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

652 A. Discussion of selected references4.8, 5.2 and 6.4 respectively for hydrated K 3 NpO 2 (CO 3 ) 2 (s), hydrated KNpO 2 CO 3 (s)and a K 2 CO 3 solution. In these conditions, the free carbonate concentration, [CO 2−3 ],was certainly less than 10 µMsoNpO + 2was not complexed, and according to the dataselected by this review, the titration reactions were thenK + + NpO + 2 + CO 2(g) + H 2 O(l) → KNpO 2 CO 3 (s) + 2H + (A.45)KNpO 2 CO 3 (s) + 2H + → K + + NpO + 2 + CO 2(g) + H 2 O(l) (A.46)K 3 NpO 2 (CO 3 ) 2 (s) + 4H + → 3K + + NpO + 2 + 2CO 2(g) + 2H 2 O(l) (A.47)Assuming that titration caused release of CO 2 (g) (bubbling, hence p CO2 ≈ 1atm),andestimating the NpO + 2concentration at the mid-point of the reaction to be about 0.03 M(these values were not reported in the publication [74VIS/VOL]), the first solubilityproduct can be very crudely estimated as∗ K s,0 (A.46) ≈ −(10.3 ± 2.0)The results from the potentiometric titrations are not particularly consistent withlater work in sodium (instead of potassium) media by the same group of authors[79VOL/VIS2], and this can be attributed to kinetic rather than thermodynamic factors(see the discussion of [79VOL/VIS2] in this appendix).The authors also reported Np(V) solubility values in 1.2 M K 2 CO 3 aqueous solutionsat 20 and 80 ◦ C. If it is assumed that this solubility was controlled by the followingequilibriumthenK 3 NpO 2 (CO 3 ) 2 (s) + CO 2−3Å 3K + + NpO 2 (CO 3 ) 5−3(A.48)log 10 K s,3 (A.48) ≈ −(3.4 ± 0.5)in 1.2 M K 2 CO 3 aqueous solutions at 20 ◦ C, andlog 10 K s,3 (A.48) ≈ −(3.1 ± 0.5)in 1.2 M K 2 CO 3 aqueous solutions at 80 ◦ C are calculated. These values are consistentwith a previous estimate (see the discussion of [66GOR/ZEN]) within the uncertainties(considering the ionic strength were not the same in the two measurements). The latervalue [74VIS/VOL] is more accurate since the solid phase was prepared more carefullyhere. However, it was not possible to verify that the solid phase did not change between20 and 80 ◦ C.