Ion selective electrodes (ISE) measure the potential of a specific ion in solution. This potential is measured against a stable reference electrode of constant potential. The potential difference between the two electrodes will depend upon the activity of the specific ion in solution. This activity is related to the concentration of the specific ion, therefore allowing the end-user to make an analytical measurement of the specific ion. Several types of sensing electrodes are commercially available. They are classified by the nature of the membrane material used to construct the electrode. It is this difference in membrane construction that makes electrodes selective for a particular ion.