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

306 16. Plutonium aqua ionsE ◦ = (0.954 ± 0.010) V by applying the SIT to data from cyclic voltammetrymeasurements at 0.5, 1, 2, and 3 M ionic strengths. Rai [84RAI] reported E ◦= (0.956 ± 0.025) V from solubility measurements. Capdevila’s value of E ◦ =(0.938 ± 0.010) V was also derived by applying the SIT treatment to their data.They calculated the ion interaction parameter ε = (0.22 ± 0.03) kg·mol −1(similar to the values for the reactions involving the corresponding uranium andneptunium species (Appendix B)). Although these three values for the standardpotential lie within their combined uncertainties, we do not select the data becausethe measurements of Riglet, Robouch and Vitorge were hampered by experimentalproblems (cf. Appendix A) as was pointed out in a later publication [95CAP/VIT]by one co-author of the initial investigation. Rai’s value was derived solely fromsolubility measurements and carries, therefore, the large uncertainty of 0.025 V.Because the value of Capdevila et al. also has a relatively large uncertainty of 0.010 V,we derive the formal potential E ◦′ (16.7, 1MHClO 4 , 298.15 K) =(0.913±0.005) V,log 10 K(16.7, 1MHClO 4 , 298.15 K) = (15.43 ± 0.08) and r G m (16.7, 1MHClO 4 ,298.15 K) =−(88.09 ± 0.48) kJ·mol −1 by combining the raw data of Capdevila andVitorge [95CAP/VIT] and Rabideau [56RAB]. These data are corrected to standardconditions using the SIT ε (= (0.22 ± 0.03) kg·mol −1 ) value of Capdevila andVitorge [95CAP/VIT]. The following values are obtained:for the reactionE ◦ (16.7, 298.15 K) = (0.936 ± 0.005) Vlog 10 K ◦ (16.8, 298.15 K) = (15.82 ± 0.09) r G ◦ m (16.8, 298.15 K) = −(90.29 ± 0.51) kJ·mol−1PuO 2+2+ 1 2 H 2(g) Å PuO + 2 + H+ (16.8)The correction to the standard pressure of 1 bar lies within the uncertainty of thevalues. The estimate of Fuger and Oetting [76FUG/OET], E ◦ = (1.016 ± 0.05) V, appearsto be too high in light of these results, and the less positive value, E ◦ = 0.936 V,is used here. Therefore, f G ◦ m (PuO+ 2, aq, 298.15 K) = −(852.6 ± 2.9) kJ·mol−1is derived from r G ◦ m (16.8, 298.15 K) and the selected value of fG ◦ m (PuO2+ 2 ,aq,298.15 K).The change in enthalpy for Reaction (16.8) is obtained from the temperature coefficientof the potential. Figure 16.1 shows the temperature variation of the derivative withrespect to temperature of the PuO 2+2 /PuO+ 2 couple formal potential (1.0 M HClO 4)forRabideau’s [56RAB] and Capdevila and Vitorge’s [95CAP/VIT] data. Because of theconflicts in the trends in temperature variation, (∂ E ◦′ /∂T )=(0.0000±0.0002) V·K −1is selected by this review. This is consistent with the variation of the temperature coefficientbetween the different investigators listed in Table 16.2. Rabideau [56RAB]reported a very small negative coefficient, whereas Capdevila and Vitorge, as well as