Abstract: Atwo-layer sandwich membrane was used as the indicator electrode to determine the formation constants of the complexes of tert-butyl-calix[6]arene hexaacetic acid hexaethyl ester with metal ions in solvent polymeric membrane potentiometrically. Anew method for the measurement of sandwich membrane potential from which the formation constant of the complex can be calculated was proposed. Calix[6]arene ester was used as ionophore and the complex formation constants of it with Li⁺, Na⁺, K⁺, Cs⁺and NH⁴⁺were determined. The logarithmic values of the formation constants were found to be 6. 14, 6. 48, 6. 74, 7. 43 and 6. 21 respectively. According to the experimental data, Cs⁺was chosen as the primary ion because of its highest value of complex formation constant and a Cs-selective electrode was prepared. The resulting logarithmic selectivity coefficients to the ions of Li⁺, Na⁺, K⁺and NH⁴⁺are found to be -3. 33, -2. 54, -1. 60 and - 2. 90 respectively by fixing interference method (FIM). From the data of both formation constants and selectivity coefficients, a good correlation between them was found. The higher the formation constant of the complex of calix[6]arene ester with the responded ion is, the higher the selectivity to it is.

Key words: Calix[6]arene ester, Two-layer sandwich membrane, Potentiometry, Complex formation con-stant