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

A. Discussion of selected references 769were evaluated using a chemical model that included the formation of three Pu(III)fluoride complexes according to the equilibriaPu 3+ + qF − Å PuF 4−qq . (A.69)The following constants were reported at I = 1M(NaClO 4 ): log 10 β 1 (A.69, q =1) = (3.58 ± 0.08), log 10 β 2 (A.69, q = 2) = (6.40 ± 1.13), andlog 10 β 3 (A.69, q =3) = (12.61 ± 0.15). The authors reported that β 2 could be omitted without reducingsignificantly the goodness-of-fit, and they concluded that K 3 > K 2 ,whereK q is thestepwise formation constant according to the equilibriaPuF 5−qq−1 + F− Å PuF 4−qq . (A.70)However, the result of the fitting exercise suggests not only K 3 > K 2 ,butalsoK 3 > K 1 : log 10 K 1 (A.70, q = 1) = 3.58, log 10 K 2 (A.70, q = 2) = 2.82, andlog 10 K 3 (A.70, q = 3) = 6.21. As a comparison, we use the constants selected in theamericium review [95SIL/BID] for the corresponding Am(III) system (where valueshave been selected for the formation of the 1:1 and 1:2 complexes only), and wecalculate the speciation at the highest fluoride concentration used by the authors: Wefind that under these conditions only 6.5% of Pu(III) was complexed with fluoride, therest was present as the free aqua ion, Pu 3+ . Certainly, it cannot be aprioriexcludedthat a metal-ligand system behaves such that the 1:3 complex is drastically more stablethan both the 1:1 and the 1:2 complex. But it is striking that the authors found verymuch the same behaviour of Sm(III) and Bi(III), with very high stabilities of the 1:3complexes, and comparatively low stabilities for the 1:1 and 1:2 complexes. This is incontradiction to the lanthanide(III) and actinide(III) fluoride complexation behaviourfound in other studies, e.g. [61PAU/GAL, 86CHO, 92FUG/KHO, 92MIL]. On thisbasis it is not possible to accept the results of this study unless they are verified byindependent measurements.[93STA/LAR]This paper describes a study of the reaction of water vapour (at 0.02 Pa) with plutoniummetal (also see the entry for [92HAS] in this appendix). A non-equilibrium surfacelayer is formed, and the composition of the outermost 5 mm was found (using X-rayphotoelectron spectroscopy (XPS)) to have a composition near PuO 2.2 . The higheroxide can apparently be formed from water adsorbed on PuO 2 , and loses oxygen toform PuO 2 at temperatures above 350 ◦ C.[93WEG/OKA]This was a qualitative spectrophotometric study of the Pu(VI) phosphate system. Nodata on complexes or solubility of a specific solid were reported, and complicationsdue to colloid formation arose during absorbance measurements. The authors havemeasured the shift of their reported 833 nm band of Pu(VI) in solutions of varyingphosphate concentrations and for pH values of 2.7 to 11.9. It was proposed that the