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

A. Discussion of selected references 779the interaction coefficients are very similar for An = U, Np and Pu: ε(IV/III) =ε (An 4+ ,ClO − 4 ) − ε (An 3+ ,ClO − 4 ) = (0.35 ± 0.06) kg·mol−1 ,(0.35 ± 0.03) kg·mol −1 and(0.33 ± 0.05) kg·mol −1 respectively, and ε(VI/V) = ε (AnO2+2 ,ClO− 4 ) − ε (AnO + 2 ,ClO− 4 ) =(0.20 ± 0.06), (0.21 ± 0.03) and (0.22 ± 0.03) kg·mol −1 respectively. For plutoniumthe uncertainty in ε(IV/III) was reported as ±0.05 kg·mol −1 in Table 4, but as ±0.03kg·mol −1 in the abstract and in Table 2. The authors increased their uncertainties forsystematic errors (considering variation of the junction potential with temperature,ionic strength and temperature corrections to the standard potential of the referenceelectrode, and use of the SIT approximation). Re-evaluation carried out in conjunctionwith the present review led to ε(IV/III) = 0.22 kg·mol −1 with an uncertainty of ≤0.03, regardless of whether only the 25 ◦ C or the data for all temperatures, smoothed to25 ◦ C, were used. Previous ε values for the plutonium couples [89RIG/ROB] wererejected because interference between the two redox couples during the measurementof E ◦′ was suspected. Published E ◦′ and E ◦ data were discussed, and possibleexplanations of outliers were given. The E ◦′ and E ◦ values [95CAP/VIT] are in accordwith the ones selected in this review. This review selects and uses the ε valuesreported in this paper in preference to the SIT coefficients for Np and Pu species thathad been derived from [89RIG/ROB] and used previously [92GRE/FUG, 95SIL/BID].The influence of changes in temperature on interaction coefficients of reaction, ε,and the ionic strength correction applicable to the enthalpies and entropies of reactionsare also calculated in this work from the temperature and ionic strength dependence ofthe formal potentials of the plutonium reversible redox couples in acidic media.[95FAN/NEC]In this report a set of Pitzer coefficients is derived for Np(V) hydrolysis species andcarbonate complexes for perchlorate and chloride media (based on data reported inearlier publications). See the discussions of [91KIM/KLE, 94MEI, 94NEC/KIM,94NEC/RUN] in this appendix.[95MOR/PRA]Pratopo’s data (also in [93PRA/MOR]) were obtained from experiments in which equilibriumwas probably not attained. Although equilibrium constants based on these datawere proposed in Pratopo’s thesis [93PRA], equilibrium constants for soluble speciesbased on these data are not accepted in the present review. All the published Np(VI)solubility measurements for these conditions ([CO 2−3] > 10 µM) are quite scattered.Some values plotted in log 10 -log 10 representation are parallel to those for UO 2 CO 3 (s)solubility curves over small domains; but extracting numbers from this type of observationleads to contradictions with other published solubility work. The slow transformationof one solid phase (NpO 2 CO 3 (s)) to another (Na 4 NpO 2 (CO 3 ) 3 (s)) could explainthese scattered results for a single series of measurements, since soluble species generallyattain equilibrium faster than transformation can occur within the bulk of the solidphase. Also see the comments in this appendix, for [93PRA/MOR, 98VIT/CAP].