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

216 12. Neptunium group 14 compounds and complexesthe fixed β 3 (12.2) andβ 6,3 (12.6) values used here are those selected in the presentreview (elsewhere in this section).This is done, even though the values were determined from an overly sparseset of solubility measurements, as pointed out by the authors. In the extrapolationto zero ionic strength the following estimated specific interaction coefficients wereused: ε (NpO2 (CO 3 )(aq),Na + ) = 0kg·mol −1 (as for all the uncharged species) andε (NpO2 (CO 3 ) 2−2 ,Na+ ) =−(0.02 ± 0.14) kg·mol−1 which is the ε (UO2 (CO 3 ) 2−2 ,Na+ ) valuewith increased uncertainty. Therefore, for the reactionsNpO 2+2+ 2CO 2−3Å NpO 2 (CO 3 ) 2−2(12.3)NpO 2+2+ CO 2−3Å NpO 2 CO 3 (aq) (12.4)the selected values of the formation constants areThen,andlog 10 β2 ◦ (12.3, 298.15 K) = (16.52 ± 0.73)log 10 β1 ◦ (12.4, 298.15 K) = (9.32 ± 0.61) r G ◦ m (12.3, 298.15 K) = −(94.28 ± 4.16) kJ·mol−1 r G ◦ m (12.4, 298.15 K) = −(53.20 ± 3.48) kJ·mol−1 f G ◦ m (NpO 2(CO 3 ) 2−2, 298.15 K) = −(1946.01 ± 7.03) kJ·mol−1 f G ◦ m (NpO 2CO 3 , aq, 298.15 K) = −(1377.04 ± 6.62) kJ·mol −112.1.2.1.2.c The carbonate trinuclear Np(VI) complexThe trinuclear complex (MO 2 ) 3 (CO 3 ) 6−6, was first identified for uranium (M = U) byCiavatta et al. [81CIA/FER]. A few years later, during a sabbatical leave in Vitorge’slaboratory, Grenthe worked with Riglet and Vitorge and used a spectrophotometrictechnique to demonstrate that neptunium and plutonium also form corresponding trinuclearcomplexes [86GRE/RIG, 90RIG].For the neptunium complex, the stoichiometry determination, sensitivity analysis,uncertainty estimation and quantitative interpretation (i.e., determination of the equilibriumconstant value) were carried out using graphical methods. In the present reviewthe original data tabulated in Riglet’s thesis [90RIG] (from experiments in 3 MNaClO 4 ) were reanalyzed. The same equilibrium constant value (but with increaseduncertainty limits) was found (Table 12.1) for the reaction3NpO 2 (CO 3 ) 4−3Å 3CO 2−3+ (NpO 2 ) 3 (CO 3 ) 6−6(12.5)The trinuclear neptunium complex is less stable than the corresponding uraniumcomplex, while the solid compounds of these two actinide(VI) (MO 2 CO 3 (s)) areof