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

A. Discussion of selected references 641although the increased solubility of the MNpO 2 SO 3 (s) precipitates at higher but unspecifiedSO 2−3concentrations suggest that a third complex may form. Unfortunately,the original data are not given and no correction can be made to these values to use anappropriate dissociation constant for H 2 SO 3 (aq).The following stable solids were isolated: NaNpO 2 SO 3·H 2 O(s),NaNpO 2 SO 3·2H 2 O(s), NH 4 NpO 2 SO 3 (s), and KNpO 2 SO 3 (s). The physicalsolubility and thermal characteristics of these compounds are given in the text,however no thermodynamic data are presented.[72CAS/MAG]This is a potentiometric study of the hydrolysis of Pu(VI) in 1 M NaClO 4 .The authors considered the species PuO 2 OH + , (PuO 2 ) 2 (OH) 2+2 , (PuO 2) 2 (OH) + 3(PuO 2 ) 2 (OH) 4 (aq), (PuO 2 ) 3 (OH) + 5 ,(PuO 2) 3 (OH) 2+4and (PuO 2 ) 3 (OH) 6 (aq). Of these,(PuO 2 ) 2 (OH) + 3 , (PuO 2) 2 (OH) 4 (aq), (PuO 2 ) 3 (OH) 2+4and (PuO 2 ) 3 (OH) 6 (aq) wererejected. This appears to be a well designed study, and it is unfortunate only a smallfraction of the experimental data were published. In this review, the uncertainties areestimated as ±0.5 inlog10 ∗ β to reflect model error and possible radiolytic effects onthe plutonium oxidation state.[72CAS/MAG2]The authors reported log10 ∗β1 =−(5.17 ± 0.03), log10 ∗β2,2 =−(6.68 ± 0.02) andlog10 ∗β5,3 =−(18.25±0.02) from potentiometric titrations of solutions of Np(VI) withNaOH and HClO 4 in a medium of 1 M NaClO 4 . The authors selected their hydrolysismodel after careful comparison of the residuals upon fitting their data to a numberof different schemes. The reported uncertainties do not reflect possible systematicerrors, and in this review the estimated uncertainties in the values of log10 ∗ β have beenestimated as ±0.20. The ion-interaction coefficients are assumed to be equal for thecorresponding uranium and neptunium hydrolysis reactionsmNpO 2+2+ nH 2 O(l) Å (NpO 2 ) m (OH) 2m−nn + nH +hence ε 2,2 =−(0.07 ± 0.10) kg·mol −1 and ε 5,3 =−(0.23 ± 0.20) kg·mol −1 .Grenthe et al. [92GRE/FUG] reported ε 1,1 =−(0.38 ± 3.7) kg·mol −1 for formationof UO 2 OH + , and derived ε (UO2 OH + ,ClO − 4 ) =−(0.06 ± 3.7) kg·mol−1 . The uncertaintylimit is chemically unrealistic, and in the absence of a more accurate value we assign aslightly more realistic uncertainty, hence ε (UO2 OH + ,ClO − 4 )=−(0.06 ± 0.40) kg·mol−1and ε 1,1 =−(0.4 ± 0.4) kg·mol −1 . From these the selected values,log ∗ 10 β1 ◦ = −(5.1 ± 0.4)log ∗ 10 β2,2 ◦ = −(6.27 ± 0.21)log ∗ 10 β5,3 ◦ = −(17.12 ± 0.22)