Complexometric back-titrations: a theoretical restatement

The requirements for complexometric titrations to be theoretically feasible are examined, employing a simple criterion based on the evaluation of the titration error in a given range of the measured variable around the equivalence point. It is shown that the feasibility of a back-titration is essentially conditioned by the complex formed by the chelating agent either with the analyte or the titrant metal ion, having the lower stability constant. In contrast, the ratio between the stability constants of such complexes is not so important as is commonly believed; however, it determines the choice of a suitable indicator.The same conclusions are reached by using newly developed logarithmic diagrams of equilibrium concentrations for back-titration systems.