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

A. Discussion of selected references 777result from several causes: including the small systematic errors noted in this appendix,incorrect theoretical methodology (i.e., problems with the SIT or the currently acceptedε values), chemical problems with the solid phases or other experimental difficulties.[94RIZ/RAO]The paper presents results of a calorimetric study at 25 ◦ C of the titration of 242 Pu(VI)in initially acidic aqueous solution with CO 2−3-free NaOH. It provides estimates ofthe enthalpies of hydrolysis of PuO 2+2to form the monohydroxydioxoplutonium(VI)( r H m = (28.0 ± 5.0) kJ·mol −1 ) and the dihydroxydioxoplutonium(VI) complexes( r H m = (42.6 ± 7.0) kJ·mol −1 ).PuO 2+2+ aH 2 O(l) Å PuO 2 (OH) (2−a)+a (aq) + aH + (A.76)The raw data are not available for recalculation, and this is unfortunate because thepartition of the enthalpy change between the reactions to form different hydrolysisspecies is dependent on the selected hydrolysis constants, or at least on their relativevalues. The hydrolysis constant values actually used were from Pashalidis et al.[93PAS/KIM2]. Recalculation as discussed in Appendix A of the present review, suggestsonly slightly different values, log10 ∗β11 =−5.14 and log10 ∗β12 =−10.03in1MNaClO 4 at 20 ◦ C, but these (particularly ∗ β 12 )maybeinerror[95PAS/KIM]. However,if the entropies of reaction as reported are correct, it appears that regardless ofother problems, the log ∗10 β 1i values can be considered to be independent of temperaturebetween 20 and 25 ◦ C. If the results from Pashalidis et al. [95PAS/KIM] are considered,it would appear the enthalpy changes observed by Rizkalla et al. should beattributed to the first hydrolysis reaction, and (probably) to some undesired polymerisationreaction(s), at least for the pH ranges shown in Table 2. The suggested value of28 kJ·mol −1 is accepted here for the first hydrolysis step, but with the uncertainty increasedto ±15 kJ·mol −1 . It is not clear which hydrolysis reaction should be properlyassociated with the second enthalpy, and it is not accepted in the present review.[94VEI/SMI]This is a qualitative study of Pu(IV) nitrate complexation using a variety of spectroscopicmethods at an unspecified temperature and at 2.34 molal ionic strength, probablyperchloric acid (this is not clearly stated, if the total molar concentration wasconstant, the molality changes in the mixtures). The major finding from the absorptionspectroscopy measurements is the presence of two near-isobestic points at 471.2 and472.3 nm. This is interpreted as proof of the existence of three species, that is, theaqueous ion and its two complexes with one and two nitrates. Furthermore, the twonitrate complexes have identical maximum molar absorptivity at 476 nm. This reviewdoes not consider the observed effect as sufficient proof of the formation of the dinitratecomplex. Preliminary EXAFS measurements were also been reported, but a morecomplete discussion was carried out in [96ALL/VEI].