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

678 A. Discussion of selected referencesthe determination of the formation constant of Np(CO 3 ) 6−5. In this review, calculationshave been done assuming that the potential was controlled by the two limiting complexes,NpO 2 (CO 3 ) 5−3and Np(CO 3 ) 6−5(see the discussion on [81WES/SUL] inthisappendix). The equilibrium can then be writtenNpO 2 (CO 3 ) 5−3+ e − + 2CO 2 (g) Å Np(CO 3 ) 6−5Even if we assume an overall uncertainty of only 0.2 V (and the authors seem tohave estimated an uncertainty of 0.1 V for the instability of the measurement and 0.1 Vfor possible junction potential problems without even considering the p CO2 problem),with ε (Np(CO3 ) 6−5 ,Na+ )as estimated in Chapter 12.1.2.1.4, and other values selected inthis review, log 10 β5 ◦(Np(CO 3) 6−5) = (38.1 ± 3.7) is calculated. This assumes theexperimental solutions were in equilibrium with air, an assumption which is consistentwith later measurements [95VIT, 96DEL/VIT, 98VIT/CAP] (see Chapter 12.1.2.1.4).For comparison, the value for the corresponding uranium reaction is (34.1 ± 1.0)[92GRE/FUG, 95GRE/PUI], and the value selected by Capdevila [92CAP] for the plutoniumreaction was (32.2 ± 2.0). The values are in marginal agreement, but the uncertaintyin the value from this paper [79FED/PER] is particularly large.b) Np(IV)/Np(III) redox potentialFedoseev, Peretrukhin and Krot also measured the formal potential of theNp(IV)/Np(III) couple at a dropping mercury electrode in 1 M K 2 CO 3 solution. Theyproposed −1.32 V at 25 ◦ C. This value was later confirmed [84VAR/HOB]. Theuncertainty must be quite substantial, since “all the waves [of Np and Pu(IV)/(III)]in carbonate media are irreversible”. There is no information on the liquid junctionpotential, and we assume an uncertainty of 0.1 V (i.e., that it was the same as in themeasurement of the formal potential of the Np(V)/Np(IV) couple). By analogy withAm(III) [95SIL/BID], we assume that the Np(IV)/Np(III) redox equilibrium can herebe writtenNp(CO 3 ) 6−5+ e − Å Np(CO 3 ) 3−3+ 2CO 2−3Exposure of the 1 M K 2 CO 3 solution to atmospheric CO 2 (g) has little influence onthe free carbonate concentration and on this equilibrium. The required specific interactioncoefficient ε (Np(CO3 ) 3−3 ,K+ ) =−(0.15 ± 0.07) kg·mol−1 , is estimated by analogywith the Am(III) - Na + system, i.e., ε (Np(CO3 ) 6−5 ,K+ )= −(0.73 ± 0.68) kg·mol −1 ,by analogy with Na + solutions [95VIT, 96DEL/VIT] (cf. Section 12.1.2.1.4.a). Valuesfor the uncertainties have been increased because the values are estimated. Usingthese and the standard potential of the Np 4+ /Np 3+ redox couple selected in this review(Chapter 7.4), we calculate log 10 (β5 ◦(Np(CO 3) 6−5 )/β◦ 3 (Np(CO 3) 3−3)) = (20.13±2.27). Hence, log 10 β3 ◦(Np(CO 3) 3−3 ) = (15.49 ± 2.51) using log 10 β◦ 5 (Np(CO 3) 6−5 ) =(35.62 ± 1.07) as estimated in this review (Chapter 12.1.2.1.4.a).