Dioxouranium(VI) oxalate complexes

Abstract

The complex formation between dioxouranium(VI) and oxalate ion has been investigated, at 25.00 °C, in 2 and 3 M NaClO₄, by measuring the potentials of a glass and of a Hg₂C₂O₄–Hg electrode. In the analysed solutions the upper limit of the uranium(VI) concentration did not exceed 0.03 M, the oxalate concentration was imposed by the modest solubility of its sodium salt in the ionic media (about 0.015 M in 3 M NaClO₄) and the acidity was kept at levels higher than 0.01 M in order to hinder formation of hydrolysed and possible ternary species. The measurements have been interpreted by assuming the formation of mononuclear complexes only, according to the reaction UO₂²⁺ + p C₂O₄²⁻ ⇌ UO₂(C₂O₄)_p^{(2 − 2p)+}. The equilibrium constants have been extrapolated to infinite dilution by applying the Specific Interaction Theory (SIT) and, in weight molarity (molality) units, are log ⁰β₁ = 7.38 ± 0.07, log ⁰β₂ = 11.7₂ ± 0.1, log ⁰β₃ = 13.6 ± 0.2. Under the same experimental conditions the protolysis constants of oxalic acid and several interaction coefficients, necessary for extrapolation of the results to the thermodynamic scale of activities, through the SIT, have also been determined.