Studies on metal carbonate equilibria. 22. A coulometric study of the uranium(VI)-carbonate system, the composition of the mixed hydroxide carbonate species

Abstract

Complex formation in the uranium(VI) - H2O - CO2(g) system has been studied at 25 °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)2CO22- = CO32-: UO22+ 2H2O + CO2(g) ⇋ UO2(OH)2CO2(aq) + 2H+ log ∗β121 = -8.71±0.05 2UO22+ + 2H2O ⇋(UO2)2(OH)22+ + 2H+ log ßβ2,2,0=-6.07±0.15 3UO22+ + 5H2O ⇋ (UO2),(OH)5+ + 5H+ log ßβ3.5.0=-16.40±0.22 UO22+ + 4H2O + 2CO2(g) ⇋ UO2(OH)4(CO2)22- + 4H+ log ßβ1,4,2=-19.57±0.30 3UO,2+ + 12H2O + 6CO2(g) ⇋ (UO2)(OH)12(CO2)66- + 12H+ log ∗β3,12,6=-49.68±0.17 2UO22+ + 5H2O + CO2(g) ⇋ (UO2)2(OH)5CO2- + 5H+ log ∗β2,5,1 = -19.40±0.11 UO22+ + 6H2O + 3CO2(g) ⇋ UO2(OH)6(CO2)34- + 6H+log ∗β1,6,3=-29.45±0.08 11UO22+ + 24H2O + 6CO2(g) ⇋ (UO2)11(OH)24(CO2)62- + 24H+ log ∗β11,24,6= -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 3sigma;, 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.