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

A. Discussion of selected references 679c) Np(V)/Np(III) redox potentialThe value of β3,Np(III) ◦ /β◦ 3,Np(V)(11.2 ± 1.8) can be calculated from the two measurementsof Fedoseev, Peretrukhin and Krot, and this should lead to a more satisfactoryvalue for β3,Np(III) ◦ because the major uncertainties (the values of the liquid junctionand estimated stability of the limiting complex of Np(IV)) cancel. Using the valuelog 10 β3 ◦ = 5.50 for Np(V) from the present review, log 10 β◦ 3= 16.7 for the Np(III)carbonate complex is calculated. This compares well with the value (15.2±0.6) for theformation constant of the corresponding americium complex [95SIL/BID], and withlog 10 β3 ◦ (15.49 ± 2.51) based on the Np(IV/III) couple in carbonate solutions (cf. Section12.1.2.1.5).d) Pu formal potentialsPotentiometric measurements of the irreversible formal potential of the Pu(IV)/Pu(III)couple in 1 M K 2 CO 3 were reported. The formal potential is −(0.495 ± 0.030) V vs.SHE and has a variation with temperature of −0.63 mV/ ◦ C. On the basis of theseresults the Gibbs energy of oxidation was found to be 47.7 kJ·mol −1 and the entropyof reaction to be −60.8 J·K −1·mol−1 . These values appear reliable but due to the absenceof independent information concerning the stoichiometry of the Pu(III) limitingcomplex, the irreversibility of the formal potential and the lack of information concerningthe ionic strength dependence of the stability of the limiting and precursor Pu(IV)complexes, this value cannot be used to determine the nature and stability of the Pu(III)carbonate complex(es). A reduction potential was reported for the Pu(V)/Pu(IV) couple(+0.6 V), but no details were given concerning the experiments giving rise to this value,and it differs significantly from other reported values.[79KUS/GAN]This is a summary of earlier work of this research group and a short presentation ofnew results from solvent extraction studies. The constants reported for the formation ofNpI 3+ were obtained from extraction studies using HTTA and HDNNS (dinonylnaphthalenesulphonicacid): β 1 = 0.8 (HTTA) and 1.3 (HDNNS). The aqueous solutioncontained 1 M H + and had an ionic strength of 2 M (probably using perchlorate asbackground anion as was done in earlier studies), and the experiments were carried outat 25 ◦ C. Formation constants of iodide complexes of Th(IV) were also reported, as wellas of oxalate complexes of Th(IV), Np(IV) and Pu(IV), of nitrite complexes of Am(III),Cm(III), of nitrate complexes of Np(IV), and of thiocyanide complexes of Tm(III). Althoughthe experimental details are not given in this paper, it can be assumed that theextraction studies were carried out at the same level of care as the earlier studies of thisgroup, e.g., [75PAT/RAM, 76BAG/RAM2]. We therefore accept the following valueswith our estimated uncertainties: log 10 β 1 (NpI 3+ , I = 2M) =−(0.10 ± 0.30) (fromthe HTTA experiment) and (0.11 ± 0.30) (from the HDNNS experiment).The values reported for the association constants of Np 4+ with nitrate (β 1 = 3.4,β 2 = 4.8, at I = 6 M) are new. However, because the measurements were carriedout at such a high ionic strength, and few experimental details were provided, thesereported values are not accepted in the present review.