Complexation of uranium(vi) with acetate at variable temperatures

Abstract

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.

Variable denticity in carboxylate binding to the uranyl coordination complexes

“…The bidentate coordination concluded for Na[UO 2 (OAc) 3 ] agreed with the previously measured X-ray crystal structure [6,19]. Interestingly, EXAFS of crystalline [UO 2 (OAc) 2 •2H 2 O] showed both monodentate and bidentate coordination: one acetate is bidentate, while the other two are bridging monodentate [7,15].…”

“…Bidentate coordination is favored in mono-acetato complexes [15], low OAc/UO 2 ratios, and low pH [16]. For bis-acetato complexes, EXAFS has also indicated bidentate coordination [17], however not all studies have reached definitive conclusion regarding these complexes [15]. Monodentate complexes tend to be formed at higher pH values and higher OAc concentrations [16].…”

“…Extended X-ray absorption fine structure spectroscopy (EXAFS) showed that monodentate and bidentate coordination in solution can be differentiated on the basis of differences in the U-O axial bond distances, and that the coordination mode was influenced by the solution chemistry and complex stoichiometry. Bidentate coordination is favored in mono-acetato complexes [15], low OAc/UO 2 ratios, and low pH [16]. For bis-acetato complexes, EXAFS has also indicated bidentate coordination [17], however not all studies have reached definitive conclusion regarding these complexes [15].…”

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

“…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

“…Water from a Milli-Q system was used in preparing all solutions. Details on the preparation and standardization of the U(VI) stock solution were provided elsewhere [7]. Gran's potentiometric method [8] was used to determine the concentration of free acid in the U(VI) stock solution.…”