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

220 12. Neptunium group 14 compounds and complexesIn the present review, the influence of the ionic strength on the values was confirmedas being negligible from a calculation using the data at the end of the titration wherethe limiting complex had already formed, and only the ionic strength was being varied(Appendix A). The uncertainty was increased to the same value previously estimatedfor the uranium system [92GRE/FUG] r Hm ◦ (12.2, 298.15 K) = −(41.9 ± 4.1) kJ·mol−1From this value, f Hm ◦ (NpO2+ 2 , 298.15 K) =−(860.7 ± 4.7) kJ·mol−1 (Section7.2)and f Hm ◦ (CO2− 3 , 298.15 K) =−(675.23 ± 0.25) kJ·mol−1 (Table 5.1) f Hm ◦ (NpO 2(CO 3 ) 4−3, 298.15 K) =−(2928.32 ± 6.25) kJ·mol−1is calculated. From r Hm ◦ (12.2, 298.15 K) = −(41.9 ± 4.1) kJ·mol−1 and r G ◦ m (12.2, 298.15 K) = −(110.57 ± 1.09) kJ·mol−1 selected above in thissection, r Sm ◦ (12.2, 298.15 K) = (230.32 ± 14.23) J·K−1·mol−1 is calculated.This value is (51.73 ± 19.80) J·K −1·mol−1 less than the corresponding uraniumvalue (282.05 ± 13.79) J·K −1·mol−1 [92GRE/FUG]. r Sm ◦ (12.2, 298.15) =(230.32 ± 14.23) J·K −1·mol−1 , Sm ◦ (NpO2+ 2, aq, 298.15) = −(92.4 ± 10.5)J·K −1·mol−1 (Section 7.3) andSm ◦ (CO2− 3, aq, 298.15 K) =−(50 ± 1) J·K−1·mol−1(Table 5.1)giveSm ◦ (NpO 2(CO 3 ) 4−−13, aq, 298.15 K) =−(12.07 ± 17.92) J·K−1·molwhich is (46.0 ± 23) J·K−1·mol−1less than the corresponding uranium valueSm ◦ (UO 2(CO 3 ) 4−3 , aq, 298.15 K) = (33.9 ± 14.4) J·K−1·mol−1 [92GRE/FUG].12.1.2.1.2.f DiscussionThe values selected in this review (see Table 12.3) for the Np(VI) carbonate complexes(and compound, see below) are quite similar to those for the corresponding uraniumand plutonium species. Small regular variations are usually observed in the U, Np,Pu series. However, these can lead to real differences in behaviour of the species insolution as discussed above for the stability of the carbonate trinuclear complexes.12.1.2.1.3 Np(V) carbonate and mixed hydroxide-carbonate complexesAs part of their study of potassium dioxoneptunium(V) carbonate solids Gorbenko-Germanov and Zenkova [66GOR/ZEN] published absorption spectra of their aqueoussolutions. The spectra were later confirmed [75UEN/SAI] and can be interpretedas primarily resulting from the limiting complex NpO 2 (CO 3 ) 5−3[90RIG]. Simakin[77SIM] deduced the correct stoichiometry of this limiting Np(V) carbonate complex,from solubility measurements of a hydrated Na 3 NpO 2 (CO 3 ) 2 (s) compound at constantionic strength (3 M NaNO 3 ) in concentrated carbonate aqueous solutions. The fulldemonstration of this stoichiometry was only confirmed later (see Appendix A discussionsfor [77SIM, 90RIG]). Simakin’s measurements were the first reliable work on