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

756 A. Discussion of selected referencesdata in this paper are consistent (below pH values of 11) with, among others, those ofEwart et al.[86EWA/HOW]andLierseet al.[85LIE/TRE]. The reported experimentalprocedure suggests the initial neptunium concentrations in the low pH solutions mayhave been too low to cause precipitation. The solubilities for the four lowest pH valuesare rejected in the present review.[92BEN/HOF]This is a recapitulation of the work described in [90BEN]. For the reasons previouslydiscussed (see entry for [90BEN] in this appendix), the reported stability of the firsthydrolysis product is taken only as an upper bound and the first carbonate complex isaccepted (with some revision of interaction coefficients) for the purpose of this review.[92CAP]Portions of the experimental work described in this thesis were used in various otherreports and papers [89RIG/ROB, 90CAP/VIT, 92CAP/VIT, 94GIF/VIT, 95CAP/VIT,96CAP/VIT, 97CAP/VIT].a) Standard potentials of Pu redox couples in non-complexing mediaCapdevila determined the disproportionation constant of PuO + 2by studying the equilibrium(Equation 16.11, Section 16.4)3PuO + 2 + 4H+ Å 2PuO 2+2+ Pu 3+ + 2H 2 O(l)using spectrophotometry, at five different ionic strengths (I = 0.1, 0.5, 1, 2, 3 M). Froman SIT treatment, she derived the equilibrium constant at standard state log 10 K V =(2.6 ± 0.5) and ε = (0.09 ± 0.1) kg·mol −1 . K V is the nomenclature used in thethesis for K(16.11). She also measured the extent of the disproportionation equilibrium(Equation 16.5, Section 16.2)3Pu 4+ + H 2 O(l) Å 2Pu 3+ + PuO 2+2+ 4H +by mixing solutions of PuO 2+2and Pu 3+ having the same acidity (1 M HClO 4 )andfollowing the change of the different oxidation states spectrophotometrically. Fromthe data, log 10 K IV (1M) = −(2.09 ± 0.5) (K IV is the nomenclature used in thethesis for K(16.5)) was determined. This part of the work formed the basis of apaper [92CAP/VIT] and was used later (with corrections) by Capdevila and Vitorge[97CAP/VIT].Furthermore, the redox potentials of Pu 4+ /Pu 3+ and PuO 2+2 /PuO+ 2were determinedby cyclic voltammetry in 1 M HClO 4 at 298.15 K and at several different ionicstrengths (I = 0.5, 1, 2, 3 M) and temperatures between 5 ◦ C and 65 ◦ C in five-degreesteps. SIT treatment of the data yielded the standard potentials, temperature and ioninteraction coefficients. The results are summarised in Table A.26.For the Pu 4+ /Pu 3+ couple, the formal potential E ◦′ = (0.978 ± 0.007) Vagrees with the formal potentials E ◦′ = (0.982 ± 0.002) V of Connick and