An ion-selective electrode (ISE) is an example of an electrochemical sensor utilizing the principle of potentiometry, or measurement of the cell potential (i.e., ISE against a standard reference electrode) at near-zero current. The information on the composition of the sample is achieved through the measurement of the potential difference across two electrodes. Under these conditions, the boundary potential at the ISE–solution interface is governed by the laws of electrochemical thermodynamics and is compliant with the famous Nernst equation. To ensure a compliance with the Nernst equation, the very best commercially available potentiometers utilize a high input impedance > 1013 ?, and an appropriate operational amplifier in the electrometer to handle the associated minute currents. https://www.sciencedirect.com/topics/materials-science/ion-selective-electrode