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

378 18. Plutonium group 17 (halogen) compounds and complexesSawant et al. [85SAW/CHA] also reported formation constants for the 1:3 complex,NpO 2 F − 3. Again, these constants reported for I = 0.1 MandI = 1 M are notcompatible with the specific ion interaction coefficients in Appendix B. For the correspondingU(VI) system we have ε(18.11, q = 3) =−0.52 kg·mol −1 [92GRE/FUG],and by using this value to correct the data of [85SAW/CHA] toI = 0 we obtainlog 10 β3 ◦ (18.11, q = 3) = 9.6 and 8.4 for the experiments carried out at I = 0.1 Mand I = 1 M, respectively. In order to obtain consistent log 10 β3 ◦ values from thetwo determinations, we would have to use ε =+0.8 kg·mol −1 , and this would beincompatible with the commonly observed ionic strength dependence of comparableequilibria. The formation of NpO 2 F − 3needs closer examination, and no selection ismade at this time.The Gibbs energy of formation values are derived from the above selected constantsusing the selected f G ◦ m values for PuO2+2and F − . f G ◦ m (PuO 2F + , aq, 298.15 K) = −(1069.9 ± 3.1) kJ·mol −1 f G ◦ m (PuO 2F 2 , aq, 298.15 K) = −(1366.8 ± 4.1) kJ·mol −118.2.2 Aqueous plutonium chlorine complexes18.2.2.1 Aqueous plutonium chlorides18.2.2.1.a Aqueous Pu(III) chloridesQuantitative information on chloride complexation of Pu 3+ according to the equilibriaPu 3+ + qCl − Å PuClq 3−q(18.12)has been published by Ward and Welch [56WAR/WEL], Martin and White[58MAR/WHI] and Shiloh and Marcus [64SHI/MAR], and is summarised inTable 18.3. The study of Ward and Welch [56WAR/WEL] was carried out at ionicstrengths of 0.207, 0.5 and 1.0 M, respectively (cf. Appendix A). The formationconstant reported by Martin and White [58MAR/WHI] is not the result of anindependent measurement, but rather the result of an extrapolation of Ward andWelch’s [56WAR/WEL] value at I = 0.207 M (HCl) using the Davies equation.This value was used to derive the enthalpy of Reaction (18.12, q = 1) (see AppendixA and discussion below). The data of Ward and Welch [56WAR/WEL] reportedat “room temperature”, which Martin and White [58MAR/WHI] reported as 21 ◦ C, arethus corrected to t = 25 ◦ C(cf. Appendix A), using the enthalpy of reaction selectedbelow. Shiloh and Marcus [64SHI/MAR] used very high and variable ionic strengthsin their spectrophotometry experiments. These values cannot be used to select a formationconstant at I = 0. Giffaut [94GIF] noted that spectrophotometry is not a suitablemethod to study chloride complexation of Pu 3+ , due to the very small changes in theabsorption spectrum up to a chloride concentration of 4 M. It is probable that thiscomplexation could equally well be described as activity effects. The number of availableformation constants is insufficient for a linear extrapolation to I = 0. We adopt