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

166 9. Neptunium group 17 (halogen) compounds and complexes9.2.2 Aqueous neptunium chloride complexes9.2.2.1 Aqueous Np(III) chloridesThe only quantitative information on chloride complexation of Np 3+ according to theequilibriaNp 3+ + qCl − Å NpCl 3−qq (9.9)has been published by Shiloh and Marcus [64SHI/MAR]. They observed changes inthe visible absorption spectrum and concluded that NpCl 2+ and NpCl + 2were formedat LiCl concentrations between 5 and 12.7 M. Under these conditions they determinedlog 10 β 1 (9.9, q = 1) =−(2.4 ± 0.1) and log 10 β 2 (9.9, q = 2) =−(5.0 ± 0.1). Thehigh and varying ionic strengths used do not allow the selection of any value for I = 0.9.2.2.2 Aqueous Np(IV) chloridesThe experimental equilibrium data available for the reactionsNp 4+ + qCl − Å NpCl 4−qq (9.10)are listed in Table 9.6. For reasons mentioned in Appendix A, we do not considerthe data in [58STR/PEE] and[62SYK/TAY], as well as [73BAR/MUR] (very highand variable ionic strength) and [76SOU/SHA] (variable ionic strength). If we use thedata of Shilin and Nazarov [66SHI/NAZ] (assuming, due to the lack of enthalpy data,that the temperature correction to 25 ◦ C is negligible), Danesi et al. [71DAN/CHI],and Patil and Ramakrishna [75PAT/RAM] (as re-evaluated by Giffaut [94GIF]) ina linear regression analysis, we obtain, for the first formation constant at I = 0,log 10 β1◦ = (1.55 ± 0.17) and ε = −(0.10 ± 0.08)kg·mol−1 . This ε value isabout the same as obtained for the corresponding Pu(IV) system, cf. Figure 18.1:ε(9.10, q = 1) =−(0.09±0.07)kg·mol −1 , but it differs from the value of the correspondingU(IV) system, ε =−(0.29 ± 0.13)kg·mol −1 [92GRE/FUG]. Consideringthat the parameter Ba j = 1.5 in the specific ion interaction equation was optimised forionic strengths below 3.5 m, cf. Appendix B, and that values measured at higher ionicstrengths should thus be given a lower weight or not be included at all if possible, weredo the extrapolation to I = 0 by omitting the value of Danesi et al. [71DAN/CHI],and we obtain log 10 β1 ◦ = (1.42 ± 0.23) and ε =−(0.19 ± 0.14)kg·mol−1 .Theεvalue is now between that of the corresponding Pu(IV) and U(IV) systems. We preferto select an intermediate value and to assign it an uncertainty that covers the range ofexpectancy of the two results, which has the effect of lowering the weight of the valueat I = 4M[71DAN/CHI] without disregarding it completely.log 10 β1 ◦ (9.10, q = 1, 298.15 K) = 1.5 ± 0.3For ε we select −(0.15 ± 0.18)kg·mol −1 following the same procedure. Figure9.3 shows the expected range of the value of log 10 β 1 (+8D) based on the selected