Chelate complex formation as a basis for titration processes

Metal cations can be looked upon as LEWIS acids and their reaction with ligands during complex formation can be compared to the addition of the proton to a base. The complex formation reactions, however, lack sharp and definite endpoints and metal cations therefore cannot be titrated with ammonia, cyanide and other ligands in the same way as the hydrogen ion can be titrated alkalimetrically. The theoretical reason for this fact is discussed and it is shown that complex formations gain the characteristic properties of neutralisation reactions if the simple ligands are replaced by a polydentate group which is able to satisfy not only one, but several of the coordination points of the metal cation. A great number of titration processes can be based on such reactions.