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

A. Discussion of selected references 681dealing with the stability of weak complexes. There is also uncertainty as to the identityof the complexes, in the case of PO 3−4and CO 2−3complexes, as it is impossibleto determine the number of protons associated with a complex based on experimentsperformed at constant pH. In the case of NpO + 2 -PO3− 4there is an additional problemwith the assignment of stability constants: the data in the paper are more compatiblewith a series of higher order complexes of the form of either NpO 2 (HPO 4 ) 1−2nn orNpO 2 (PO 4 ) n−3n ,withn=1− 2 or greater (the protonation of the ligand cannot be determinedfrom these experiments), than the single NpO 2 PO 2−4species proposed. Thevariation in the ionic medium is sufficiently high and the experimental details sufficientlysparse in all of the experiments that this data cannot be used to select stabilityconstants. Nonetheless, the results concerning the SO 2−3complexes, where the experimentalproblems are fewer, and the remaining qualitative and semi-quantitative resultsare useful for the purposes of this review.[79RAO/GUD]The authors used a solvent extraction technique, with dinonylnaphthalenesulphonicacid (HDNNS) in benzene as the extractant, to study the complex formation betweenNpO + 2 and Cl− ,NO − 3 ,F− ,NO − 2 ,SO2− 4 ,SCN− and IO − 3 at 25◦ C. In the aqueous phasethe ionic strength is maintained at 2 M (NaClO 4 ). This is a careful study, and theextraction of uncharged complexes into the organic phase was verified by spectroscopy.Three extraction experiments were carried out for iodate as a ligand, at iodate concentrationsof 0.05, 0.10 and 0.15 M. The constant obtained is β 1 (NpO 2 IO 3 , aq) =(2.1 ± 0.1). According to this constant, the proportions of NpO 2 IO 3 (aq) formed in thethree experiments are 9, 17 and 24%, respectively. The amounts of complex formedare thus not very large, and the uncertainty in the resulting constant is high at suchlow complexation degrees. We hence accept the reported formation constant of the 1:1complex, but we assign it a larger uncertainty: log 10 β 1 (NpO 2 IO 3 , aq, I = 2M) =(0.32 ± 0.30).In the case of fluoride, the authors discussed some other studies, e.g.[70AL-/WAI],in which it was concluded that the neutral complex NpO 2 F(aq) did not form. Theyrelated these observations to the high acid concentrations in the experiments, and theyshowed that at pH = 5 the complex formation of Np(V) with fluoride is significant.They found a constant of β 1 (NpO 2 F, aq) = (9.8 ± 0.8). We accept the formationconstant reported here, but assign it a larger uncertainty, because there was a slighttrend in the resulting values in Table 3 [79RAO/GUD]. In our evaluation, we selectlog 10 β 1 (NpO 2 F, aq, I = 2M) = (0.99 ± 0.10), cf. Section 9.2.1.3.Only one sulphato complex was found. Recalculation using the tabulated data and,assuming that pH ≥ 3 (and therefore [SO 4 ] TOT=[SO 2−4 ]), yields log 10 β 1 = (0.45 ±0.10). For the nitrate complexation study, the experiments were carried out at pH 5 tomaintain neptunium as Np(V). The aqueous phase consisted of 2 M Na (ClO 4 ,NO 3 )mixtures with a maximum nitrate ion concentration of 1 M. Only 4 measurements ofK d were performed. A spectrophotometric test for the Np(V) peak in the organic phasewas run after extraction with TBP from 6 M NaNO 3 — a much higher concentrationof ligand than the one used in the measurements. The authors reported β 1 = (0.28 ±