Determination of the equilibrium formation constants of two U(vi)–peroxide complexes at alkaline pH

Abstract

The formation of uranyl-peroxide complexes was studied at alkaline media by using UV-Visible spectrophotometry and the STAR code. Two different complexes were found at a H₂O₂/U(VI) ratio lower than 2. A graphical method was used in order to obtain the formation constants of such complexes and the STAR program was used to refine the formation constants values because of its capacity to treat multiwavelength absorbance data and refining equilibrium constants. The values obtained for the two complexes identified were: UO₂²⁺ + H₂O₂ + 4OH⁻ ⇆ UO₂(O₂)(OH)₂²⁻ + 2H₂O: log β^°_1,1,4 = 28.1 ± 0.1   (1)UO₂²⁺ + 2H₂O₂ + 6OH⁻ ⇆ UO₂(O₂)₂(OH)₂⁴⁻ + 4H₂O: log β^°_1,2,6 = 36.8 ± 0.2   (2) At hydrogen peroxide concentrations higher than 10⁻⁵ mol dm⁻³, and in the absence of carbonate, the UO₂(O₂)₂(OH)₂⁴⁻ complex is predominant in solution, indicating the significant peroxide affinity of peroxide ions for uranium and the strong complexes of uranium(VI) with peroxide.