The stability of the metal complexes of cyclic tetra-aza tetra-acetic acids.

The stability constants of the complexes formed by three tetra-aza macrocyclic complexones (DOTA, TRITA and TETA) with Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+) and Pb(2+) were determined with an automated titration instrument with data acquisition and the calculations were performed with the Superquad program, confirming and extending the range of values previously available. Both 1:1 and 2:1 metal-to-ligand complexes were now considered including their protonated species. The results show that DOTA is a powerful but unselective ligand whereas TETA, although not so powerful as DOTA, is an interesting selective ligand for pairs of metal ions, e.g., Cd(2+) and Pb(2+).