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

A. Discussion of selected references 685and Np(OH) 4 (aq) or Np(IV) polymeric hydrolysis products do not seem to have beentreated as possible major equilibrium species. Although the value is reasonable, and isbased on a fit to the experimental transient conductivity data as a function of pH, it isconcluded in the present review that there were too many unknown (and unconfirmed)parameters in the data analysis for this value to be accepted. No blank conductivitymeasurements (in Np(V) solutions at different pH and without radiolysis) are reported.Also, there is insufficient evidence that Np(OH) 2+2is a major species of Np(IV) in thepH range of the experiment.[80SOL/ZAK]Aqueous mixtures of < 2.7 M HNO 3 and < 4.4 M acetic acid in various proportionswere extracted into n-dodecane containing 30 vol% TBP. From the molar ratios inthe organic phase, the concentrations being determined by potentiometric titration, itwas concluded that nitric acid is extracted as the species HNO 3·TBP, and acetic acidas (CH 3 COOH) 2·TBP. Equilibrium constants were calculated, assuming that aceticacid only appeared as a dimer, and that no HNO 3 ion pairs were formed. Calculateddistribution ratios for HNO 3 and acetic acid agreed well with predicted values for thismodel.In the presence of Pu(IV), it was concluded that the extracted species werePu(OH) i (NO 3 ) (4−i)·TBP or Pu(OH) 2 (CH 3 COOH) 2 . The Pu concentration was 420mg·dm −3 , i.e., about 1.8·10 −3 M, but no information is given about how the valencestate was ascertained, nor about the extraction technique used, nor the procedures forevaluation of the experimental results. With i=0, the coordination number N of Pu is10, while for i=3, N=7; the reviewer therefore doubts the formula for the extractedPu-species. Using activity factors taken from references, the “thermodynamicconstant” for the reaction Pu 4+ + 4OH − Å Pu(OH) 4 (aq) was calculated to be 3.5·10 56 . Using this value and the equilibrium constants from Solovkin [74SOL], thedistribution ratio of Pu(IV) was calculated and found to agree well with experimentaltests. The distribution measurements also yielded formation constants for the mixedhydroxy and nitrate or acetate complexes, assuming the activity coefficients are 1.00for uncharged species at all acid concentrations (see Table A.9).Table A.9: Formation constants for neutral plutonium(IV) complexes as given by[80SOL/ZAK].ComplexβPu(NO 3 ) 4 380Pu(OH)(NO 3 ) 3 4.8·10 4Pu(OH) 2 (NO 3 ) 2 1.2·10 3Pu(OH) 3 (NO 3 ) 9.8Pu(OH) 2 (CH 3 COOH) 2 93No comparison was given with other possible models, and insufficient information