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

A. Discussion of selected references 727to be properly measured hydrogen ion activities, were incorporated by the authors toobtain mixed constants, log10 ∗β◦∗1=−(10.4±0.4) and log10 β◦ 2 =−(22.0±0.5),wherethe increased uncertainties have been estimated in the present review, in part to reflectthe difficulties in estimating the activity coefficients of the neptunium species.In the absence of other evidence confirming its existence, the proposed speciesNpO + 2 ·H 3O + is not credited in the present review. Although complexes of cationswith NpO + 2exist, this particular species, for which the authors propose a formationconstant of 10 4 , is unlikely to have escaped detection in studies of the potential of theNpO + 2 /NpO2+ 2couple.[88CAN]The values for the formation constants of the mono- and bis-carbonato complexes ofPu(III), log 10 β 1 = 7.5andlog 10 β 2 = 12.4, were calculated from an empirical powerseries function of the ionic radii of the trivalent lanthanides. There are no other experimentalor any other empirical data concerning these complexes in the literature.Therefore these results may be useful to some investigators. However, we cannot recommendthese results.[88KLE/KIM]The work involved a study of Pu(IV) at 3.3×10 −7 Min1MNa 2 CO 3 ,pH= 11.2. Themain adsorption band is at 484 nm, similar to that found in more concentrated solutions[85EIS/KIM, 86LIE/KIM, 92CAP]. There was no attempt made to identify the speciesresponsible for this absorption band.[88NAG/TAN]The results of this study are in qualitative agreement with other recent work on Np(V).However, using pK w =−13.93 and −13.8 for the 0.005 M and 0.1 M ionic strengthmedia, respectively, the values −8.2and−7.8 are calculated for log10 ∗β1.Thesevaluesare considerably less negative than found in all of the other studies. Results from thestudies of Schmidt et al. [80SCH/GOR] and Kraus and Nelson [48KRA/NEL] wouldhave been significantly different had the first hydrolysis constant been this large. Thevalues from [88NAG/TAN] are rejected in the present review.[88NAK/ARI]The solubility of NpO 2 OH(am) was measured as a function of pH. Solutions wereequilibrated for periods from 1 day to 2 months and then filtered through a 10000 MWcutoff filter. There is no discussion about how the glass electrode was calibrated.The rather scattered solubility results were similar for solutions that had equilibrated(T = ?) for 1 week and 2 months, indicating equilibrium had been established. Thehigh solubilities for pH values above 9 suggest the presence of carbonate in the solutionsdespite the precautions taken by the authors. The CO 2 concentration in the