Ion-Selective Electrodes Based on Two Competitive Ionophores for Determining Effective Stability Constants of Ion−Carrier Complexes in Solvent Polymeric Membranes

A new potentiometric method to determine effective complex formation constants in organic membrane phases is reported in detail. It demands measurements with two different membranes, one containing a highly selective reference ionophore (in this case for H+) and anionic sites and another containing the same components and additionally the lipophilic ionophore to be characterized. The response characteristics of both electrodes can be compared and related to the effective complex formation constant of the ion carrier in the membrane. Potentiometric experiments with membranes containing a series of highly selective H+-ionophores confirm that alkali metal ions are not complexed by these ionophores. The ionophores valinomycin, BME-44, ETH 2120, tert-butylcalix[4]arene tetraethyl ester, and ETH 1810 are characterized using this potentiometric technique with potassium, sodium, and lithium ions in the sample. The complex formation constants are generally large and correspond very well to data obtained with a previously established optical method.