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

9.2 Aqueous neptunium group 17 (halogen) complexes 1699.2.2.3 Aqueous Np(V) chloridesSome authors have published formation constants for NpO 2 Cl(aq) and NpO 2 Cl − 2ationic strengths from 2 to 5 M (cf. Table 9.7). The stability of these complexes isobviously very low. The cation exchange study of Gainar and Sykes [64GAI/SYK]can be interpreted equally well by formation of NpO 2 Cl(aq) or activity factor effects,cf. the discussion in Appendix A about the nitrate experiments described in the samepaper [64GAI/SYK]. It should be mentioned that Danesi et al.[71DAN/CHI] reportedformation constants for these two complexes, which they withdrew later [74DAN/CHI]after re-evaluations based on an improved value for the liquid junction potential oftheir electrode system, cf. Appendix A. Vodovatov et al. [76VOD/KOL] could notobserve any chloride complexes of Np(V) by the proton relaxation method. Rao et al.[79RAO/GUD] discussed the studies reported in the literature and concluded that highchloride concentrations are necessary (> 3 M HCl) to distinguish the complexationeffects from the experimental errors. The same group [81PAT/RAM] used a very highionic strength of I = 8.5 M(NaClO 4 ) and chloride concentrations up to 0.75 M andinterpreted their extraction data in terms of the formation of NpO 2 Cl(aq). Neck, Kimand Kanellakopulos [94NEC/KIM] reported formation constants for the 1:1 and the 1:2complexes, but the ionic strength is high (I = 5 M) and in the absence of precise estimatesof the interaction coefficients, the error in the extrapolation to I = 0 will be dominatedby the error of our estimates of the ionic interactions. Giffaut [94GIF] observedno significant changes in the visible absorption spectrum of NpO + 2 in 1 M HClO 4 and4 M NaCl. Hence, it would be highly speculative to make a selection on the basis ofthe existing data.9.2.2.4 Aqueous Np(VI) chloridesThe influence of chloride concentration on the isotopic exchange rate between Np(V)and Np(VI) has been studied by Cohen, Sullivan and Hindman [55COH/SUL] at0,5and 10 ◦ C in 3 M perchloric acid solution. At chloride concentrations above 1.5 M thefitting of the experimental data was improved by taking into account the formation ofa 1:2 complex, NpO 2 Cl 2 (aq), although this is more likely to be a medium effect.NpO 2+2+ qCl − Å NpO 2 Cl 2−qq (9.11)Other values for q = 1 and 2 were reported from potentiometric measurements[70AL-/WAI2, 74DAN/CHI] and from solvent extraction experiments[71DAN/CHI] (cf. Table 9.8). In the careful experiment of Al-Niaimi, Wain andMcKay [70AL-/WAI2], the potential changes caused by the limited formationof NpO 2 Cl + were very small, resulting in a low precision, although the experimentalmethod seems reliable. Due to insufficient information, the results ofthe study of Sykes and Taylor [62SYK/TAY] are not used in selecting a recommendedvalue. We use the constants reported in [55COH/SUL] (extrapolated to25 ◦ C), and in [70AL-/WAI2, 71DAN/CHI, 74DAN/CHI, 78BED/FID] for ourselection, but a statistical treatment seems difficult. We therefore adopt the εvalue of the corresponding Pu(VI) system and increase its uncertainty by 0.05: