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- pH and ORP Measurement
| Inductive probes The inductive magnetic field probe, also referred to as an electrode-less probe, operates on a different principle from the electrode probe. The inductive probe is made up of two completely enclosed electrical coils. During operation, the probe is entirely immersed in the process liquid. The probe is enclosed in a corrosion- resistant coating. The instrument sends an alternating current through the primary coil. This current creates an alternating magnetic field that induces an alternating current 120 Field Instrumentation GEN Rev) in the pickup coil. The conductivity of the process liquid affects the magnetic coupling between the coils. The induced current in the pickup coil is directly proportional to the conductivity of the liquid. No direct contact is necessary between the coils and the solution, thus reducing potential maintenance problems. This system normally transmits a 4 to 20 mA dc signal that is proportional to the measured conductivity. To change the cell constant, change the probe. pH and ORP Measurement pH is a measure of the number of hydrogen ions in a process liquid. ORP (Oxidation-Reduction Potential) measures the ratio of ions that oxidize to those that reduce other chemicals. 7.2 pH Measurement The pH value of an aqueous (water-containing) liquid is an indication of positive hydrogen ion (H) activity. Readings of pH in non-aqueous liquids are not measures of hydrogen ion activity, but are useful in specific situations. The 121 Field Instrumentation GEN Rev) molecules that makeup a liquid, whether water alone or water with another chemical dissolved in it, can be partially broken down into smaller particles referred to as ions. Some of the molecules in water (HO) breakdown into two different kinds of ions, H+ (hydrogen) and OH- (hydroxyl). By definition, pH is the negative log of the hydrogen ion concentration of a solution. It is important to recognize that pH measures only the concentration of hydrogen ions in a solution, and not the total acidity or alkalinity of the liquid. Liquids with a pH of less than seven have more H+ than OH- ions. They are referred to as acids. Acid solutions increase in strength as the pH value fall below 7 (down to 0). Liquids with a pH greater than seven are referred to as bases and have many more OH- ions than H+ ions. Basic solutions increase in strength as the pH values rise above 7 (up to 14). Temperature has a significant effect on pH, because the dissociation constant separation into ions) changes with changes in temperature. Instrumentation normally includes a temperature detector and is capable of calculating the pH, 122 Field Instrumentation GEN Rev) by means of a temperature correction factor, as if the temperature actually were Cb ORP Measurement Oxidation maybe defined as the loss of electrons by one molecule and reduction as the absorption of electrons by another. Every liquid has both oxidizing and reducing ions, but their balance varies from liquid to liquid. Those liquids that tend to reduce have an excess of electrons. Those that tend to oxidize have a shortage of electrons. The ORP of a process liquid indicates by its voltage polarity whether the process liquid has an oxidizing potential or a reducing potential. A liquid with an oxidizing potential has a positive polarity relative to the reference voltage. A liquid with a reducing potential has a negative polarity relative to the reference voltage. The unit of ORP measurement—the voltage potential measured between two electrodes submerged in a liquid—is the milli volt. Download 3.37 Mb. Share with your friends:
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