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

9.2 Aqueous neptunium group 17 (halogen) complexes 161Table 9.4: Experimental equilibrium data for the neptunium(V) fluoride system.Method Ionic medium t ( ◦ C) log 10 β (a) q log 10 βq◦(a)NpO + 2 + Å F− NpO 2 F(aq)Referencesp 2 M NaClO 4 25 (1.35 ± 0.30) (b) (1.18 ± 0.37) [78RAO/PAT]dis 2 M NaClO 4 25 (0.99 ± 0.10) (b) (0.82 ± 0.24) [79RAO/GUD]dis 1 M NaClO 4 23 (1.26 ± 0.30) (b) (1.37 ± 0.32) [84CHO/RAO]dis 1 M NaClO 4 25 (1.39 ± 0.30) (b) (1.50 ± 0.32) [85INO/TOC2]ise-F − 0.1 M NaClO 4 21 (1.51 ± 0.50) (b) (1.70 ± 0.50) [85SAW/RIZ]1MNaClO 4 21 (1.15 ± 0.80) (b) (1.26 ± 0.81)NpO + 2 + 2F− Å NpO 2 F − 2dis 1 M Na(ClO 4 ,F) 25 (2.07 ± 0.50) (b) [85INO/TOC2](a) Refers to the reactions indicated, the ionic strength and temperature given in the table. log 10 β ◦ qis in molal units, at I = 0 and 298.15 K.(b) Uncertainty estimated in the present review (cf. Appendix A).documentation of experimental measurements, but the reported formation constant lieswell in the range of other values.For the extrapolation to I = 0weuseε(9.6) =−(0.27 ± 0.10)kg·mol −1 fromthe interaction coefficients of ε (NpO+2 ,ClO − 4 ) = (0.25 ± 0.05)kg·mol−1 and ε (F − ,Na + ) =(0.02 ± 0.02)kg·mol −1 , and by increasing the uncertainty in ɛ by 0.05. Table 9.4shows that there is considerable variation in the resulting log 10 βq ◦ values. Although thestudy of Rao et al.[79RAO/GUD] appears very reliable, its resulting value at I = 0isincompatible with the result of [85INO/TOC2] and the 0.1 M result of [85SAW/RIZ].TheIAEAreview[92FUG/KHO] selected the results of [85SAW/RIZ] and recommendedlog 10 β1 ◦ (9.6, q = 1, 298.15 K) = (1.9 ± 0.2). The weighted average of the sixvalues listed in Table 9.4 is log 10 β1 ◦ (9.6, q = 1, 298.15 K) = (1.20 ± 0.14), andtheunweighted average is (1.3 ± 0.9), where the uncertainty covers the entire range ofexpectation of all values. Though the constants in Table 9.4 are not entirely consistent,the disagreement is not serious, and we prefer to select the rounded weighted averagewith an increased uncertainty:log 10 β1 ◦ (9.6, q = 1, 298.15 K) = 1.2 ± 0.3Only one study reported a value for the 1:2 complex, NpO 2 F − 2[85INO/TOC2] atfluoride concentrations up to 0.25 M. However, we cannot accept this single reportedlog 10 β 2 value, as it is correlated with the authors’ value for log 10 β 1 [85INO/TOC2].Our selected average value for log 10 β 1 is somewhat different, and the value of log 10 β 2value from [85INO/TOC2] would not be consistent with the selected value of log 10 β 1 .The Gibbs energy of formation is calculated using the selected f G ◦ m values for NpO+ 2and F − . f G ◦ m (NpO 2F, aq, 298.15 K) =−(1196.1 ± 5.9) kJ·mol −1