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

122 8. Neptunium oxygen and hydrogen compounds and complexesthe solids, it is further calculated that accepting this enthalpy value means NpO 2.59 (cr)would exclude NpO 2 OH(am) (or Np 2 O 5 (cr) based on the value from Fuger and Merli[94MER/FUG]) from the neptunium-oxygen-water phase diagram – at least near roomtemperature. Considering the large uncertainties in the enthalpy values, the difficultyin carrying out the analyses [76FAH/TUR] and the lack of agreement between f Hm ◦ (NpO 2(OH) 2 (cr)) as similarly determined by Belyaev, Smirnov and Taranov[79BEL/SMI] and the value determined from the simpler experiment by Fuger andBrown [69FUG/BRO], only the value from Merli and Fuger [94MER/FUG] (for acompound assumed to be stoichiometric Np 2 O 5 (cr)) is used in the present review, andis selected. f Hm ◦ (Np 2O 5 , cr, 298.15K) =−(2162.7 ± 9.5) kJ·mol −1This is done with the recognition that the stable compound may have a stoichiometrythat differs slightly from NpO 2.50 . The selected value of f H ◦ m (Np 2O 5 , cr, 298.15 K)is compatible, within the uncertainty limits, with marginal stability with respect todecomposition to NpO 2 . Also, this value is reasonable compared to the estimated valuesfor f H ◦ m (NpO 3, cr, 298.15 K), −1068 (this review) or −(1070 ± 6) kJ·mol −1([82MOR/FUG] based on UO 3 and experimental data on the homologous U - Np species),if one takes uncertainty limits into account and assumes that the entropy changeforNpO 3 (cr) → 0.5Np 2 O 5 (cr) + 0.25O 2 (g)is only due to the formation of O 2 (g). Pan and Campbell [95PAN/CAM] have measuredthe solubility of Np 2 O 5 (cr) in aqueous solutions as a function of pH. Their measurementsfor pH values below 8 can be used to obtain a value of log 10 ∗ K s,0 = (3.90 ±0.02) for the reaction0.5Np 2 O 5 (cr) + H + Å NpO + 2 + 0.5H 2O(l) (8.10)and f G ◦ m (Np 2O 5 , cr, 298.15 K) =−(2008 ± 11) kJ·mol −1 . From this and the selectedvalue for f Hm ◦ , S◦ m (Np 2O 5 , cr, 298.15 K) = (95 ± 46) J·K −1·mol−1 is calculated.This value is not in agreement with values of Sm ◦ (Np 2O 5 , cr, 298.15 K)previously estimated by Merli and Fuger [94MER/FUG] ((186 ± 15) J·K −1·mol−1 )and Lemire [84LEM] ((163 ± 23) J·K −1·mol−1 ) and the value is almost certainlytoo small for well-crystallised bulk Np 2 O 5 (cr). Indeed, it is much less than twice theexperimentally determined value for crystalline NpO 2 (cr) (Section 8.2.5.1), and lessthan twice Sm ◦ (UO 2.25(cr)) = (83.53 ± 0.17) J·K −1·mol−1 and Sm ◦ (UO 2.6667(cr)) =(94.18 ± 0.17) J·K −1·mol−1 [92GRE/FUG]. The difference may simply indicate thatthe surface of the Np 2 O 5 (cr) samples used in the solubility experiments had large numbersof active sites. It may also indicate the selected entropy for NpO + 2(aq) in thepresent review is too negative. In the present review the valueSm ◦ (Np −12O 5 , cr, 298.15 K) = (174 ± 20) J·K−1·molis accepted as a value for “ideal”, crystalline NpO 2.5 (cr), and hence, f G ◦ m (Np 2O 5 , cr, 298.15 K) = −(2031.6 ± 11.2) kJ·mol −1