Studies on stability constants of europium(III) carbonate complexes and application of SIT and ion-pairing models

A synergic solvent extraction method using 1-nitroso-2-naphthol and trioctylphosphine oxide (TOPO) in chloroform has been applied to study the carbonate complexes of Eu(III). These studies have been performed at 1.0 M ionic strength and at varied total carbonate (ΣCO2 = 1x10-3 - 0.5 M) concentrations and pH (8.0 - 9.0). The following species have been identified: EuCO3+, Eu(CO3)2-, Eu(CO3)33- and Eu(CO3)45-; and their formation constants (log β) have been calculated as 5.88 ± 0.05, 10.03 ± 0.06, 12.4 ± 0.08 and 14.3 ± 0.1, respectively. The specific ion interaction theory (SIT) and ion-pairing model have been used to extrapolate the stability constants of EuCO3+ and Eu(CO3)2- to zero ionic strength. The corresponding log β0 values are 8.06 and 12.91 by SIT and 7.91 and 13.07 by the ion-pairing model. (orig.)