The pH value is used as a unit of measurement for the acidity or alkalinity of a liquid medium. The pH-sensitive element of glass electrodes is a glass bulb that supplies an electrochemical potential which is dependent upon the pH value of the medium. This potential is generated because small H+ ions penetrate through the outer layer of the membrane while the larger negatively charged ions remain in the solution.
Glass pH probes contain an integrated Ag/AgCl reference system that serves as the required reference electrode. The pH value is calculated from the potential difference between the reference system and the measuring system using the Nernst equation