A thermodynamic study of the complex formation of trivalent lanthanides with hydroxyethylethylenedieminetriacetic acid and other amoniacetic acids

The partial stability constants of the mixed and 1:2 complexes Ln-HEDTA-L (where Ln = La, Ce, ... and L = glycine, IMDA, NTA, HEDTA and EDTA) have been determined by potentiometric titration at 25degC and at a constant ionic strength of 1(KCl). The changes in stability of the complexes studied (1:1, 1:2, mixed, protonated and unprotonated) vs. atomic number of the lanthanide are discussed. The changes observed in the trends of the partial and overall stability constants across the lanthanide series are attributed to the decrease in the number of water molecules in the 1:1 LnHEDTA.xH2O from x = 3 for light lanthanides to x = 2 for heavy ones. However, in this 1:1 complex, HEDTA seems to be a hexadente ligand in the La-Sm range of the series and a pentadentate ligand in the Gd-Lu range. Significant differences have been found between the complexes containing four nitrogen atoms, i.e. L = HEDTA, EDTA, and those with three nitrogen atoms, i.e. L = glycine, IMDA, NTA