pH is the universally accepted scale for the concentration of hydrogen ions in an aqueous solution. By measuring pH a liquid can be categorised as either acidic, neutral or alkaline. Most often used pH electrodes are glass electrodes. 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.

pH Glass Electrode Probes

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.

pH Sensors for pH Meters & pH Measurement

pH Sensors for pH Meters & pH MeasurementpH sensors are usually designed with pH glass electrodes and probes. Typical pH sensor includes a measuring electrode, a reference electrode, and a temperature sensor, a preamplifier, and an analyser or transmitter. A pH measurement loop is essentially a battery where the positive terminal is the measuring electrode and the negative terminal is the reference electrode. The measuring electrode, which is sensitive to the hydrogen ion, develops a potential (voltage) directly related to the hydrogen ion concentration of the solution. The reference electrode provides a stable potential against which the measuring electrode can be compared.

When immersed in the solution, the reference electrode potential does not change with the changing hydrogen ion concentration. A solution in the reference electrode also makes contact with the sample solution and the measuring electrode through a junction, completing the circuit. Output of the measuring electrode changes with temperature (even though the process remains at a constant pH), so a temperature sensor is necessary to correct for this change in output. This is done in the analyser or transmitter software.