Calorimetric determination of the enthalpies of the carbonate complexes of U(VI), Np(VI), and Pu(VI) in aqueous solution at 250C

Enthalpies of carbonate complex formation with UO2+2, NpO2+2, and PuO2+2 were determined by microcalorimetric titration of solutions containing UO2(NO3)2, NpO2SO4, and PuO2(ClO4)2 with sodium carbonate in the presence of excess sulfate. The standard enthalpies of formation at 298.15 K of the tricarbonato complex ions were obtained from titrations at high carbonate to actinyl ratios at ionic strengths varying between 0.27 and 1.08 M. Enthalpies of formation for the dicarbonato complexes were calculated from the calorimetric data at lower CO3-2/AnO2+2 ratios with the aid of previously published and estimated association quotients of the mono-, di-, and tricarbonato complexes, as well as the reported hydroxycarbonato complexes. No unequivocal experimental evidence for the presence of the monocarbonate or hydroxycarbonate complexes was found. (orig./RB)