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

696 A. Discussion of selected referencesTable A.13: Values of temperature coefficients for standard potentials from Blanc andMadic [84BLA/MAD].(∂ E ◦ /∂T ) therm / V·K −1 (∂ E ◦ /∂T ) isotherm / V·K −1 r S ◦ / J·K−1·mol−1Pu 4+ /Pu 3+ ∼ (0.0021 ± 0.0004) ∼ (0.0019 ± 0.0004) (183±39)Np 4+ /Np 3+ ∼ (0.0021 ± 0.0004) ∼ (0.0019 ± 0.0004) (183±39)PuO 2+2 /PuO+ 2 0.00085 0.00065NpO 2+2 /NpO+ 2 0.00043 0.00023couples at I=1 are approximately +0.2 and +0.6 mV.K −1 for Np and Pu respectively.Blanc and Madic then claimed values of +0.43 and +0.85 mV.K −1 for I=0. Usingthe authors’ equation 5 [59DEB/LIC] leads to values of ∂ E ◦ /∂T of +0.23 and +0.65mV.K −1 as in the table above. In the present review, because of the cycles used,a more useful value would be ∂ E ◦′ /∂T (1 M HClO 4 ). Based on the graphs in thepaper, the values of (∂ E ◦′ /∂T ) might be expected to be approximately 0.0 and +0.4mV.K −1 , respectively, for the Np and Pu couples. These values differ by no more than0.5 mV.K −1 from those determined from measurements of the potentials at differenttemperatures (cf. Figure 16.1), but the theoretical basis for the thermoelectric powervalues is not as strong. Therefore, the values of Blanc and Madic [84BLA/MAD] arenot used in in this review in determining the selected values of the entropies for theaqua ions.[84CHO/RAO]The authors used a solvent extraction technique to determine the formation constantsof UO 2 F + and NpO 2 F(aq) and a potentiometric method with a fluoride ion selectiveelectrode to determine the formation constants of UO 2 F + ,NpO 2 F + and PuO 2 F + .Allresults refer to the following type of reaction (An=actinide):AnO + 2 /AnO2+ 2+ F − Å AnO 2 F(aq)/AnO 2 F +The experiments were carried out at 23 ◦ C, at pH = 2.96 and at a constant ionic strengthof 1 M (NaClO 4 ). Apparently, the two independent results of Choppin and Rao forUO 2 F + were missed in the uranium review of the present series [92GRE/FUG]. Theformation constants, log 10 β 1 (UO 2 F + , I = 1M), were reported by Choppin and Rao[84CHO/RAO] as(4.52 ± 0.10) from solvent extraction and (4.70 ± 0.07) from potentiometry.The first value is fully consistent with the selection of the uranium review[92GRE/FUG], while the second one is slightly higher. The authors did not mentionwhat value they used for the protonation constant of fluoride, and this could bethe source of a small error, but is insufficient to explain the difference. However,the results of this study seem to be reliable, and we include the following values inour selection procedure in Parts III and IV, Sections 9.2.1.3, 9.1.2.4 and 18.2.1.4:log 10 β 1 (NpO 2 F + , I = 1M) = (4.27 ± 0.15),log 10 β 1 (PuO 2 F + , I = 1M) = (4.22 ±0.20), andlog 10 β 1 (NpO 2 F, aq, I = 1M) = (1.26 ± 0.30). We have increased the re-