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

A. Discussion of selected references 701and finally using, as above, log 10 β3 V = (8.54 ± 0.3) [83MAY]()log 10 β3 VI in 1 M NaClO 4 = (20.59 ± 0.76)for3CO 2−3+ NpO 2+2Å NpO 2 (CO 3 ) 4−3Fixing the E ◦′value at (0.426NpO 2 (CO 3 ) 4−3 /NpO 2(CO 3 ) 5−3 ± 0.044) V/SHE instead3of using the value from the best least-squares fit, has a relatively small effect onthe value of log 10 (β3 VI/βV3in1MNaClO 4 ), but there is more influence on thevalues of the second and third (NpO 2+2 /CO2− 3) dissociation constants. The valueof log 10 (β 2 /β 3 ) =−(4.85 ± 1.05) is changed to −(5.72 ± 1.96) and the value oflog 10 (K 2·K 3 ) =−(11.47 ± 2.14) is changed to −(11.14 ± 1.97).Hence,log 10 β 1 (1MNaClO 4 ) ≤ (9.46 ± 2.11)log 10 β 2 (1MNaClO 4 ) ≤ (14.87 ± 2.10)The β i values estimated here for Np(VI) are of the same order of magnitude as those ofuranium, log 10 β 1 (1MNaClO 4 ) = (8.44 ± 0.07),log 10 β 2 (1MNaClO 4 ) = (15.67 ±0.13) recalculated using values in [92GRE/FUG]. The value of β 3 is slightly smallerfor Np(VI) than for U(VI), but the measurements of β 1 and β 2 are not accurate enoughto demonstrate any differences between the U and Np systems. As already mentioned,the values of β i /β 3 calculated here (and hence the β i values deduced from them) areonly rough estimates.b) Potentiometric titrations of Np(VI) in carbonate mediaAqueous HClO 4 was added to a Np(VI) carbonate solution in which carbon dioxidegas, pre-equilibrated with an aqueous solution of the same ionic strength, was bubblingthrough the working solution to maintain a constant value of p CO2 . The ionicstrength was constant (1 M NaClO 4 ), and log 10 [H + ] was measured with a glass electrodethat had been calibrated using concentration standards. The author tried differentmodels (sets of complexes) to interpret his potentiometric data, and using the knownformation constants of some of the simple hydroxide complexes [72CAS/MAG2] asfixed parameters, gave the corresponding curve-fitting results. It was concluded thatin addition to the known simple hydrolysis products, the limiting complex of Np(VI)reacted to form NpO 2 (CO 3 ) 2−2and (NpO 2 ) 2 CO 3 (OH) − 3. Formation constants werecalculated for these species. As mentioned by Maya, the curve-fitting results themselves,are not sufficient to determine a unique model or set of species. Independentqualitative chemical information was used to choose an interpretation, and the selectedmodel was probably not correct for the following reasons:First, the statistical analysis was not really correct because:• The values of some fitted parameters were correlated, indicating some of the experimentalparameters had not been varied sufficiently (approximately the same