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

170 9. Neptunium group 17 (halogen) compounds and complexesTable 9.7: Experimental equilibrium data for the neptunium(V) chloride system.Method Ionic medium t ( ◦ C) log 10 β q(a) ReferenceNpO + 2 + Cl− Å NpO 2 Cl(aq)cix 2 M H(Cl,ClO 4 ) 25 −(0.29 ± 0.05) [64GAI/SYK]emf 4 M (Na,H)(Cl,ClO 4 ) 25 −(2.5 ± 0.4) (b) [71DAN/CHI]emf 4 M (Na,H)(Cl,ClO 4 ) 25 “no complexation” [74DAN/CHI]prx ? RT? < −0.7 [76VOD/KOL]dis 2 M Na(Cl,ClO 4 ) 25 −(0.42 ± 0.04) [79RAO/GUD]dis 8.5 M Na(Cl,ClO 4 ) 25 0.48 [81PAT/RAM]sp 4 M NaCl 25 “no complexation” [94GIF]dis 5 M Na(Cl,ClO 4 ) 25 −(0.05 ± 0.02) [94NEC/KIM]NpO + 2 + 2Cl− Å NpO 2 Cl − 2emf 4 M H(Cl,ClO 4 ) 25 −(1.55±0.05) (b) [71DAN/CHI]emf 4 M H(Cl,ClO 4 ) 25 “no complexation” [74DAN/CHI]dis 5 M Na(Cl,ClO 4 ) 25 −(0.71 ± 0.04) [94NEC/KIM](a) Refers to the reactions indicated, as well as to the ionic strength and temperaturegiven in the table.(b) These values were withdrawn later by the same authors [74DAN/CHI].ε(9.11, q = 1) =−(0.08 ± 0.13) kg·mol −1 . The constants are converted to molalunits and then corrected to I = 0, cf. Table 9.8. The resulting values at I = 0arecompatible. Their large uncertainties originate from the error propagation in theextrapolation procedure. We select the weighted average of these values:log 10 β1 ◦ (9.11, q = 1, 298.15 K) = 0.40 ± 0.17.As a comparison, the IAEA review [92FUG/KHO] selected log 10 β1 ◦ = (0.4±0.1),which agrees with the present selection.Two publications [55COH/SUL, 78BED/FID] reported constants for the formationof the 1:2 complex. Both these studies seem to be reliable, cf. Appendix A. If we adoptthe ε value accepted from Giffaut’s [94GIF] evaluation for the corresponding Pu(VI)system, ε(9.11, q = 1) =−(0.43 ± 0.20)kg·mol −1 , we obtain log 10 β2◦ =−0.7and −0.2, respectively, cf. Table9.8. These small values are likely due to inter-ionicinteractions, rather than to the formation of NpO 2 Cl 2 (aq).The Gibbs energy of formation value is derived from the above selected constantusing the selected f G ◦ m values for NpO2+2and Cl − . f G ◦ m (NpO 2Cl + , aq, 298.15 K) = −(929.4 ± 5.7) kJ·mol −1