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

304 16. Plutonium aqua ions298.15 K) = −(290.91 ± 0.85) kJ·mol −1 is used to calculate f G ◦ m from the valuesselected for Pu 3+ and CODATA auxiliary values. f G ◦ m (PuO2+ 2, aq, 298.15 K) = −(762.4 ± 2.8) kJ·mol−1Using the Gibbs-Helmholtz relation and r Hm ◦ = −(822.0 ± 6.6) kJ·mol−1 , r Sm ◦ (16.4, 298.15 K) = −(200.2 ± 22.1) J·K−1·mol−1 . From this number, thestandard molar entropy value of PuO 2+2is calculated as:S ◦ m (PuO2+ 2 , aq, 298.15 K) = −(71.2 ± 22.1) J·K−1·mol −1 .using Sm ◦ (Pu,α,298.15 K) = (54.46 ± 0.80) J·K−1·mol−1 from this review andCODATA auxiliary values.Lemire and Campbell [96LEM/CAM] have used the results of their study of apparentmolar heat capacities of NpO 2 ClO 4 (aq) with values of the partial molar heat capacitiesof other electrolytes [89HOV/NGU, 96LEM/CAM2] to estimate the variation ofthe potential of the Pu(VI)/Pu(V) couple as a function of temperature. The apparentmolar heat capacities of PuO 2+2and PuO + 2as a function of temperature were assumedequal to those for UO 2+2and NpO + 2, respectively [89HOV/NGU, 96LEM/CAM].The mean value proposed by Capdevila and Vitorge [95CAP/VIT] for r C p,m (16.8) is approximately −400 J·K −1·mol−1 . This was calculated by doubledifferentiation of a smoothed function of EVI/V ◦ (T) (based on experimental potentialmeasurements for temperature from 5 to 65 ◦ C), and is much more negative than the−70 J·K −1·mol−1 calculated by the method of Lemire and Campbell [96LEM/CAM].The uncertainties in that method cannot accommodate the Capdevila and Vitorge value[95CAP/VIT], and suggest the uncertainty in the entropy value proposed for that workshould also be increased. To a first approximation, this review accepts that the partialmolar heat capacities of all the aqueous dioxoactinide(V) and dioxoactinides(VI)ions can be assumed identical to those determined for UO 2+2and NpO + 2 , respectively[89HOV/NGU, 96LEM/CAM].16.3 PuO + 2The standard Gibbs energy of formation of the dioxoplutonium(V) cation is calculatedby this review from the standard potential of the reactionPuO 2+2+ e − Å PuO + 2(16.7)Several independent determinations of the PuO 2+2 /PuO+ 2potential in perchloricacid solution are available. They are summarised in Table 16.2. The formal potentialsin 1 M HClO 4 at 25 ◦ C were determined by Rabideau [56RAB],andbyCapdevilaet al. [92CAP, 92CAP/VIT, 95CAP/VIT], as E ◦′ = (0.9164 ± 0.0002) V, and(0.912 ± 0.007) V, respectively.The value of Riglet, Robouch and Vitorge [89RIG/ROB], (0.941 ± 0.0052) V,was determined at 20 ◦ C, and not at 25 ◦ C, the standard temperature. They derived