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

A. Discussion of selected references 733and the lack of other data, this review accepts this value.A general problem arises from the standpoint of strict thermodynamics whenformal potentials are determined against a reference electrode other than the standardhydrogen electrode. For example, Riglet, Robouch and Vitorge used the cellAg | AgCl | 3MNaClO 4 | 0.02 M NaCl || NpO 2+2| NpO + 2 | lMHClO 4 | PtTo compare the potential of this cell to a cell arrangement using a SHE referenceelectrode the potential of the following cell must be known:Pt | H 2 (1atm) | lMHClO 4 || 3MNaClO 4 || 3MNaClO 4 | 0.02 M NaCl | AgCl | AgRiglet, Robouch and Vitorge used the activity coefficient of HClO 4 for the lefthalf-cell and the Cl − activity coefficient calculated with the SIT for the right half-cellto correct to the standard potential. However, H + ,Cl − ,Na + ,andClO − 4may have beentransferred across the junction. The system is strictly not at equilibrium, because theNaClO 4 and NaCl concentrations in the left half-cell are very close to zero.b) Pu 4+ /Pu 3+ systemRiglet, Robouch and Vitorge determined the formal potential of the Pu 4+ /Pu 3+ couplein 1 M HClO 4 at 293.15 K by cyclic voltammetry using a platinum electrode asE ◦′ (293) = (0.959±0.005) V. Connick and McVey [51CON/MCV] and Rabideau andLemons [51RAB/LEM] determined for the Pu 4+ /Pu 3+ couple the formal potentialsof E ◦′ = (0.982 ± 0.002) VandE ′ = (0.9821 ± 0.0005) V, respectively. The dataof these investigators were derived from junction potential-free emf measurementswhich are precise and accurate. To compare their non-standard (293.15 K) formalpotential to these earlier derived potentials at standard temperature, Riglet, Robouchand Vitorge used a temperature correction of (11 ± 2) mV which they indicated wasderived by Connick and McVey [51CON/MCV]. This is incorrect. Connick andMcVey’s value is (dE/dT ) isotherm = (0.00125 ± 0.00004) V·K −1 . Capdevila, Vitorgeand Giffaut [92CAP/VIT] reported (dE/dT ) isotherm = (0.00168 ± 0.00007) V·K −1 ,that was determined over a wide temperature range. This review selects(dE/dT ) isotherm = (0.00149 ± 0.00008) V·K −1 (see text). Therefore, a correctionof (0.0075 ± 0.0004) V should be applied for the temperature change ofthe formal potential from 293.15 to 298.15 K. Correcting Riglet, Robouch andVitorge’s potential by this amount results in the formal Pu 4+ /Pu 3+ potential atstandard temperature, E ◦′ = (0.966 ± 0.0005) V. This potential is smaller by 16 mVthat the potentials determined earlier, E ◦′ = (0.982 ± 0.002) V by Connick andMcVey [51CON/MCV], and E ◦′ = (0.9821 ± 0.005) V by Rabideau and Lemons[51RAB/LEM]. The reason for this discrepancy may be found in the problem ofRiglet, Robouch and Vitorge’s reference electrode, which is discussed above for theNpO 2+2 /NpO+ 2system, or in the possible presence of polymeric Pu(IV) in solutionwhich would shift the potential to more negative values. However, the most likelyreason, as suggested later by Capdevila and Vitorge [95CAP/VIT], is interferencebetween the two reversible redox couples during the measurements.