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

12.1 Neptunium carbon compounds and complexes 271However, as this review neared completion, a new study on Np(IV) solubilitywas used to select data from which log 10 β5(IV) ◦ (12.23) = (35.62 ± 1.07) andε (Np(CO3 ) 6−5 ,K+ )= −(0.73 ± 0.68) mol·kg −1 were obtained (Section 12.1.2.1.4).By extrapolation of E ◦′ (12.29, 1 M K 2 CO 3 ) to zero ionic strength (assumingε (K + ,Np(CO 3 ) 3−3 ) = ε (Na + ,Am(CO 3 ) 3− )) 3E ◦ (12.30, 298.15 K) =−(0.962 ± 0.134) V/SHE r G ◦ m (12.30, 298.15 K) = (92.82 ± 12.93) kJ·mol−1are calculated and selected. From the latter, f G ◦ m (Np(CO 3) 6−5, 298.15 K) (Section12.1.2.1.4) and f G ◦ m (CO2− 3, 298.15 K) (Table 5.1) f G ◦ m (Np(CO 3) 3−3, 298.15 K) =−(2185.95 ± 15.45) kJ·mol−1Then using values of E ◦ (12.30) and the standard potential of the Np 4+ /Np 3+redox couple E ◦ (7.9) as selected in this review (Section 7.5), log 10 (β5(IV) ◦ (12.23)/β3(III) ◦ (12.31, M = Np)) = (19.96 ± 2.27) and log 10 β◦ 3(12.31, M = Np) =(15.66 ± 2.51) are calculated. The corresponding values for the americium system[95SIL/BID] are (20.1 ± 0.9) and (15.2 ± 0.6), respectively.12.1.2.2 Solid neptunium carbonates12.1.2.2.1 Solid neptunium(VI) carbonatesThere have been several publications on neptunium(VI) solubility in carbonate/bicarbonateaqueous solutions. The available experimental information issummarised in Table 12.1, where the results of reanalysis of data carried out in thepresent review are also provided.In this review the solubility measurements of Gorbenko-Germanov and Klimov[66GOR/KLI] have been interpreted, and those of Moskvin [71MOS2] have been reinterpreted(see Table 12.1), as discussed by Vitorge and Capdevila [98VIT/CAP] consideringthe following equilibria:(NH 4 ) 4 NpO 2 (CO 3 ) 3 (s) Å NpO 2 (CO 3 ) 4−3+ 4NH + 4(12.32)K 4 NpO 2 (CO 3 ) 3 (s) Å NpO 2 (CO 3 ) 4−3+ 4K + (12.33)This was done even though the solid phase K 4 NpO 2 (CO 3 ) 3 (s) was not properlycharacterized. The corresponding equilibrium constants (Table 12.1) were extrapolatedto zero ionic strength (see Appendix A) by performing linear SIT regressions with ε(12.32), −(0.70±0.14) kg·mol −1 and ε (12.33) −(0.54±0.37) kg·mol −1 from whichε (NpO2 (CO 3 ) 4−3 ,NH+ 4 ) = −(0.78 ± 0.25) kg·mol −1ε (NpO2 (CO 3 ) 4−3 ,K+ )= −(0.62 ± 0.42) kg·mol −1