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

A. Discussion of selected references 659of this paper was on the determination of the formation constants of the aqueous TTAcomplexes of Np(IV) and Pu(IV), respectively. In addition to the constants ∗ K 1 (A.51,M=Np)=(48 ± 2) and ∗ K 1 (A.51,M=Pu)=(83 ± 5),M 4+ + HTTA(aq) Å MTTA 3+ + H +(A.51)the complexation constants of the 1:1 and 1:2 complexes of fluoride and sulphate havebeen reported for Np(IV) and Pu(IV).From the neptunium HSO − 4experiments, sufficient data are provided to allow recalculationusing a non-linear least squares model, but the errors in log 10 β 1 and log 10 β 2are large because of the small number of data points. Because of the large errors inthese determinations, the results, although consistent with previous results, are ignoredin the calculation of best values.The constants for the reaction of Np 4+ with HF(aq), corrected for aqueousTTA complexing, are used in the selection procedure in Part III, Section 9.2.1.2with estimated uncertainties: log ∗ 10 β 1 (NpF 3+ , I = 2M) = (4.70 ± 0.15) andlog ∗ 10 β 1 (NpF 2+2, I = 2M) = (7.38 ± 0.30).In the plutonium experiments, 238 Pu or 239 Pu was used. Based on the data given inthis paper log 10 β 1 (I)(19.4) = (2.73 ± 0.18) and (2.80 ± 0.13) and log 10 β 2 (I)(19.5)= (4.66 ± 0.07) and (4.47 ± 0.08). We have not made any correction for aqueousTTA complexing of Pu(IV) on these data or any other data set used for evaluating thecomplexation constants for the sulphato complexes. The maximum effect is likely tobe approximately 6 % in β 1 (I)(19.4) or β 2 (I)(19.5) (i.e., it is within the range of thevariation of the measurements themselves). The constants for the reaction of Pu 4+ withfluoride, corrected for aqueous TTA complexing, are used in the selection procedure inPart IV, Section 18.2.1.2 with estimated uncertainties: log ∗ 10 β 1 (PuF 3+ , I = 2M) =(4.64 ± 0.15) and log ∗ 10 β 1 (PuF 2+2, I = 2M) = (7.62 ± 0.30).[76BAG/RAM2]The purpose of this study was to review earlier solvent extraction studies of Pu(IV)complexation by different authors, as well as to remeasure these systems, in particularwith regard to controlling the Pu(IV) oxidation state before and after the experiments.In the absence of a holding oxidant (which in principle may interact with other componentsin the system and thus introduce systematic errors), Pu(IV) may be reduced toPu(III) which is much less readily extractable than Pu(IV). The presence of this extraunextracted plutonium may be mistakingly interpreted as evidence for the formation ofhydrophilic complexes. Also, in earlier studies, the complex formation of Pu(IV) andTTA − in the aqueous phase was neglected. Such complexation was taken into accountin this study [76BAG/RAM2], although it should be mentioned that the correctionfactor is very small.The method used was solvent extraction, with HTTA, dinonylnaphthalenesulphonicacid (HDNNS), and tributylphosphate (TBP) as extractants, and an aqueous phasecontaining varying proton concentrations, as well as sulphate, nitrate, chloride and fluoride,respectively, as aqueous ligands. The temperature was reported as (25 ± 0.1) ◦ C