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The complexation of plutonium(IV) with sulfate at variable temperatures has been investigated by solvent extraction method. A NaBr03 solution was used as holding oxidant to maintain the plutonium(IV) oxidation state throughout the experiments. The distribution ratio of Pu(1V) between the organic and aqueous phases was found to decrease as the concentrations of sulfate were increased. Stability constants of the 1 : 1 and 1:2 Pu(IV)-HS0; complexes, dominant in the aqueous phase, were calculated from the effect of [HSO;] on the distribution ratio. The enthalpy and entropy of complexation were calculated from the stability constants at different temperatures using the Van't Hoff equation.

Plutonium(IV) / Fluoride / Complexation / Stability constants / Temperature effect / Solvent extractionSummary. Complexation of Pu(IV) with fluoride was studied by solvent extraction at 25, 40 and 55 • C in 2.2 mol kg −1 HClO 4 . The distribution ratio of Pu(IV) between the organic and aqueous phases decreased as the concentration of fluoride was increased due to the formation of Pu(IV)-F complexes in the aqueous phase. Two complexes, PuF 3+ and PuF 2 2+ , were identified under the conditions in this work and their stability constants at 25, 40 and 55 • C and I = 2.2 mol kg −1 HClO 4 were determined from the distribution data. The specific ion interaction approach (SIT) was used to extrapolate the constants to the state of infinite dilution. Data from this work indicate that the complexation of Pu(IV) with fluoride is endothermic and entropy-driven. The complexation becomes stronger at higher temperatures.

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