A glass electrode is a potentiometric sensor made from glass of a specific composition. All glass pH electrodes have extremely high electric resistance from, 50 to 500 MOhm. There are different types of pH glass electrode. Some of them have improved characteristics for working in very alkaline or acidic medium. But almost all electrodes can operate in the 1 to 12 pH range.
A typical pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The pH electrode is essentially a galvanic cell. The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and out with a ~10nm layer of a hydrated gel. These two layers are separated by a layer of dry glass and the potential is created by the equilibrium in H+ ions across the membrane.
Schematic description of a typical pH glass electrode (Courtesy Cole Parmer)
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