·mol -1 at 25 o C, compared well with those in the literature.…”

“…The measured value, -47.7 ± 0.3 kJ·mol -1 at 25 o C, compared well with those in the literature. 19,22 Multiple titrations were conducted with different concentrations of UO 2 (ClO 4 ) 2 and HClO 4 at 25 °C. For each titration run, n experimental values of the total heat produced in the reaction vessel (Q ex,j , j = 1 -n) were calculated as a function of the mass of the added titrant.…”

Effect of Temperature on the Complexation of Uranium(VI) with Fluoride in Aqueous Solutions

“…The result was -(47.7 ± 0.2) kJ·mol -1 at 25ºC, in excellent agreement with the value in the literature. Details of the microcalorimeter are provided elsewhere [10].…”