The pH meter measures the pH of a solution using an ion-selective electrode (ISE) that responds to the H+ concentration of the solution. The pH electrode produces a voltage that is proportional to the concentration of the H+ concentration, and making measurements with a pH meter is therefore a form of potentiometry. The pH electrode is attached to control electronics which convert the voltage to a pH reading and displays it on a meter.
A pH meter consists of a H+-selective membrane, an internal reference electrode, an external reference electrode, and a meter with control electronics and display. Commercial pH electrodes usually combine all electrodes into one unit that are then attached to the pH meter.
Picture of a pH meter