Determination of formal complex formation constants of various Pb2+ ionophores in the sensor membrane phase

Formal complex formation constants of seven ionophores with Pb2+ and a series of interfering cations have been determined in the organic membrane phase. To this purpose, potentiometric selectivities of the target ions relative to tetramethylammonium are compared for ion-exchanger- and ionophore-based ion-selective electrode membranes. Furthermore, information on the stoichiometry of the complexes formed is obtained from measurements with two membranes of different ionophore concentrations. Surprisingly, the results show that ionophores having thioamide are less efficient in coordinating Pb2+ than corresponding ones with oxamide. The formal complex formation constants found are in excellent agreement with those determined by bulk optode measurements. A generally applicable method is proposed to gain more insight with a view to designing ligands and optimizing sensor membrane compositions. (C) 1999 Elsevier Science B.V. All rights reserved.