chemical thermodynamics of neptunium and plutonium - U.S. ...

18.2 Aqueous plutonium group 17 (halogen) complexes 387The Gibbs energy of formation is calculated using the selected values for Pu 4+ andBr − . f G ◦ m (PuBr3+ , aq, 298.15 K) =−(591.0 ± 3.2) kJ·mol −1The 1:2 complex, PuBr 2+2, seems to be more stable than its homologue inthe Pu(IV) chloride system, cf. Danesi, Orlandini and Scibona [66DAN/ORL](Appendix A). However, β 2 is small and there is no evidence for the existence of the1:2 complex. We hence do not select any value for the formation constant of the 1:2complex.18.2.3.3 Aqueous Pu(V) and Pu(VI) bromidesNo aqueous species of the form PuO 2 Br 1−qqidentified.18.2.4 Aqueous plutonium iodine complexes18.2.4.1 Aqueous plutonium iodidesor PuO 2 Br 2−qq , respectively, have beenStudies with the purpose of investigating the stability of iodide complexes of plutoniumare not available. However, in an investigation of Pu(III) thiocyanate complexes[74KHO/MAT] (cf. Appendix A), iodide was used as a holding reductant forplutonium, and the formation of Pu(III) iodide complexes had to be taken into accountquantitatively in the evaluation of the extraction data. Hence, Khopkar and Mathur[74KHO/MAT] carried out a separate solvent extraction experiment at 30 ◦ CandI = 1M(NH 4 ClO 4 ) to investigate the complexation behaviour of Pu(III) with iodide.They found evidence for the formation of only one complex up to an iodide concentrationof 0.7 M:Pu 3+ + I − Å PuI 2+ (18.17)They reported an equilibrium constant of log 10 β 1 (18.17, I = 1M, 303.15 K) =(0.04 ± 0.03). As an attempt to extrapolate this constant to I = 0, we can use the sameestimation for the ionic strength dependence as for the Pu(III)-SCN system, namelyε(18.17) =−0.15 kg·mol −1 . In this way we obtain log 10 β1 ◦ (18.17, 303.15 K) =1.09. This constant is of the expected order of magnitude. In spite of the paucityof data, we feel that we can recommend this value as a guideline, assuming that thetemperature dependence of log 10 β 1 (18.17) is negligible:log 10 β1 ◦ (18.17, 298.15 K) = 1.1 ± 0.4We assign a large uncertainty to reflect that only a single determination is availableand that the ionic strength extrapolation is based on estimations only.18.2.4.2 Aqueous plutonium iodatesNo aqueous complexes of iodate with any oxidation state of plutonium have been identified.