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

Formal complex formation constants between lipophilic ligands and cati ons 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 a re traced for membranes with and without ionophore. A theoretical disc ussion is given as well as experimental results with the ionophores va linomycin and BME-44 (for K+), ETH 4120 (for Na+), ETH 1001 and ETH 12 9 (for Ca2+), and ETH 7025 (for Mg2+). Complexes with various stoichio metries are formed for the Mg2+ and Ca2+ complexes of ETH 7025, which make the realization of a selective magnesium optode with this ionopho re not possible. With this novel method, the key parameters determinin g the performance of ionophore-based ion-selective optodes and electro des become directly accessible.