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

110 8. Neptunium oxygen and hydrogen compounds and complexeslower neptunium concentrations, in the pH range above 10, if the solutions were passedthrough a filter with a small pore size (∼ 1 nm). It was suggested [85LIE/TRE] thatsorption of neutral species on the filter might lead to incorrect (low) solubility values.This may have happened in the work of Ewart et al.. It is also possible the water usedby Ewart et al. contained reducing agents after equilibration with the concrete.This said, the values summarised by Neck, Kim and Kanellakopulos [92NEC/KIM]were refitted using our selected value of ε (NpO+2 ,ClO − 4 ) = 0.25 kg·mol−1 . The resultingvalue for ε (Na + ,NpO 2 (OH) − 2 ) =−(0.01 ± 0.07) kg·mol−1 is accepted. Because thevalues for β 1 and β 2 from each set of solubility experiments are undoubtedly correlated,no attempt was made to weight the results from the different studies [85LIE/TRE,92ITA/NAK, 92NEC/KIM]. Instead the refitted values based on those tabulated byNeck, Kim and Kanellakopulos [92NEC/KIM] wereused(cf. Table 8.5). The uncertaintieswere based on the original experimental uncertainties and, for β 2 , increased tobe consistent with the values from the work of Itagaki et al. [92ITA/NAK].Thus, the formation constants (I = 0) have been estimated aslog10 ∗ β◦ 1 (8.1) =−(11.3 ± 0.7)(8.2) =−(23.6 ± 0.5)log ∗10 β◦ 2Thus,NpO + 2 + H 2O(l) Å NpO 2 OH(aq) + H + (8.1)NpO + 2 + 2H 2O(l) Å NpO 2 (OH) − 2 + 2H+ (8.2) f G ◦ m (NpO 2OH, aq, 298.15 K) = −(1080.4 ± 6.9) kJ·mol −1 f G ◦ m (NpO 2(OH) − 2, aq, 298.15 K) = −(1247.3 ± 6.3) kJ·mol−1Although there is evidence for formation of additional anionic hydrolysis speciesin strongly basic solutions [90TAN, 94TAN], the available data are not adequate tobe used to derive reliable values of the formation constants. Sets of Pitzer-equationion-interaction parameters have recently been reported for NpO + 2and Np(V) hydrolysisspecies [95FAN/NEC, 95NEC/FAN, 95NOV/ROB, 96RUN/NEU], and may beparticularly useful for modelling the behaviour of neptunium(V) in solutions containingNaCl at high molalities. However, no evaluation of the Pitzer-equation parametershas been carried out in the present review.The NpO + 2 ·H 3O + species proposed by Rösch et al. [87RÖS/MIL] is not acceptedin the present review. If this species actually forms, it is not clear why it would nothave been detected in previous potentiometric studies.A value for the enthalpy of hydrolysis of NpO + 2was reported by Sullivan, Choppinand Rao [91SUL/CHO] but, as discussed in Appendix A, the measured heat is probablynot the heat of this hydrolysis reaction. Recently, the entropy values of NpO 2 OH(aq)and NpO 2 (OH) − 2have been estimated by Lemire [84LEM] and Lemire and Garisto[89LEM/GAR]. The value Sm◦ (NpO 2OH, aq, 298.15 K) = (25 ± 60) J·K−1·mol−1