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

16.2 PuO 2+2303After incorporating changes in the auxiliary data, and using the selected value of f H ◦ m (Pu3+ , aq, 298.15 K) (cf. Section 16.5), the present review accepts f Hm ◦ (PuO2+ 2, aq, 298.15 K) = −(822.0 ± 6.6) kJ·mol−1We also adopt the approach of the IAEA review that based their standardmolar entropy value for PuO 2+2on data of Connick and McVey [53CON/MCV]and Rabideau [53RAB] for the Pu 4+ disproportionation equilibrium reaction, r G m (16.5, 1MHClO 4 , 298.15 K) = (11.68 ± 0.54) kJ·mol −1 ,3Pu 4+ + 2H 2 O Å PuO 2+2+ 2Pu 3+ + 4H + (16.5)and on E ◦′ (Pu 4+ /Pu 3+ ),andS ◦ m (Pu3+ , aq, 298.15 K) (cf. Section 16.5). Capdevila andVitorge [92CAP/VIT, 95CAP/VIT] recently confirmed the disproportionation equilibriumconstant and derived E ◦ =−(0.988±0.020) V for the standard potential accordingtoReaction(16.6).PuO 2+2+ 4H + + 3e − Å Pu 3+ + 2H 2 O (16.6)Because of the large uncertainty in this standard potential, this review prefers to derive r G m (16.6, 1MHClO 4 , 298.15 K) = (295.95 ± 0.56) kJ·mol −1 and log 10 K(16.6,1MHClO 4 , 298.15 K) =−(51.85±0.10) by combining the value of the formal Gibbsenergy of reaction for the Pu 4+ disproportionation equilibrium Reaction (16.5) withthe formal Gibbs energy of reaction corresponding to the Pu 4+ /Pu 3+ redox potential(0.9821 V, see Section 16.4). The values are converted to molality units and extrapolatedto the standard condition, I = 0, using the specific interaction theory (SIT). Inthe absence of reliable values for the interaction coefficients for Pu 3+ and PuO 2+2withClO − 4 , the values of the homologous ions Nd3+ , ε (Nd 3+ ,ClO − 4 )= (0.49±0.03) kg·mol−1[83SPA, 95SIL/BID], and UO 2+2 , ε (UO 2+2 ,ClO− 4 ) = (0.46 ± 0.03) kg·mol−1 are used, butwith the uncertainties of the interaction coefficients estimated by analogy raised to0.05 kg·mol −1 (values of several interaction coefficients [92GRE/FUG, 95SIL/BID]involving plutonium species have been revised in the present review for reasons discussedin footnotes to Appendix B).The following selected values are obtained (the contribution to this potential ofconversion to a standard pressure of 1 bar (+0.00017 V) is within the uncertainty of thevalue):log 10 K ◦ (16.6, 298.15 K) = (50.97 ± 0.15)E ◦ (16.6, 298.15 K) = (1.005 ± 0.003) V r G ◦ m (16.6, 298.15 K) = −(290.91 ± 0.85) kJ·mol−1E ◦ (PuO 2+2 /Pu3+ ) = (0.988 ± 0.020) V for the reduction potential that wasderived by Capdevila and Vitorge [92CAP/VIT] is consistent with the selected valueE ◦ (16.6, 298.15 K) = (1.005 ± 0.003) V. The Gibbs energy of reaction r G ◦ m (16.6,