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

18.2 Aqueous plutonium group 17 (halogen) complexes 371and thusCp,m ◦ −1(PuOI, cr, 298.15 K) = (75.6 ± 10.0) J·K−1·mol18.2 Aqueous plutonium group 17 (halogen) complexes18.2.1 Aqueous plutonium fluoride complexes18.2.1.1 Aqueous Pu(III) fluoridesIn two studies from the same group [86MAH/SAW, 93SAW/MAH], data were reportedon fluoride complexation of Pu 3+ obtained by potentiometry using a fluoride ionselective electrode. In the later study [93SAW/MAH], the credibility of the earlierresult was questioned due to the evaluation method used then, which was unsuitableif higher complexes were present. The results of the recent study, however, are verysurprising as they suggest that the 1:3 complex, PuF 3 (aq), and other MF 3 (aq) complexesare by far more stable than the 1:1 and the 1:2 complexes. This is contrary tothe behaviour of Am(III) [92FUG/KHO, 95SIL/BID], and independent verification ofthe results of Sawant et al. [93SAW/MAH] is required. Appendix A contains a moredetailed discussion of this paper. No data are selected for the Pu(III) fluoride system.It is worth mentioning that the trivalent actinide ions exhibit a very similar complexationbehaviour, see for example [92FUG/KHO], which resembles that of the trivalentlanthanide ions, e.g. [86CHO, 92MIL]. Hence, the stability constants of the Pu(III) fluoridecomplexes are expected to be of magnitudes similar to those of other actinide(III)and lanthanide(III) fluoride complexes.18.2.1.2 Aqueous Pu(IV) fluoridesExperimental equilibrium data have been published for the following reactionsPu 4+ + qHF(aq) Å PuF 4−qq + qH + (18.8)Pu 4+ + qF − Å PuF 4−qq (18.9)Most experiments were carried out in strongly acidic solutions, and the relevantequilibrium is thus Reaction (18.8). The data reported in [90SAW/CHA2] are convertedto refer to Reaction (18.8) by taking the authors’ protonation constant of F − (cf.Appendix A). All reported constants are listed in Table 18.1.The values for log 10 ∗ β 1 are in good agreement, but an extrapolation to I = 0bylinear regression is not reasonable, because the available data refer to only two differentionic strength values. We use ε(9.4, q = 1) =−(0.12 ± 0.09) kg·mol −1 from thecorresponding Np(IV) system (the uncertainty increased by 0.05), cf. Section 9.2.1.2,