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

21.1 Plutonium carbon compounds and complexes 437is calculated to be log 10 K ◦ (21.9, 298.15 K) = (52.7±1.6). Because of the uncertaintyin the assignment of interaction coefficients in the Pu(VI) system, however, this valueshould be used with caution at any ionic strengths much different from that of theoriginal experiment.21.1.2.1.2 The Pu(V) carbonate complexesA stability constant for the reactionPuO + 2 + CO2− 3Å PuO 2 CO − 3(21.10)was determined by laser photoacoustic spectrometry by [90BEN] and[92BEN/HOF]:log 10 β 1 (21.10, I = 0.5MNaClO 4 , 296.15 K) = (4.60 ± 0.03). Using the recommendedvalue of ε (NpO2 CO − 3 ,Na+ ) as an estimate of ε (PuO 2 CO − 3 ,Na+ )=−(0.18 ± 0.18) (withthe uncertainty expanded), ε(21.10) =−(0.34 ± 0.19) kg·mol −1 . Using this valueof ε and assuming that the measurements at 296.15 K can be used at 298.15 K:log 10 β1 ◦ (21.10, 296 K) = (5.12 ± 0.14) r G ◦ m (21.10, 298.15 K) = −(29.2 ± 0.8) kJ·mol−1which, in the absence of an enthalpy determination for correction to standard state conditions,are accepted as the recommended values at 25 ◦ C. Although no such analogousspecies has been reported in the U system, in the Np system log 10 β ◦ (NpO 2 CO − 3 ) =(4.96 ± 0.06) in fairly good agreement with the reported result. There is no experimentalbasis for the assignment of an entropy or enthalpy to reaction 21.10.The stability of the limiting Pu(V) carbonate complex at high carbonate concentrationand 25 ◦ C was determined from measurements of the formal potential of thePu(VI)/Pu(V) couple in 0.3-1.5 M (Na 2 CO 3 ) medium by Capdevila [92CAP]. On thebasis of the reversibility of the couple (determined by cyclic voltammetry) and thestoichiometry of the limiting PuO 2 (CO 3 ) 4−3complex, the limiting Pu(V) complex wasassumedtohavetheformPuO 2 (CO 3 ) 5−3and be formed by a single electron transfer.It was recognised that the PuO 2 (CO 3 ) 5−3complex could lose a CO 2−3subsequent toreduction. It is possible that the reaction at the electrode involves both electron andcarbonate ion exchange:PuO 2 (CO 3 ) 4−3+ e − Å PuO 2 (CO 3 ) −(5−2n)3−n+ nCO 2−3(21.11)where n = 1, 2. Given the high concentrations of CO 2−3in solution this reactionmight continue to be reversible at the voltametric electrode. Although there is no independentevidence of the stoichiometry of the stable limiting Pu(V) carbonate complexin solution, the tricarbonato stoichiometry of the limiting complex (n = 0inEq. (21.11)) is accepted by analogy to the Np(V) species (see Section 12.1.2.1.3).Capdevila [92CAP] gave the reversible potential of the Pu(VI)/Pu(V) couple in 1 MNa 2 CO 3 as (339 ± 7) mV vs. SHEat25 ◦ C and reported the results of similar experimentsat temperatures between 5 and 65 ◦ C and in solutions ranging from 0.3