Large organic cation-selective electrodes

In a cation-selective electrode for use in measuring the concentration of an organic cationic species in an aqueous test solution as a function of potentiometric response, comprising a cation-selective membrane adapted for interfacing with said test solution and either formed as a layer on a conductive substrate or in electrical contact with an internal reference electrode element, said membrane being composed of a polymeric matrix having dispersed or dissolved therein a cation-exchange material, the improvement which comprises said cation exchange material having as its counter-anion a high molecular weight alkyl or alkaryl sulfonate or sulfate of the formula R(O).sub.n SO.sub.3.sup.-, wherein n is 0 or 1, R is an alkyl group having at least 13 carbon atoms or an alkaryl group of the formula R.sub.1 --Ar--R.sub.2, Ar is an aryl group, R.sub.1 is an alkyl group, and R.sub.2 is an alkyl group or H, with the proviso that when Ar is monocyclic and R.sub.1 has less than 12 carbon atoms, R.sub.1 and R.sub.2 are each the same or different alkyl groups having at least 9 carbon atoms, and when Ar is polycyclic and R.sub.1 has less than 10 carbon atoms, R.sub.1 and R.sub.2 are each the same or different alkyl groups having at least 7 carbon atoms, whereby said electrode has a high degree of selectivity for large organic cationic species with respect to both smaller organic cationic species and inorganic cationic species.