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

328 17. Plutonium oxygen and hydrogen compounds and complexesTable 17.2: Hydrolysis and solubility data for PuO + 2n : m Methods t Ionic strength log10 ∗βn,m log10 ∗β◦n,m Reference( ◦ C) (medium)sol RT var. Cl − (log 10 K s,0 =−8.6) [48KRA/NEL]sol RT self ClO − 4(log 10 K s,0 =−9.3)[64GEL/ZAI][68ZAI/ALE]1:1 pot RT 3 × 10 −3 M −9.7 −9.7 [49KRA/DAM]ClO − 41:1 LPAS 23 0.1 M ClO − 4 −9.73 −9.73 [92BEN/HOF]17.1.4 Plutonium(III) hydroxide complexesPu(III) is unstable in aqueous solutions towards oxidation by air and becomes lessstable as the pH increases. Excluding air and/or adding a reductant will lead to ratherstable Pu(III) solutions in the absence of species that can form strong complexes withPu(IV). Solvent extraction with strongly complexing extractants (like HTTA), will shiftthe equilibrium towards the IV state.Table 17.4 summarises the ∗ β 1 Pu(III) hydrolysis data for the reactionPu 3+ + H 2 O Å PuOH 2+ + H + (17.3)There are really only six experimental studies (although two different valuesare reported on the basis of one of them [46KRA/DAM2, 49KRA/DAM3]).There are three kinds of data: (i) potentiometric titrations done at Oak Ridgeand Los Alamos ([46KRA/DAM2], [49KRA/DAM3], [50BUS/COW]) duringthe U.S. Manhattan Project, (ii) a potentiometric titration [82NAI/CHA] done atTrombay, and (iii) studies done using the radio-tracer solvent extraction technique([75HUB/HUS, 76HUB/HUS]).The potentiometric measurements by Kraus et al. were carried out using Pu solutionsin ≤ 0.002 M NaCl or NaClO 4 at (probably) room temperature. Kraus et al.alsostudied the hydrolysis (and complexation) of plutonium in all oxidation states, so thisgroup was familiar with problems resulting from oxidation state changes, precipitation,as well as possible polymerization during hydrolysis at these Pu concentrations (seeAppendix A). The titrations were carried out with a protective atmosphere, avoidingcarbonate in the titrant, and using spectrophotometry to control the relative concentrationsof Pu(III) and Pu(IV) species. The experiments seem very reliable up to pH ≤ 7,when precipitation began to occur. With modifications, the experiments were repeatedseveral times with the same result. Studies of Am(III) suggest such solutions maybe supersaturated [95SIL/BID]. The potentiometric titrations by [82NAI/CHA] werecarried out in an inert atmosphere up to pH 6, when the solutions became opalescentdue to “oxidation and precipitation” of Pu(IV). The Pu concentration was rather low,0.04 mM, which would required high precision in order to yield reliable hydrolysisdata up to a ligand number of 0.7. When the derived log10 ∗β1 =−(5.55 ± 0.03) is