Ion selective electrodes (ISEs) are electrochemical sensors that allow potentiometric determination of the activity of certain ions in the presence of other ions in the sample solution. Potentiometric sensors are essentially passive electrochemical devices, in which changes in the electromotive force (emf) are monitored under virtually zero current conditions. In so-called direct potentiometry, the emf is ideally a function of the activity of only one chosen sample-ion so that it can be selectively assessed in the presence of other ions. Achieving sufficient selectivity to measure one ion in...