Studies of metal carbonate equilibria. 20. Formation of tetra(carbonato)uranium(IV) ion, U(CO 3) 44−, in hydrogen carbonate solutions

Abstract

The equilibrium between the tetracarbonate and pentacarbonate complexes of the U(IV) ion has been studied at 25 °C in CO 2/HCO 3 − solutions of different ionic strength ( I = 0.5; 1.0; 2.0; 3.0 M NaClO 4), using Vis-UV spectrophotometry. The data have been explained by assuming the equilibrium U(CO 3) 4 4− + CO 3 2− ⇌ U(CO 3) 5 6− where K 5 is the (ionic strength dependent) equilibrium constant. The molar absorptivities for the penta- and tetracarbonate at 660 nm were constant and equal to ϵ 1 = 17.2 ± 0.2 and ϵ 2 = 35.5 ± 0.5, respectively. By using the specific ion interaction theory (S.I.T.), and the equilibrium constants at different ionic strength, the equilibrium at zero ionic strength: 1g K 5(0) = −1.12 ± 0.25, was obtained. This stepwise equilibrium constant is small and the formation of U(CO 3) 5 6− seems to be mainly an ionic strength effect. Hence, very small amounts of U(CO 3) 5 6− (and possibly also similar complexes of the other tetravalent actinides) can form in natural waters.