The solubility of U(VI) was measured in highly basic solutions (pOHc = -logfOH"] ranges from 3 down to 0) at the ionic strength I = 0.5-2.0 Μ (Μ: mol dm -3 ) over a wide range of carbonate ion concentrations C = IX 10~3-0.5 Μ by both oversaturation and undersaturation methods. In oversaturation experiments, the solubility of U(VI) decreased with increasing equilibration time from 1 week to 1 year, suggesting an increase in the crystallinity of the solid phase with aging. The solid phase was identified as Na 2 U 2 0 7 · χ H 2 0 (x = 3-5) by X-ray diffraction. Undersaturation experiments conducted for 1 month with this solid phase indicated a rapid attainment of equilibrium. These data were well interpreted by considering the formation of [U0 2 (0H) 3 ]", [U0 2 (0H) 4 ] 2 " and [U0 2 (C0 3 ) 3 ] 4 ", and the apparent equilibrium constants for [UO 2 (0H) 3 ]~ and [U0 2 (0H) 4 ] 2 " were determined with fairly small deviations. Using the evaluated solubility product of Na 2 U 2 0 7 · χ H 2 0 (χ = 3-5), the stability constants for the above species were evaluated. The upper limit values of the stability constants of the carbonatohydroxodioxouranium(VI) complexes were also evaluated.

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