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

A. Discussion of selected references 661Np(IV) at 25 ◦ C. Np(IV) was studied by solvent extraction with dinonylnaphthalenesulphonicacid (HDNNS) at I = 1M(HClO 4 ):Np 4+ + HF(aq) Å NpF 3+ + H + .(A.52)The experiments were carried out at 10, 25 and 40 ◦ C to obtain some informationon r H m . The authors evaluated their measurements in terms of the formationof a 1:1 complex and reported the following values for the formation ofNpF 3+ at I = 1 M (HClO 4 ): log ∗ 10 β 1 (A.52, 283.15 K) = (4.71 ± 0.03),log ∗ 10 β 1 (A.52, 298.15 K) = (4.60 ± 0.02) and log ∗ 10 β 1 (A.52, 313.15 K) =(4.52 ± 0.02). No check of the linearity of the distribution coefficient was presented,and the reader has thus no assurance that the oxidation state Np(IV) was stable duringthe experiment. The similarity of these values to the corresponding Th(IV) constantand to other Np(IV) constants from the literature gives some confidence that the +4oxidation state was maintained. The constant at 25 ◦ C is considered in the evaluationof log ∗ 10 β1 ◦ in the present review, using an uncertainty of ±0.20 in log 10 ∗ β 1 .From the three constants at 10, 25 and 40 ◦ C we obtain an average enthalpy of r H m (A.52) =−(10.7 ± 2.0) kJ·mol −1 , where the error spans the range of valuesobtained. Assuming that this value does not depend strongly on the ionic strength,we can combine it with r Hm ◦ of the protonation reaction of F− , r Hm ◦ = (12.2 ±0.3) kJ·mol −1 [92GRE/FUG], and we obtain r Hm ◦ (A.53) = (1.5 ± 2.0) kJ·mol−1 .Np 4+ + F − Å NpF 3+(A.53)This value is slightly lower than the one of Choppin and Unrein [76CHO/UNR], becausethey used a different value for the enthalpy of protonation of F − . This value isaccepted in the present review.[76FAR/BUC]This is a report of a very careful ion exchange study at 25 ◦ C in 1 and 2 MH(ClO 4 ,HSO 4 ). Hydroquinone was used as the holding reductant under an N 2atmosphere (in preference to ascorbic acid which was tried but had a number ofundesirable properties). The results were interpreted as indicating that the protonatedSO 2−4ligand does not participate in the complexing of Pu(III). The plutonium isotopewas not specified but distribution coefficients were evaluated using β-active 241 Pu.The dissociation constant of HSO − 4was taken from experiments of [68AHR/BRA2](20 ◦ C, I = 1M)and[51ZEB/ALT](25 ◦ C, I = 2 M), and extrapolated to temperatureusing the enthalpy of ionization determined by [59ZIE] (see [74FAR/PEA] fordiscussion of this choice of data). The raw experimental data were not provided andtherefore the values have been accepted without re-evaluation.[76FRO/CHI]This paper describes a study of oxidation of neptunium(IV) by persulphate ions incarbonate solutions. The equilibrium p CO2 in this medium was practically constant,