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.

The most important specification for ion selective electrodes is the type of ion the user needs to detect. Also important are the concentration range and accuracy, pH range through which the electrodes can operate and the response time, which is typically given as the time needed to reach 95% of the final value. Ion selective electrodes may be half-cell models that require a separate reference electrode, or combination models composed of two parts, the measuring electrode and the reference electrode.

Handheld or portable configurations allow ion selective electrodes to be used with ease in laboratories, where the operator may be testing several different samples. Insertion style electrodes are often inserted into process piping through a tapped hole in the pipe or bulkhead. Flow-through styles fit directly into the pipeline and become an integral part of it via some connection such as a flange or other fitting.

Samples read by ion selective electrodes may be aqueous, non-aqueous, or dry, and the electrodes may have many different types of membranes. Each type of membrane has its own unique characteristics that make it the best choice for a particular application. The most common choices for membranes are glass, solid state, liquid-ion exchange, PTFE/gas sensing, ISFET and plastic. Typical features for ion selective electrodes are built-in temperature sensors and temperature compensation.