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

434 21. Plutonium group 14 compounds and complexes21.1.2.1.1.b The dicarbonato complexThe evidence for the existence of the dicarbonato complex is based on the fitting of amodel curve to solubility data [87ROB/VIT] and enthalpy titration data [88ULL/SCH].In both cases the stability range of this complex is small.Neither study provides evidence for the existence of a mixed hydroxide-carbonatespecies over the narrow range of pH in which measurements were reported.The association constant for the reactionPuO 2+2+ 2CO 2−3Å PuO 2 (CO 3 ) 2−2(21.4)was estimated from solubility experiments in 3 M NaClO 4 solutions at (20 ± 1) ◦ Cby Robouch and Vitorge [87ROB/VIT] (log 10 β 2 = (13.6 ± 1.4)). Using theinteraction coefficients for the analogous UO 2+2species and ClO − 4(with the uncertaintiesexpanded by 0.05) and the tabulated value of ε (Na + ,CO 2−3 )to calculateε(21.4) =−(0.26±0.17) kg·mol −1 , the measured value can be extrapolated to I = 0,log 10 β 2 (21.4, 20 ◦ C) = (14.7±1.4). Ullman and Schreiner [88ULL/SCH] determinedlog 10 β2 ◦ (21.4, 298.15 K) = (15.1 ± 1.2) using a procedure inconsistent with theprocedures adopted by this review (cf. Appendix A). For the purposes of this review,the uncertainty in this result is expanded to log 10 β2 ◦ (21.4, 298.15 K) = (15.1 ± 2.2) toaccount for this inconsistency. The enthalpy of reaction determined by [88ULL/SCH],however, r Hm ◦ (21.4) =−(27 ± 4) kJ·mol−1is accepted by this review as measured.The determination of log 10 β 2 (21.4, 20 ◦ C) of Robouch and Vitorge [87ROB/VIT]can be corrected to 25 ◦ C using the enthalpy determined in [88ULL/SCH]. The resultof this correction yields a second estimate, log 10 β2 ◦ (21.4) = (13.9 ± 1.4). Anaverageof the two available experimental results yieldslog 10 β2 ◦ (21.4, 298.15 K) = (14.5 ± 2.6)which is accepted by this review.On the basis of these parameters, the remaining thermodynamic parameters,as listed in Table 4.2, can be calculated: r G ◦ m (21.4, 298.15 K) = −(82.767 ±14.841) kJ·mol −1 and r Sm ◦ (21.4, 298.15 K) = (187.042 ± 51.553) J·K−1·mol−1 .21.1.2.1.1.c The tricarbonato complexA spectrophotometric study [78WOO/MIT] demonstrated that the principal carbonatecomplexin0.15MNa(ClO − 4 ,OH− ) at pH = 11.4 is a tricarbonato species. The shape ofthe calorimetric titration curve of PuO 2+2with Na 2 CO 3 indicates that the tricarbonatocomplex is the dominant and limiting species at [CO 2−3 ] T :[PuO 2+2 ] T ratios of 3 orgreater in strongly basic solution [88ULL/SCH]. The existence of the tricarbonatocomplex is also consistent with the curve fitting procedure in the solubility experiments[87ROB/VIT].