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

A. Discussion of selected references 773TOPO(tri-n-octylphosphine oxide) as the organic phase extractant. Potassiumpermanganate was used as a holding oxidant. In the experiments performed atpH = 2.5 the sodium form of HDNNS was used as the extractant, and the data wereexplained in terms of formation of the complex NpO 2 H 2 PO + 4 ,NpO 2+2+ H 2 PO − 4Å NpO 2 H 2 PO + 4(A.73)log 10 β(A.73, 1MNaClO 4 , 298.15 K) = (2.52 ± 0.07)Since the data were obtained at constant pH, no information about the proton contentof the species formed can be obtained. Analogy with the dioxouranium(VI) phosphatesystem at acidic pH would suggest the formation of H 3 PO 4 and mixed H 3 PO 4 −H 2 PO − 4complexes, which the authors have not considered at pH = 2.5 (although they did consideredHPO − 4species). The concentrations of the various phosphate ions reported intheir Table 1 differ from the ones calculated with the values of the constants reportedat the bottom of the same table. For these reasons, the constant corresponding to EquationA.73 has been accepted only with a substantially increased uncertainty. In view ofthe analogies between uranium(VI) and neptunium(VI) systems, a correction to infinitedilution similar to that used for Equation V.134 in [92GRE/FUG] was applied. This,together with the corresponding correction of K 1 for phosphoric acid, yields:log 10 β ◦ (A.73, 298.15 K) = (3.32 ± 0.50)For the experiments performed at pH = 5.14, TOPO was used as the extractant,and the extraction mechanism was verified - a slope of 2 was obtained in the plot oflog 10 1/D vs.log 10 [DBM]. The data have been explained with the equilibria:NpO 2+2+ HPO 2−4Å NpO 2 HPO 4 (A.74)NpO 2+2+ 2HPO 2−4Å NpO 2 (HPO 4 ) 2−2(A.75)with the constants: log 10 β(A.74, 1MNaClO 4 , 298.15 K) = (4.54 ± 0.13) andlog 10 β(A.75, 1MNaClO 4 , 298.15 K) = (7.49 ± 0.13).No details of the pH electrode calibration are reported, and the influence of anactivity scale calibration to the calculated concentrations is apparently more importantatpH=5.14. Distribution coefficients much higher than those reported in Fig. 1of [94MAT/CHO] seems to have been used in the same system according to Fig. 3of the reference. Thus, D 0 values of the order of 500 can be estimated from Fig. 3,while in Fig. 1 the D 0 values vary between 0.03 and 1. The model function (Equation9 in [94MAT/CHO]) used to obtain the formation constants for HPO 2−4complexescontains a linear term in HPO 2−4multiplied by the constant for the H 2 PO − 4 complex,thus propagating its error here. No allowance has been made for the hydrolysis ofthe dioxoneptunium(VI) ion at pH 5.14, and there is not enough information to permitrecalculation of the data.Hence, the values reported have been selected with substantially increased uncertaintylimits:log 10 β(A.74, 1MNaClO 4 , 298.15 K) = (4.54 ± 0.70)log 10 β(A.75, 1MNaClO 4 , 298.15 K) = (7.5 ± 1.0)