STUDIES ON METAL CARBONATE EQUILIBRIA .22. A COULOMETRIC STUDY OF THE URANIUM(VI)-CARBONATE SYSTEM, THE COMPOSITION OF THE MIXED HYDROXIDE CARBONATE SPECIES

Complex formation in the uranium(VI) - H2O - CO2(g) system has been studied at 25-degrees-C in a 0.50 m NaClO4 medium. The experimental method was potentiometry (H+-measurements), using coulometry to change the total concentration of hydrogen ions. The -log [H+] range covered was 3.5-5.2. The experimental data were explained using the following chemical model, where (OH)2CO2(2-) = CO3(2-): UO2(2+) + 2H2O + CO2(g) half-arrow-left-over-half-arrow-right UO2(OH)2CO2(aq) + 2H+ log *beta-121 = -8.71 +/- 0.05 2UO2(2+) + 2H2O half-arrow-left-over-half-arrow-right (UO2)2(OH)2(2+) + 2H+ log *beta-2,2,0 = -6.07 +/- 0.15 3UO2(2+) + 5H2O half-arrow-left-over-half-arrow-right (UO2)3(OH)5+ + 5H+ log *beta-3,5,0 = -16.40 +/- 0.22 UO2(2+) + 4H2O + 2CO2(g) half-arrow-left-over-half-arrow-right UO2(OH)4(CO2)2(2-) + 4H+ log *beta-1,4,2 = -19.57 +/- 0.30 3UO2(2+) + 12H2O + 6CO2(g) half-arrow-left-over-half-arrow-right (UO2)3(OH)12(CO2)6(6-) + 12H+ log *beta-3,12,6 = -49.68 +/- 0.17 2UO2(2+) + 5H2O + CO2(g) half-arrow-left-over-half-arrow-right (UO2)2(OH)5CO2- + 5H+ log *beta-2,5,1 = -19.40 +/- 0.11 UO2(2+) + 6H2O + 3CO2(g) half-arrow-left-over-half-arrow-right UO2(OH)6(CO2)3(4-) + 6H+ log *beta-1,6,3 = -29.45 +/- 0.08 11UO2(2+) + 24H2O + 6CO2(g) half-arrow-left-over-half-arrow-right (UO2)11(OH)24(CO2)6(2-) + 24H+ log *beta-11,24,6 = -72.48 +/- 0.31 The experimental data were fitted to the selected model by using the LETAGROP least-squares procedure. The uncertainties reported for the refined constants are 3-sigma, while the standard deviation in the error-carrying variable (the EMF) was 0.55 mV. The experiments have been made in such a way that the determination of the minor species is optimized. Previous experimental data have been reviewed in order to obtain a consistent set of equilibrium constants. A brief discussion of possible structures of the mixed uranium hydroxide-carbonate complexes is given.