ORP measurement can be found in diverse applications such as disinfection, wine making, electroplating and mining. This article offers a brief introduction to some of these processes.
Municipal water disinfection is often accomplished through the addition of chlorine. Chlorine can be generated through sodium hypochlorite, calcium hypochlorite or even chlorine gas in elemental form. The resulting oxidation reaction with water forms hypochlorous acid. In water treatment applications ORP is used to measure the oxidation by controlling the amount of free chlorine (either hypochlorous acid or hypochlorite ions) until a known bacteria killing ORP level is achieved. ORP and hypochlorous acid is utilized in a similar manner to maintain pool chlorination.
Dissolved oxygen is an important measurement parameter in power plants, bioprocessing, and aquatic applications and is normally measured using a dedicated dissolved oxygen (DO) analyzer. Oxygen in solution acts as an oxidizer, and a system transitioning to an oxygen deficient environment will display a decreased ORP value. In water treatment applications ORP may be used in conjunction with DO for biological phosphate and nitrogen removal using bacteria. ORP levels help monitor the bacteria’s metabolic activity present in aerobic, anoxic, and anaerobic environments to optimize the removal process.
ORP has also been successfully applied in winemaking fermentation to control aeration during yeast fermentation. ORP levels are used to maintain an aerobic environment to inhibit the formation of hydrogen sulfides and volatile sulfur aromas which are undesirable in the final product.
Redox reactions are common practice in industrial wastewater treatment for either the reduction or oxidation of components before discharge. Cyanide wastewater treatment is a common example of an oxidation reaction in metal processing applications. Cyanide is a well-known carcinogen and cannot be released to the environment. ORP is utilized to monitor the oxidation potential for two chemical reactions; first, an oxidation of cyanide to cyanate at alkaline pH, and second, a conversion of cyanate to carbonate at a more neutral pH value. In both reactions, the oxidation potential is directly measured with ORP, with plateaus in ORP indicating complete oxidation of components and transition to the next stage of treatment.
Chromate is a commonly used chemical in electroplating of metals to alter chemical properties. The compound is toxic and needs to be removed from wastewater to limit environmental release. Chromate reduction from hexavalent chromium to trivalent chromium is pH controlled at acidic conditions and monitored with ORP, with a dramatic decrease in ORP value serving as an indication of complete reduction. A change in pH through lime addition precipitates out chromium hydroxide, which is separated through settling.