The complexation between uranium() and acetate in 1.05 mol kg Ϫ1 NaClO 4 was studied at variable temperatures (25, 35, 45, 55 and 70 ЊC). The formation constants of three successive complexes, UO 2 (OOCCH 3 ) ϩ , UO 2 (OOCCH 3 ) 2 and UO 2 (OOCCH 3 ) 3 Ϫ , and the molar enthalpies of complexation were determined by potentiometry and calorimetry. Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) provided structural information to identify the coordination modes of the acetate in the complexes in solution, which helped to interpret the trends in the enthalpy and entropy of the complexation. The effect of temperature on the stability of the complexes is discussed in terms of the electrostatic model.

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.

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In acidic aqueous solutions, gluconate protonation is coupled with lactonization of gluconic acid. With the decrease of pC H , two lactones (δ/γ) are sequentially formed. The δ-lactone forms more readily than the γ-lactone. In 0.1 M gluconate solutions, if pC H is above 2.5, only the δ-lactone is generated. When pC H is decreased below 2.0, the formation of the γ-lactone is observable although the δ-lactone predominates. At I = 0.1 M NaClO 4 and room temperature, the deprotonation constant of the carboxylic group, using the NMR technique, was determined to be log K a = 3.30 ± 0.02; the δ-lactonization constant, by the batch potentiometric titrations, was obtained to be log K L = -(0.54 ± 0.04). Using ESI-MS, the rate constants of the δ-lactonization and the hydrolysis at pC H ~ 5.0 were estimated to be k 1 = 3.2 x 10 -5 s -1 and k -1 = 1.1 x 10 -4 s -1 , respectively.

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The complexation of uranium(VI) and samarium(III) with oxydiacetate (ODA) in 1.05 mol kg(-1) NaClO(4) is studied at variable temperatures (25-70 degrees C). Three U(VI)/ODA complexes (UO(2)L, UO(2)L(2)(2-), and UO(2)HL(2)(-)) and three Sm(III)/ODA complexes (SmL(j)((3-2)(j)+) with j = 1, 2, 3) are identified in this temperature range. The formation constants and the molar enthalpies of complexation are determined by potentiometry and calorimetry. The complexation of uranium(VI) and samarium(III) with oxydiacetate becomes more endothermic at higher temperatures. However, the complexes become stronger due to increasingly more positive entropy of complexation at higher temperatures that exceeds the increase in the enthalpy of complexation. The values of the heat capacity of complexation (Delta C(p) degrees in J K(-1) mol(-1)) are 95 +/- 6, 297 +/- 14, and 162 +/- 19 for UO(2)L, UO(2)L(2)(2-), and UO(2)HL(2)(-), and 142 +/- 6, 198 +/- 14, and 157 +/- 19 for SmL(+), SmL(2)(-), and SmL(3)(3-), respectively. The thermodynamic parameters, in conjunction with the structural information from spectroscopy, help to identify the coordination modes in the uranium oxydiacetate complexes. The effect of temperature on the thermodynamics of the complexation is discussed in terms of the electrostatic model and the change in the solvent structure.

Uranium(VI) / Malonate / Complexation / Temperature effect / Stability constants / Enthalpy / EntropySummary. The complexation between uranium(VI) and malonate in 1.05 mol kg −1 NaClO 4 was studied at variable temperatures (25,35, 45, 55 and 70 • C). The formation constants of three successive complexes, UO 2 (OOCCH 2 COO), UO 2 (OOCCH 2 COO) 2 2− and UO 2 (OOCCH 2 COO) 3 4− , and the molar enthalpies of complexation were determined by potentiometry and calorimetry. The heat capacity of the complexation, ∆C o p,m(ML j ) , is calculated to be 96 ± 12, 195 ± 15 and 267 ± 22 J K −1 mol −1 for j = 1, 2 and 3, respectively. Extended X-ray Absorption Fine Structure Spectroscopy helped to characterize the coordination modes in the complexes in solution. UV/Vis absorption and luminescence spectra at different temperatures provided qualitative information on the temperature effect. The effect of temperature on the complexation between uranium(VI) and malonate is discussed in terms of the electrostatic model and compared with the complexation between uranium(VI) and acetate.