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pH and ORP Reference Electrodes
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- 7.5 pH and ORP Measurement Electrodes
| 7.4 pH and ORP Reference Electrodes A pH probe is made up of two separate electrodes, as is an ORP probe. The reference electrode is the same for both pH and ORP probes, but the measurement electrodes are different. The reference electrode is designed to provide a constant voltage potential despite changes in pH or temperature. This electrode is used to monitor any change in the total liquid potential. The lead wire is in an inner tube that contains silver metal and silver chloride paste. This paste is in contact with a saturated solution of potassium chloride (KCI), which acts as an electrical bridge to the solution being measured. The potential of the reference electrode depends on temperature and on the KCI concentration. The KCL slowly migrates from the reference electrode to the solution being measured by means of a liquid junction consisting of a porous ceramic material near the bottom of the electrode. Crystals of solid KCL in the bottom of the electrode ensure that the solution stays saturated. A portal is provided for replenishment of the KCI, and a rubber sleeve protects the portal from contamination. 123 Field Instrumentation GEN Rev) 7.5 pH and ORP Measurement Electrodes The pH measurement electrode produces a potential of 59.2 mV per pH unit, which relates directly to the solution in which it is submerged. Measurement of the potential requires a current through a very high impedance amplifier and back to the process liquid. Electrical connection of the measuring circuit to the liquid is through a liquid junction—a calibrated leak of liquid from the reference electrode into the process—that is part of the reference electrode. The pH measurement electrode has a thin walled glass bulb filled with a liquid of known pH, usually potassium chloride. A silver-silver chloride electrode is immersed in the liquid. The hydrogen ions on the inside of the electrode are of a different concentration than the hydrogen ions on the outside. These ions want to reach equilibrium by migrating through the glass wall of the electrode until the concentrations are the same. This migration is prevented by the glass, and thus a potential is established. A potential difference across the thin bulb will occur when the H+ activity inside the electrode is different from the H+ activity in the liquid to be measured. Because glass is an insulator, the bulb wall must be very thin to permit accurate voltage readings. The pH measurement electrode (which is usually attached to a high-impedance voltmeter) reacts to any change in voltage and reports it as a pH reading. The pH measurement electrode has a watertight seal at the top to keep out any liquid or moisture that could affect the probe’s operation. Both reference and measurement electrodes are contained in the same housing in the probe. 124 Field Instrumentation GEN Rev) The ORP measurement electrode is simply a platinum wire exposed at its bottom end to the process liquid. The ORP measurement electrode reacts to variations in voltage. Voltage variations caused by variations in the activity of the oxidizing and reducing ions in the process liquid are amplified and displayed as the ORP of the liquid. |