Some limiting factors of the ion-exchange equilibrium method for determining apparent formation constants of mononuclear complexes

Some limiting analytical conditions of the ion-exchange equilibrium method are discussed. The most reliable apparent formation constants are obtained when the range of the experimental complexant concentration brackets the reciprocal value of the stability constant for ML-type complexes. This working range will ensure significant analytical differences in the total metal remaining in solution for successive experiments with increasing complexant concentrations. For accurate determination of stepwise formation constants for ML 2-type complexes, the intermediate complex species ML + must not be exchanged by the resin to any extent. Otherwise Schubert's equations are no longer valid, and the partition ratios of all cationic species present should be taken into account.