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

436 21. Plutonium group 14 compounds and complexesevidence of this species was not found in either the calorimetric experiments of Ullmanand Schreiner [88ULL/SCH] or the spectrophotometric experiments of Woods,Mitchell and Sullivan [78WOO/MIT]. No evidence of this species was found in thesolubility experiments of Robouch and Vitorge [87ROB/VIT] at high ionic strengthand moderate total Pu(VI) concentrations. This value is accepted for the conditions atwhich it was measured, but because of the large variation in the estimated interactioncoefficients for the analogous U(VI) and Np(VI) species (see Section 12.1.2.1.2.c and[92GRE/FUG]), this value cannot be reliably extrapolated to the standard state. Giventhe evidence that the stability of this complex is limited to high Pu(VI) concentrationand high ionic strength, the extrapolation, if possible, would require caution.Together with the enthalpy and ε of Reaction (21.5), log 10 β3 ◦ (21.5, 298.15K) canbe extrapolated to the ionic strength and temperature of the [86GRE/RIG] experiment:log 10 β 3 (21.5, I = 3MNaClO 4 , 22 ◦ C) = (19.2 ± 1.4). This permits the expression ofthe stability of the trimer in standard form:3PuO 2+2+ 6CO 2−3Å (PuO 2 ) 3 (CO 3 ) 6−6(21.7)At I = 3 M the accepted value for log 10 β 6,3 (21.7, 3M) = (50.1 ± 2.5).21.1.2.1.1.e Mixed U(VI) carbonate complexGrenthe, Riglet and Vitorge [86GRE/RIG] noted that a mixed PuO 2+2 -UO2+ 2trimeralso forms in high-ionic-strength solutions. They reported the stability constant for thereaction:2UO 2 (CO 3 ) 4−3+ PuO 2 (CO 3 ) 4−3Å (UO 2 ) 2 PuO 2 (CO 3 ) 6−6+ 3CO 2−3(21.8)in3MNaClO 4 and at (22 ± 1) ◦ Caslog 10 K(21.8) =−8.8, with no uncertainty assigned.Grenthe et al. [92GRE/FUG] re-evaluated the uncertainties in the reportedstability constant, assumed that the experimental value could be applied to 25 ◦ C, andextrapolated the reported value to I = 0togivelog 10 K ◦ (21.8) =−(8.8 ± 0.2). Ina later correction [95GRE/PUI] to the TDB uranium volume, revised interaction coefficientsfor the U(VI) system were proposed, and it was assumed that the interactioncoefficients of the U(VI) system could be applied, by analogy, to the Pu(VI) system tore-evaluate the extrapolation to I = 0 (ε = (0.16 ± 0.36) kg·mol −1 ).Thisrevisedvalue is accepted:Hencelog 10 K ◦ (21.8) =−(8.2 ± 1.3) f G ◦ m ((UO 2) 2 PuO 2 (CO 3 ) 6−6, 298.15 K) =−(6135.7 ± 10.5)Using this selected value and the selected stability constants for UO 2 (CO 3 ) 4−3andPuO 2 (CO 3 ) 4−3the formation constant for the reaction:2UO 2+2+ PuO 2+2+ 6CO 2−3Å (UO 2 ) 2 PuO 2 (CO 3 ) 6−6(21.9)