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

322 17. Plutonium oxygen and hydrogen compounds and complexes17.1.3 Plutonium(IV) hydroxide complexesThere have been a number of potentiometric [51RAB/LEM, 57RAB], solubility[90PAZ/KUD], extraction [74SOL, 76MET/GUI] and spectroscopic[50KRA/NEL, 60RAB/KLI, 96NIT/SIL] studies of the first hydrolysis constant ofPu(IV). These are discussed in Appendix A. Measured values may be time dependentbecause of slow oxidation, disproportionation, radiolysis, or polymerization reactions.Therefore, the hydrolysis constant can be found only from studies in which theelectrochemical potential was carefully controlled, or in which the equilibrium totalplutonium(IV) in solution was measured, or the measurements were carried outrapidly.The values of ∗ β 1 for reaction:Pu 4+ + H 2 O(l) Å PuOH 3+ + H + (17.2)reported from the two extraction studies [74SOL, 76MET/GUI], are in relatively goodagreement, but the source of Solovkin’s values [74SOL] is not clear (see Appendix A).Metivier and Guillaumont’s value [76MET/GUI] differs by an order of magnitudefrom the value found potentiometrically in a similar medium [51RAB/LEM]. Conversely,the spectrophotometric results of Rabideau and Kline [60RAB/KLI] arealmosthalf an order of magnitude larger than the value reported from potential measurementsby Rabideau [57RAB]. In this complicated system, spectrophotometric measurementsare probably less prone than extraction or potentiometric measurements tosome of the possible systematic errors that can originate from slow disproportionationof Pu(IV). There is only one study in chloride solution. Even for perchlorate solutionsthere are insufficient data, and the data are too badly scattered, to calculate an interactionε term for the hydrolysis reaction. An estimated value of ε (PuOH 3+ ,ClO − 4 ) =0.50 kg·mol −1 was used to calculate ε =−0.18 kg·mol −1 and used with the experimentallog ∗ 10 β 1 values from the three spectroscopic studies in perchlorate media[50KRA/NEL, 60RAB/KLI, 96NIT/SIL] (equally weighted) to obtain the selectedlog10 ∗ β◦ 1 (17.2, 298.15 K) =−(0.78 ± 0.60)The uncertainty has been increased to overlap with the values from the potentiometricstudies [51RAB/LEM, 57RAB]. Within the uncertainties, the value is the same asthose selected for corresponding reactions involving uranium [92GRE/FUG] and neptunium(Section 8.1.4) species. Hence, f G ◦ m (PuOH3+ , aq, 298.15 K) = −(710.7 ± 4.4) kJ·mol −1Two studies [57RAB, 60RAB/KLI] provide information on the temperature dependenceof the first hydrolysis constant of Pu 4+ . The reported values of r H m forthe reaction 17.2 are (30.5 ± 2.1) [57RAB] and (35.6 ± 3.8) [60RAB/KLI], both determinedin perchlorate media of approximately 2 M ionic strength. Both these valueswere obtained from changes in equilibrium constants over quite small temperature differences,and the actual hydrolysis constants reported in the two studies are only in