EDTA: Chemistry and Properties

In complexation reactions, although ligands of various denticities can be used, polydentate ligands are generally preferred, as they form highly stable metal–ligand complexes due to the chelate effect.

EDTA, a well-known chelator, contains six potential coordinating sites with lone pairs that can chelate with metal ions to form metal–EDTA complexes.

These complexes are cage-like structures with geometries that differ depending on the size of the metal ions.

Neutral EDTA is a tetraprotic acid, although, at very acidic pH, the last two carboxyl groups undergo protonation to give a hexaprotic system. 

This species undergoes successive dissociation into various forms, and their relative amounts vary with the pH of the solution.

At very low pH, hexa-, penta-, and tetraprotic species predominate. As the pH increases, the tri-, di- and monoprotic species gain predominance.

Finally, above pH 10, the most dominant form is the fully unprotonated species, which is the active form of EDTA and the most important complexing species.