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

374 18. Plutonium group 17 (halogen) compounds and complexesAppendix A, and combination with the enthalpy of protonation of the fluoride ion, f H ◦ m = (12.2±0.3) kJ·mol−1 , results in the following selected values: r Hm ◦ (18.9, q = 1, 298.15 K) = (9.1 ± 2.2) kJ·mol−1 r Hm ◦ (18.9, q = 2, 298.15 K) = (11 ± 5) kJ·mol−1From this value, we derive the enthalpies of formation and the entropies of PuF 3+and PuF 2+2 : f H ◦ m (PuF3+ , aq, 298.15 K) = −(866.1 ± 3.9) kJ·mol −1 f Hm ◦ (PuF2+ 2, aq, 298.15 K) = −(1199.6 ± 6.0) kJ·mol−1S ◦ m (PuF3+ , aq, 298.15 K) = −(229 ± 13) kJ·mol −1Sm ◦ (PuF2+ 2, aq, 298.15 K) = −(105 ± 20) kJ·mol−118.2.1.3 Aqueous Pu(V) fluoridesNo fluoride complexes of PuO + 2 have been identified. Values for the NpO+ 2 complexesmay be used as a guideline.18.2.1.4 Aqueous Pu(VI) fluoridesAs in the case of neptunium(VI), the investigation of fluoride complexation of plutonium(VI)is complicated by the fact that reduction of PuO 2+2to Pu(IV) is favouredin acidic medium and in the presence of fluoride ion. The constants reported in theliterature refer to either of the following reaction types,PuO 2+2+ qHF(aq) Å PuO 2 F 2−qq + qH + (18.10)PuO 2+2+ qF − Å PuO 2 F 2−qq , (18.11)depending whether the fluoride ion was protonated or deprotonated under the experimentalconditions. The constants are listed in Table 18.2. As in the case of Np(VI),the study published by Krylov, Komarov and Pushlenkov [68KRY/KOM2] yields constantsthat are unrealistically high, probably again due to reduction of Pu(VI) duringthe experiment (cf. Appendix A). The study of Savage [74SAV] is a rather approximateone reporting an even higher formation constant of PuO 2 F + , calculated via theformation of a hydroxide complex, and must be discarded as well. Since the resultsof Patil and Ramakrishna [76PAT/RAM] are also discarded (cf. Appendix A), due toa very strong trend in the results, the only remaining data are those of Choppin andRao [84CHO/RAO] and Sawant et al. [85SAW/CHA] referring to Reaction (18.11).Correction of these constants to I = 0byusingε(18.11, q = 1) = −(0.19 ±0.10) kg·mol −1 , as was done for the corresponding neptunium system above, yieldsvalues that are not incompatible (cf. Table 18.2). We select the weighted average ofthese constants, but we increase the resulting uncertainty of ±0.10 to ±0.20 to reflect