Colloid formation and solubility of U(IV) in acidic HClO4/NaClO4 solutions are investigated by coulometric titration. Quantification of traces of U(VI) by laser fluorescence spectroscopy proves that the tetravalent state of uranium had been maintained. Laser-induced breakdown detection (LIBD) is applied for the detection of traces of uranium colloids as the pH is increased. The pH values at the onset of colloid formation are used for thermodynamic calculations aimed at determining the solubility products K°sp of crystalline and amorphous uranium dioxides. The particle size of the colloids determined by means of LIBD is taken into account since it influences the solubility product. The results indicate the formation of crystalline UO2(cr) at low pH∼1 (logK°sp= -59.6±1.0) whereas the amorphous hydrous oxide UO2·xH2O(am) is formed at pH ∼ 3 (logK°sp = -54.1 ± 1.0). Measurements by EXAFS, XRD and REM confirm the occurrence of these different phases. The obtained solubility products fit well in the known series of solubility products of the other tetravalent actinides.
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