DETERMINATION OF COMPLEX-FORMATION CONSTANTS OF NEUTRAL CATION-SELECTIVE IONOPHORES IN SOLVENT POLYMERIC MEMBRANES

Formal complex formation constants between lipophilic ligands and cations have been determined within the solvent polymeric membrane phase. The method is based on spectrophotometric measurements on a 1-2-mu um thin membrane phase (optode) containing a H+-selective chromoionophore. Ion exchange equilibria between the membrane and the aqueous phase are traced for membranes with and without ionophore. A theoretical discussion is given as well as experimental results with the ionophores valinomycin and BME-44 (for K+), ETH 4120 (for Na+), ETH 1001 and ETH 129 (for Ca2+), and ETH 7025 (for Mg2+). Complexes with various stoichiometries are formed for the Mg2+ and Ca2+ complexes of ETH 7025, which make the realization of a selective magnesium optode with this ionophore not possible. With this novel method, the key parameters determining the performance of ionophore-based ion-selective optodes and electrodes become directly accessible.