The solubility of uranium(VI) is investigated in the pH range 2.8 to 4
.6 in equilibrium with 8.0% and 0.3% CO2 partial pressure and in the p
H range 3.8 to 7.0 in equilibrium with 0.03% CO2 partial pressure at I
= 0.1 M NaClO4 and 25 degrees C. UO2CO3(s) under 8.0% and UO3 . 2 H2O
under 0.3% CO2 and 0.03% CO2 partial pressures at pH < 7 are characte
rized as solubility limiting solid phases. Above pH 7, a spontaneous p
hase transformation of UO3 . 2 H2O(s) in a sodium containing poorly cr
ystalline material is observed. Thermodynamic solubility products lg K
-sp(o) = -14.91 +/- 0.10 for UO2CO3(s) and K-s(o) = 4.68 +/- 0.14 and
lg K-s(o) = 5.14+/-0.05 for UO3 . 2 H2O(s) under 0.3% and 0.03% CO2 pa
rtial pressures, resp. are obtained. The evaluation of UO3 . 2 H2O(s)
solubilities on basis of spectroscopic speciation by TRLFS are discuss
ed and a formation constant lg K-35' = -17.14 +/- 0.13 is obtained.