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

17.1 Aqueous plutonium hydroxide complexes 321The data for the the anionic species in basic solutions, are too uncertain for hydrolysisconstants to be recommended at this time. f G ◦ m (PuO 2OH + , aq, 298.15 K) = −(968.1 ± 4.0) kJ·mol −1 f G ◦ m ((PuO 2) 2 (OH) 2+2, aq, 298.15 K) = −(1956.2+6.1−8.0 ) kJ·mol−1 f G ◦ m (PuO 2(OH) 2 , aq, 298.15 K) = −(1161.3 +3.9−9.0 ) kJ·mol−1Based on the work of Rizkalla et al. [94RIZ/RAO], but with an increased uncertainty(cf. Appendix A), the value r H m (Appendix A, Equation A.76, a = 1) =(28 ± 15) kJ·mol −1 is accepted. Hence: f H ◦ m (PuO 2OH + (aq)) =−(1080 ± 16) kJ·mol −117.1.2 Plutonium(V) hydroxide complexesThere are few studies of the hydrolysis of the Pu(V). This is due to problems associatedwith possible disproportionation or precipitation in the pH range where hydrolysis islikely to occur. One series of studies [46KRA/DAM, 48KRA/NEL, 49KRA/DAM,56KRA] provides a limiting value of log ∗ β1 ◦ =−9.7 for the first hydrolysis constant:PuO + 2 + H 2O Å PuO 2 OH(aq) + H + (17.1)A recent thesis and paper [90BEN, 92BEN/HOF] proposes log ∗ β1 ◦ (298 K) ≈log ∗ β 1 (296 K, 0.1 M) is − (9.73 ± 0.10), but for reasons outlined in Appendix Aentry for [90BEN], this value probably reflects an upper limit for the stability of thiscomplex. The selected upper limitlog ∗ β1 ◦ (17.1, 298 K) ≤−9.73from [90BEN, 92BEN/HOF] is consistent with the value selected for the analogousNp(V) species (log ∗ β1 ◦(8.1) =−(11.3±0.7)). If a value is required for log 10 ∗ β1 ◦(17.1)rather than a limit, a value equal to that for the corresponding neptunium reaction butwith an increased uncertainty, log ∗ 10 β1 ◦ (17.1) =−(11.3 ± 1.5), would be reasonable.Recently Peretrukhin, David and Maslennikov [94PER/DAV] determined E ◦ =−0.6 V for the reduction on Pu(V) to Pu(IV) in 1 M LiOH, indicating Pu(V) is unlikelyto disproportionate in basic solutions. Their work also suggested possible formationof an anionic Pu(V) hydrolysis species in strongly basic solutions (> 0.5 M LiOH),but provides no quantitative evidence of the stability of this or these species. Furtherwork is required on the stability of Pu(V) hydrolysis products. Peretrukhin, Davidand Maslennikov [94PER/DAV] andNeuet al. [94NEU/HOF], have concluded thatsignificant solution concentrations of Pu(V) species can be found in basic and neutralaqueous solutions of plutonium.