Existence of protonated complexes of ethylenediaminetetraacetic acid with lanthanides in aqueous solutions

Ethylenediaminetetraacetic acid (EDTA) is widely used in various areas of radiochemistry, including the separation of rare earth elements (REE) by ion exchange and extraction methods. The discovery of the stoichiometry and determination of the constants of formation of complexes of REE with EDTA have been the subject of a large number of studies. We demonstrated the fact that no stable (pKA = 1-2) complex acids of the lanthanides with EDTA exist in aqueous solutions. We calculated the equilibrium concentration of H+ ions considering the dissociation of the complex LnL- and the protonation of free EDTA5-. Then the results obtained were treated and the reproducibility of the values of the dissociation constants KA was detected both according to the series of lanthanides from Ce3+ to Lu3+ and according to the dependence of KA on the pH