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

12.1 Neptunium carbon compounds and complexes 275Kim et al. [91KIM/KLE], Meinrath [94MEI] and Neck and co-workers[94NEC/KIM, 94NEC/RUN, 95FAN/NEC, 95NEC/FAN, 95NEC/RUN] did not reportany such problems with the solid phase they used for their solubility measurements.As discussed above in the selection of formation constants for the aqueous complexes(see Appendix A), the reported values for the NaNpO 2 CO 3 (s) solubility product,K s,0 (12.11) are first extrapolated to zero ionic strength, and the resulting standardvalue, Ks,0 ◦ (12.11), is used in this review to derive the selected value.NaNpO 2 CO 3·xH 2 O(s) Å Na + + NpO + 2 + CO2− 3+ xH 2 O (12.11)The value of the corresponding ε for Eq. 12.11 fitted in this review fromthese K s,0 (12.11) determinations at I = 0.1 to 3 M (see Appendix A)[91KIM/KLE, 94NEC/RUN], and using the values of ε (Na + ,ClO − 4 ) and ε (Na + ,CO 2−3 ) fromAppendix B, results in the calculated value ε (NpO+2 ,ClO − 4 ) = (0.34 ± 0.10) kg·mol−1 ,while (0.18 ± 0.03) kg·mol −1 is calculated in the present review from the liquid-liquidextraction data of Neck et al. [94NEC/KIM, 95NEC/FAN]. Similar values canbe obtained from calculated parameters proposed by the authors (e.g., thevaluesε (NpO+2 ,ClO− 4 ) = (0.31 ± 0.08) or (0.20 ± 0.05) kg·mol−1 from ε = (0.21 ± 0.03)kg·mol −1 [94NEC/RUN] or from the Pitzer parameters [95FAN/NEC], respectively).The mean, ε (NpO+2 ,ClO− 4 ) = (0.255 ± 0.185) kg·mol−1 , of the two recalculated (statisticallyinconsistent) values from the two different experimental studies is in agreementwith the value used in the present review ε (NpO+2 ,ClO − 4 ) = (0.25 ± 0.05) kg·mol−1 .Differences in the estimates of ε (NpO+2 ,ClO− 4 )probably originated, at least in part, in theauxiliary values (Table 12.5). The ionic strength corrections proposed byε (CO2−3 ,Na+ )the authors were not used in the present review. It cannot be ruled out that whateverproblems is at the origin of the apparent inconsistency in the ε (NpO+2 ,ClO− 4 ) valuesextracted from the solubility product determinations, K s,0 (12.11), was also present inthe rest of the solubility work. For this reason similar ionic strength corrections forthe complexation constants, calculated from these solubility studies, were not used inthis review (see Section 12.1.2.1.3.a). The same type of problem was found for thedeterminations of K s,0 (12.39), the solubility product forNa 3 NpO 2 (CO 3 ) 2 · xH 2 O(s) Å (12.39)3Na + + NpO + 2 + 2CO2− 3+ xH 2 OInformation in the one of the later papers from this group [96FAN/NEC] suggestedthat there may still be some minor systematic problems in their calibration procedures.As suggested by the authors, it is also possible that their work indicates a need to eventuallyrevise auxiliary data (e.g.,the value for ε (Na + ,CO 2−3)) used in the present review(see the discussion of [96FAN/NEC] in Appendix A).Lemire, Boyer and Campbell [93LEM/BOY] carried out solubility experimentswith hydrated sodium and potassium Np(V) carbonate salts at 30 to 75 ◦ C, but theirdata are very scattered, especially at higher temperatures. They attributed this to slow