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

A. Discussion of selected references 757Table A.26: Temperature dependence of redox couples [92CAP].Couple I E (dE/dT ) isotherm ε(M) (V vs. SHE) (V·K −1 ) (kg·mol −1 )1 (0.978±0.007) (0.00184±0.00010)±0.005 (a) (0.00147±0.00011) (a)PuO 2+2 /PuO+ 20 (1.044±0.010) (0.00168±0.0003) (0.35±0.10)(0.00167±0.00014) (a) ±0.05 (a)1 (0.912±0.007) (0.00039±0.00010) −(0.913±0.005) (a) (0.00016±0.00004) (a)0 (0.938±0.010) (0.00019±0.00030) (0.22±0.10)(0.00034±0.00004) (a) ±0.03 (a)(a) Values reported in [95CAP/VIT] from the same set of experimental data, recalculated using asecond order polynomial regression versus temperature. Simple linear regression was used in[92CAP].McVey [51CON/MCV] andE ◦′ = (0.9821 ± 0.0005) V of Rabideau and Lemons[51RAB/LEM]. The difference between the standard potential and the formalpotential,E ◦ − E ◦′ = (0.064 ± 0.012) Vagrees with the (0.064 ± 0.011) V reported for the analogous Np 4+ /Np 3+ system byRiglet, Robouch and Vitorge [89RIG/ROB]. The results of this cyclic voltametry study(including temperature effects) were cited in [92CAP/VIT] (which was written beforethe thesis was completed) and reported in [95CAP/VIT].Using the constants K V and K IV for the disproportination equilibrium of PuO + 2or Pu 4+ , respectively, and the corresponding data for the reversible redox couples,Capdevila calculated the values in Table A.27 for the irreversible plutonium couples.b) Standard potential of the Pu(VI)/Pu(V) redox couple in carbonate mediaThis report also gives results of an apparently very thorough study on Pu(V) in acidicand basic carbonate media. Pu(V) was prepared electrolytically, and characterizedspectrophotometrically. The data from cyclic voltammetric experiments (25 ◦ C, 1 Msodium carbonate) and previously determined stability constants of PuO 2 (CO 3 ) 4−3[87ROB/VIT] were used to calculate a value for the formation of the PuO 2 (CO 3 ) 5−3at I = 0 using SIT and interaction coefficients determined experimentally oflog 10 β3 ◦ = (4.9 ± 0.8). All data was presented to permit re-evaluation of errors.The value of log 10 β 3 is similar to (or a little smaller than) the value of the correspondingNp(V) tricarbonato complex (log 10 β3 ◦ = 5.50 ± 0.15) selected in this review