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

A. Discussion of selected references 707For the reaction:NpO + 2 + HPO2− 4Å NpO 2 HPO − 4(A.61)the authors reported log 10 β 1 (A.61, 0.1 M NaClO 4 ) = (3.11 ± 0.19) at 10 ◦ C,(2.36 ± 0.42) at 25 ◦ C and (2.06 ± 0.39) at 35 ◦ C. From the temperature variationof the complexation constant the authors also reported the thermodynamicquantities r H m (A.61, 0.1 M NaClO 4 , 298.15 K) = −69.9 kJ·mol −1 and r S m (A.61, 0.1MNaClO 4 , 298.15 K) =−188 J·K −1·mol−1 .There was no information provided about the recalculation of the constants of phosphoricacid at the given ionic strength and temperature, and no total phosphate concentrationswere reported. A recalculation performed in which the total phosphate wasfirst calculated from the reported HPO − 4concentrations, the pH and the thermodynamicconstants of phosphoric acid resulted in essentially no variation in the values ofthe constants for 10 and 25 ◦ C, while at 35 ◦ C no such calculations could be performed.This is because from the values in the last column of Table 3 of [84REE/DAN], a D 0value that varies between 7.7 and 9.2 has been used at 35 ◦ C, while a constant value of11.52 was used at the two other temperatures.A weighted linear regression of ln β 1 vs. 1/T (K −1 ) for the measurements at 10and 25 ◦ C, using the auxiliary values for HPO 2−4from Table 5.1 of the present review,results in r H m (A.61, 0.1 MNaClO 4 , 298.15 K) =−(52 ± 50) kJ·mol −1 and r S m (A.61, 0.1MNaClO 4 , 298.15 K) =−(127 ± 167) J·K −1·mol−1 .The recalculated value of this complexation constant at 25 ◦ C, log 10 β(A.61,0.1 MNaClO 4 ) = (2.42 ± 0.42) is used in the selection of a final value for theconstant. Extrapolation of the value to I = 0 with the SIT equations and interactioncoefficients in Appendix B (in fact the interaction coefficient term at this low ionicstrength is very small, since also ε (NpO+2 ,ClO− 4 ) = 0.25 kg·mol −1 ,(TableB.3) andε (Na + ,HPO 2−4 ) = −0.30 kg·mol −1 (Table B.5) and the interaction coefficient of thesingle charged complex is small) results in:log 10 β ◦ (A.61, I = 0, 298.15 K) = (2.86 ± 0.45)[84VAR/BEG]The authors reported quasi-reversible formal potentials for the Pu(VI)/Pu(V) and theNp(VI)/Np(V) couples in 2 M Na 2 CO 3 :0.11Vand0.23Vvs SCE, respectively.The authors did not make an assessment of uncertainties associated with their measurements(assumed to be ±0.01 V in this review). Also, the possibility of systematic errorsassociated with non-negligible junction potentials was not discussed or evaluated.The reported values are given no weight in the determination of the stability constantsof neptunium and plutonium carbonate complexes. The experimental work on the neptuniumsystem described in this report was also discussed in a paper [84VAR/HOB].