Determination of complex formation constants of lipophilic neutral ionophores in solvent polymeric membranes with segmented sandwich membranes

A potentiometric method to determine ionophore complex formation constants in solvent polymeric membrane phases, proposed originally by Russian researchers, is critically evaluated and compared to other established methods. It requires membrane potential measurements on two-layer sandwich membranes, where only one side contains the ionophore. The resulting initial membrane potential reflects the ion activity ratio at both aqueous phase--membrane interfaces and can be conveniently used to calculate complex formation constants in situ. This method is potentially useful, since it does not require the use of a reference ion or second ionophore in the measurement. In this paper, the five ionophores valinomycin, BME-44, ETH 2120, tert-butylcalix[4]arene tetraethyl ester, and S,S'-methylenebis(diisobutyldithiocarbamate) are characterized in poly(vinyl chloride) (PVC) plasticized with dioctyl sebacate (DOS) and compared with other established methods. The resulting formation constants correspond well to literature values. The influence of varying membrane concentrations and different anionic site additives is studied and found to be relatively small. Experiments are also performed with and without lipophilic inert electrolytes and with ionophore-free sandwich membranes to illustrate the effect of ion pairing and the membrane internal diffusion potential on the response of such sandwich membranes. These experiments suggest that ions are completely associated in PVC-DOS membranes, but that such ion pairs are rather nonspecific. Diffusion potentials seem to play a minor role with these systems. The results are explained with theory. This work indicates that the characterization of electrically charged ionophores, anion-selective ionophores, and ionophores in membrane matrixes other than PVC plasticized with DOS may now be experimentally accessible.