“…The recognition of specific ions by these receptors is determined by the cavity size and the kind of functional groups attached to the aromatic rings. Because of the selective ion complexation, various derivatives of calix [4]arene have been used as ionophores in membrane electrodes that are selective towards sodium [1-7], potassium [8, 91 or cesium [lo] cations. Replacements of the hard oxygen donor atoms by softer nitrogen or sulfur donor atoms in functional groups of calix [4]arenes lead to membrane electrodes that are selective towards transition metal cations, e.g., silver [l l-131, cadmium, copper [ 1 I] and lead [ 11, 141.…”