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

266 12. Neptunium group 14 compounds and complexesFor comparison purposes, the simple mean of these enthalpy of reaction values atdifferent ionic strengths is -(85.06 ± 4.63) kJ·mol −1 .From r Hm ◦ (12.21, 298.15 K) =−(84.39 ± 4.38) kJ·mol −1 and f Hm ◦ (NpO 2(CO 3 ) 5−3, 298.15 K) = −(3017.12 ±6.89) kJ·mol −1 , f Hm ◦ (Np(CO 3) 6−5 , 298.15 K) =−(3888.53 ± 8.17) kJ·mol−1 iscalculated. From these values, r Hm ◦ (12.23, 298.15 K ) = (43.54 ± 9.27) kJ·mol−1is calculated, which is very different from the corresponding (calorimetrically determined)uranium value −(20.0 ± 4.0) kJ·mol −1 [92GRE/FUG]. For this reason the calculatedenthalpy of formation value for Np(CO 3 ) 6−5is not selected.12.1.2.1.4.e Discussion: Using Np analogues to estimate missing dataTable 12.7 provides a summary of equilibrium constant and enthalpy of reaction valuesfor carbonate complexation as selected in the TDB volumes for U [92GRE/FUG,95GRE/PUI], Np, Pu and Am [95SIL/BID]. Besides the values selected in this review,it seems reasonable to use trends in the numerical values of equilibrium constants measuredwithin the actinide series (Th, U, Np, Pu and Am have been prepared in the +4oxidation state in bicarbonate-carbonate media) by the same author, even when eachvalue is not exactly the one selected in the present review. In such cases, several systematicerrors should cancel. Rai et al. [95RAI/FEL, 97FEL/RAI, 98RAI/FEL] recentlypublished data from solubility measurements of Th(IV) and U(IV). Results from similarNp(IV) studies [99RAI/HES] from this group were made available for this reviewprior to publication. Vitorge et al. [95VIT, 96CAP/VIT, 96DEL/VIT] published reliabledata from spectrophotometric measurements or literature review on U(IV) andPu(IV). They also estimated values for Np(IV), but with poorer accuracy. Hence, theirNp(IV) data are not used below.