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

330 17. Plutonium oxygen and hydrogen compounds and complexesTable 17.4: Data for the Pu(III) hydroxide system. ∗ β 1 refers to Pu 3+ + H 2 O ÅPuOH 2+ + H + .t ( ◦ C) Method Medium log10 ∗β1 (b) log10 ∗βo1 Referenceroom? pot (gl) 0.069 M NaClO 4 −7.12 −6.73 [46KRA/DAM2][48KRA/NEL2][49HIN/AME]room? pot (gl?) 0.069 M NaClO 4 −7.22 −6.83 [49KRA/DAM3]room? pot (gl?) 0.024 M NaCl −7.37 −7.06 (a) [49KRA/DAM3]room? pot (gl?) 0.040 M NaCl −7.25 −6.86 (a) [50BUS/COW]23 dis (hydrazine) 0.20 M LiClO 4 −3.80 −3.26 [75HUB/HUS]23 dis (red?) 0.10 M LiClO 4 −3.80 −3.36 [76HUB/HUS]25 pot (gl?) 1.00 M NaClO 4 −5.55 −4.74 [82NAI/CHA](a) Including corrections for Cl − complexation (0.05 and 0.08, respectively).(b) For discussion of uncertainties see textused to calculate the titration curve, systematic deviations occur from the experimentalpoints. It is not possible for the reviewer to explain the discrepancy, as too many explanationsare possible. If Pu(IV), which hydrolyses much more easily (i.e., at a lowerpH), was present, this would account for the difference between the calculated andobserved experimental values and yield too large a value for log10 ∗β1 of Pu(III).The third group, using radio-tracer solvent extraction with the extractant HTTA,obtained even larger hydrolysis constants than groups (i) and (ii), indicating that thehydrolysis starts at much lower pH values when using this technique (as compared tousing potentiometric titrations). There should be no such discrepancy between the twotechniques, but in this kind of study there can be many difficulties with radio-tracersolvent extraction. The results in [75HUB/HUS] are discussed in Appendix A. Themain difficulty is to avoid the oxidation of Pu(III) to Pu(IV); the reviewer believes thatPu(IV) was present and interfered with the results, even though care was taken to avoidoxidation (excluding air and adding hydroxylamine as a reductant). The plutoniumconcentration was quite low, 10 −5 to 10 −8 M, and the amount of Pu(IV) was not measuredduring the experiments, which also showed unexpected kinetics. Hubert et al.[76HUB/HUS] did a slightly different experiment, carried out at lower ionic strength(0.1 instead of 0.2 M LiClO 4 ) and in the presence of citrate, which gave approximatelythe same equilibrium constant. In both cases, the statistical treatment of the data wasunsatisfactory. Table 17.4 shows that it would be unwise to try to estimate ε by aSIT plot. However, using ε = (0.04 ± 0.05) kg·mol −1 estimated by analogy withlanthanide systems [95SIL/BID], would yield a log10 ∗β◦1(17.4, I = 0) =−7.0.Pu 3+ + H 2 O Å PuOH 2+ + H + (17.4)The average value of the potentiometric measurements (excluding the result from[82NAI/CHA]) is −6.73 at I ≤ 0.05 M after corrections for temperature and chloridecomplexation. In this reviewlog10 ∗ β◦ 1 (17.4, I = 0) =−(6.9 ± 0.3)