Complexation of plutonium(IV) with fluoride at variable temperatures

Complexation of Pu(IV) with fluoride was studied by solvent extraction at 25, 40 and 55 C in 2.2 mol kg-1 HClO4. 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, PuF3+ and PuF22+, 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 HClO4 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. (orig.)