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

654 A. Discussion of selected references[75IOR/SHC]NMR measurements (proton and 14 N resonances) in water/acetone mixtures at−105 ◦ C were reported in this paper. The authors detected the first nitrate complexfor Np(VI)/water ratios < 185 (approximately 0.3 M NpO 2+2if in solution in wateronly). The second complex was detected for Np(VI)/water ratios of 40-150. No nitrateconcentrations nor formation constants were reported.[75PAT/RAM]The authors used a solvent extraction technique with HTTA or dinonylnaphthalenesulphonicacid (HDNNS) in benzene. The aqueous phase contained 2 M H + , varyingconcentrations of Cl − and HF(aq), respectively, and ClO − 4as a supporting electrolyteto maintain I = 2 M. In the chloride complexation experiments with Np(IV), Fe(II)sulphamate was used to stabilise the oxidation state of neptunium. The temperature waskept constant during the experiments at 25 ◦ C. The experimental procedure seems satisfactory.However, no check was made to verify the proper oxidation state of Np(IV)at the end of the experiments.For the Np(IV) chloride system,Np 4+ + qCl − Å NpCl 4−qq , (A.49)the authors reported β 1 (A.49, q = 1) = (0.9±0.1) and β 2 (A.49, q = 2) = (0.7±0.1).Giffaut [94GIF] has re-evaluated Patil and Ramakrishna’s experiments and obtainedslightly different values due to improvement of the fit in the lower range of the chlorideconcentrations: β 1 (A.49, q = 1) = (1.0 ± 0.1) and β 2 (A.49, q = 2) = (0.6 ±0.1). We use these results in our selection procedure, but we increase the uncertainties:log 10 β 1 (A.49, q = 1) = (0.00 ± 0.20) and log 10 β 2 (A.49, q = 2) =−(0.22 ± 0.40).For the Np(IV) fluoride system,Np 4+ + qHF(aq) Å NpF 4−qq + qH + (A.50)the authors reported log 10 ∗ β 1 (A.50, q = 1) = (4.62 ± 0.01) from HDNNS extraction.The evaluation of the HTTA extraction studies was done with a one- and a two-complexmodel, resulting in log 10 ∗ β 1 (A.50, q = 1) = (4.72 ± 0.01) using only the points at[HF(aq)] < 3 × 10 −4 M, and log 10 ∗ β 1 (A.50, q = 1) = 4.60 and log 10 ∗ β 2 (A.50, q =2) = 7.49 using all the points up to [HF(aq)] =10 −3 M. The uncertainties given bythe authors are grossly underestimated. We use the independent results from this study,i.e., log 10 ∗ β 1 from the HDNNS extraction and log 10 ∗ β 1 and log 10 ∗ β 2 from the HTTAextraction, and we assign uncertainties of ±0.20 (log 10 ∗ β 1 )and±0.40 (log 10 ∗ β 2 ).[75RAG/RAM]The authors used a solvent extraction technique with HTTA or dinonylnaphthalenesulphonicacid (HDNNS) in benzene to study the bromide complexation of Th 4+ ,Np 4+and Pu 4+ . Vanadate was used as a holding oxidant for Pu 4+ rather than nitrite as usedearlier [73PAT/RAM2], because nitrite was found to oxidise bromide to bromine under