To measure pH by electrode, a measuring electrode must be available, the voltage of which varies in proportion to the hydrogen ion activity of the solution, and a reference electrode, which provides a constant voltage. More often than not, a combined electrode is used that includes both the measuring and the reference elements. The measuring electrode consists of a thin glass membrane – susceptible to hydrogen ions – blown to the end of an inert glass tube or electrode body. This electrolyte-filled tube accommodates an Ag/AgCl filament.
This part is the measuring electrode. An identical AIS membrane-free system is used as a reference electrode. A diaphragm connects the electrode body to the solution to be measured. The device then measures in millivolts the difference in potential between the reference electrode. The electronics then allow the conversion and display of these millivolts in pH units.