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

A. Discussion of selected references 687and 1.0 M. The authors corrected the resulting value of β 1 (NpI 3+ ) = 0.8 for aqueousTTA complexes and obtained β 1 (NpI 3+ ) = 1.1. There is no obvious reason to discardthe reported formation constant, and since it is not completely clear if this study isindependent of the one presented in [79KUS/GAN], we increase the uncertainty. Inour evaluation we use log 10 β 1 (NpI 3+ , I = 2M) = (0.04 ± 0.30). The authors alsoreported solvent extraction results on nitrate complexation of Np(V) at 25 ◦ C, againusing NaDNNS in benzene. The aqueous phase had a very high ionic strength (I =8.5 M) using an unspecified electrolyte (probably sodium perchlorate) at pH = 3-4. For the nitrate complex of Np(V) the value of the constant reported is exactly thesame as the one reported in [79RAO/GUD] with the same experimental conditions butI = 2 M. The measured changes in the distribution coefficient are better explained asresulting from activity coefficient changes rather than from the formation of a complex.[81RAI/SWA]The experimental work shows that the plutonium(IV) polymer seems to have solubilityproperties intermediate between those of Pu(OH) 4 and PuO 2 . The Pu(IV) polymerdoes not seem to form stable suspensions at pH values above 5.[81VOL/VIS]This work is an extension of the previous paper [81VOL/VIS4] about the exchangeof simple cations with MO + 2(V) cations. As is the case for this previous publication,[81VOL/VIS4], the discussion proposed in this paper was not supported by sufficientexperimental verification.[81VOL/VIS2]Hydrated Na 3 NpO 2 (CO 3 ) 2 (s), Na 4 NpO 2 (CO 3 ) 2.5 (s), andRb 3 NpO 2 (CO 3 ) 2 (s) wereprecipitated in (respectively) 0.3 to 1.7, 2.4 M Na 2 CO 3 and1to3.8MRb 2 CO 3solutions. The X-Ray powder diffraction patterns of these solids were obtainedand reported. This review concludes the X-ray diffraction pattern shown forNa 4 NpO 2 (CO 3 ) 2.5 (s) could be interpreted equally well as resulting from a poorlycrystallised hydrate (also see [98VIT/CAP]).[81VOL/VIS3]Crystal structures for hydrated M 3 NpO 2 (CO 3 ) 2 (s) (M = Na, K, Rb) are proposedbased on powder X-ray crystallographic data. They are not exactly isostructual, but allhave layers containing Np atoms, with the CO 2−3ions being bidentate. This Np(V)-containing layer also incorporates one third of the alkali metal cations. The rest of thealkali metal ions go into interlayer positions, and are susceptible to ion exchange (asis also the case for the Na + ions in NaNpO 2 CO 3 (s)) [79VOL/VIS2, 98VIT/CAP] (cf.the discussion of [84VIT] in this appendix).The difference in structure could explain the relatively slow transformation ofNaNpO 2 CO 3 (s) to Na 3 NpO 2 (CO 3 ) 2 (s) [66GOR/ZEN].