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

A. Discussion of selected references 677−(395.66 ± 0.5) kJ·mol −1 , and these values are used in the present calculations.Then, using f H ◦ m (H 2O, 6MHCl, 0.04 M FeCl 2 , 0.004 MNa 2 SiF 6 ) =−(286.65 ±0.04) kJ·mol −1 and f H ◦ m (HCl, 6MHCl, 0.04 M FeCl 2, 0.004 M Na 2 SiF 6 ) =−(153.40 ± 0.11) kJ·mol −1 (both values from [2000RAN/FUG] based onParker [65PAR] for 6 M HCl solutions), f H ◦ m (NpCl 4, 6MHCl, 0.04 M FeCl 2 ,0.004 M Na 2 SiF 6 ) =−(1148.2 ± 2.2) kJ·mol −1 . Hence, f H ◦ m (NpO 2(OH) 2 (s)) =−1363 kJ·mol −1 and f H ◦ m (Np 2O 5 (s)) =−2142 kJ·mol −1 .The reported analyses suggest the composition of the “Np 2 O 5 ” used in thecalorimetry experiments was actually closer to NpO 2.59 . Ifthisisassumedtobeso, f H ◦ m (NpO 2.59(s)) =−1086 kJ·mol −1 .[79CHI/TAL]The authors measured the influence of fluoride concentration on the equilibrium of thePu(IV)/Pu(III) redox couple. The experiments were performed at “room temperature”,reported as ∼25 ◦ C, and at I = 1M(HClO 4 ). Chitnis et al. [79CHI/TAL] assumedthat only Pu 4+ was complexed by fluoride, and that fluoride complexing of Pu 3+ wasnegligible. This assumption appears reasonable. The procedure is satisfactory, and theauthors used a two-complex model to interpret the data. The formation constant of the1:1 complex, PuF 3+ , in acidic solution is reported as log ∗ 10 β 1 (PuF 3+ , I = 2M) =(4.66 ± 0.07), to which we assign an uncertainty of ±0.20. The formation constant ofthe 1:2 complex, PuF 2+2, has a greater uncertainty, because it formed to less than 30%during the experiments. We accept the reported value with an increased uncertainty:log ∗ 10 β 2 (PuF 2+2, I = 2M) = (7.32 ± 0.40).[79FED/PER]a) Np(V)/Np(IV) redox potentialThe formal potential of the Np(V)/Np(IV) and Np(IV)/Np(III) redox couples weremeasured respectively by potentiometry, probably at a mercury electrode, and in 1 MK 2 CO 3 solution by polarography. There is not much experimental detail. The Np(IV)carbonate solution was prepared by adding solid oxalate. As expected the attainmentof the Np(V)/Np(IV) equilibrium was slow. The authors waited seven hours, but thismay not have been long enough for this type of equilibrium and for this methodology.The authors indicate it was difficult to obtain a stable measurement using theelectrode: “... only an extremely approximate value close to 0.1 V” was finally proposed.This value was confirmed by Varlashkin et al. [84VAR/HOB] (with no betterprecision for the same reason), and again recently by Vitorge and his co-workers[95VIT, 96DEL/VIT, 98VIT/CAP]. Thus any thermodynamic data calculated fromthis will have a very large uncertainty.The concentrations of the major species would not be strongly affected by changesof carbon dioxide partial pressure above the solutions, but if the difference betweenan open and closed system (with inert cover gas) is considered [95VIT, 96DEL/VIT],p CO2 can increase by almost 10 3 , and this would increase the redox potential of thesolution by more than 0.3 V, and induces an error of almost 6 orders of magnitude in