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

20.1 Plutonium nitrogen compounds and complexes 413Hence the following formation constant results from the extrapolation:log 10 β1 ◦ (20.2, q = 1, 298.15 K) = 1.95 ± 0.15and has been selected in this review. This value, and the value of ε =−(0.19 ±0.05) kg·mol −1 , obtained simultaneously, compare well with those for the correspondingvalues for the neptunium complex (Section 11.1.4.1) and with log 10 β1 ◦ =1.47 ± 0.13 and ε =−(0.21 ± 0.05) kg·mol −1 reported for the uranium complex[92GRE/FUG].20.1.3.1.c Aqueous Pu(V) nitratesNo nitrate complexes of dioxoplutonium(V) have been identified.20.1.3.1.d Aqueous Pu(VI) nitratesQuantitative information on the formation of nitrate complexes of plutonium(VI) accordingto the reactionPuO 2+2+ qNO − 3Å PuO 2 (NO 3 ) q 2−q(20.3)(Table 20.3) has been reported by Heisig and Hicke [52HEI/HIC], Lahr and Knoch[70LAH/KNO] and Mazumdar and Sivamakrishnan [65MAZ/SIV] using distributionmeasurements, while Krevinskaya et al. [59KRE/NIK] and Vasiliev, Andeichuk andRyukov [75VAS/AND2] have used spectrophotometric measurements. As noted in AppendixA, the data from several groups [59KRE/NIK, 70LAH/KNO, 75VAS/AND2]have been rejected due to experimental shortcomings. The apparently high value ofthe constant reported in [75VAS/AND2] has to do with the fact that it does not refer toEquation (20.3) (q = 2) but to a reaction involving water substitution (see Appendix Afor details).The dioxoplutonium(VI) nitrate complexes are weak as is also the case for the otheractinides [92GRE/FUG] (also see Section 11.1.4.1). This makes it difficult to distinguishbetween complex formation and changes in the activity factors of the speciesstudied, caused by the large changes in the composition of the ionic medium whensubstituting the inert anion (often perchlorate) by the ligand ion (nitrate). As discussedin Appendix A for both studies considered here ([65MAZ/SIV]and[52HEI/HIC]), theexperimental data can be explained at least equally well by accounting for these activityfactor changes and assuming no formation of nitrate complexes. However, evenassuming that the end user of the database requires an ion association approach for usein a calculation code, it is not reasonable to attempt to correct the values to I = 0 fromthe high ionic strengths (higher than 4.5 molal) pertinent to these two works. If a valueof β 1 forformationofPuO 2 NO + 3is required, use of the first association constant recommendedfor the analogous uranium system [92GRE/FUG] will probably not causetoo large an error.20.1.3.2 Solid plutonium nitratesThere is a limited amount of information on the solid nitrate compounds of plutonium.