which microorganism is supported for growth by a food having oxidation reduction potential of -200mv
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ORP is a numerical index of the intensity of oxidizing or reducing conditions within a system (Hem, 1985). Water samples contain different types of oxidizing and reducing substances. Because the ion species cannot be identified by the ORP value, the measurement of ORP in natural water is in millivolts (mV) rather than concentration units (U.S. Environmental Protection Agency, 2017). Positive values for ORP indicate oxidizing conditions, whereas negative values indicate reducing conditions.
ORP electrodes were first used in the 1930s to determine the relation between an oxidizing environment and bactericidal properties of water containing chlorine (Schmelkes et al., 1939). ORP is currently measured using a solid-state system constructed with a reference electrode of silver/silver chloride and a platinum measurement electrode that measures the electrical potential of a solution (U.S. Environmental Protection Agency, 2017).
ORP is used to monitor drinking water and wastewater-treatment processes for oxidation state due to the addition of halogenated disinfectants such as chlorine. These and other studies showed a strong correlation of ORP and bactericidal activity of chlorine. ORP was suggested by the World Health Organization (WHO) for monitoring the oxidation levels of free chlorine in treated drinking water (World Health Organization, 1971). ORP also is used as an indicator of the levels of disinfectant in wastewater effluent. The presence of chlorine will result in a positive ORP value and the presence of hydrogen sulfide will result in a negative ORP value. ORP sensors were used in the ORSANCO water-quality monitoring network on the Ohio River in the early 1960s as a sanitary indicator (Cleary, 1967).
ORP also is commonly measured as a master variable in groundwater systems to characterize groundwater geochemistry. If the sample water contains a dominant oxidation/reduction system, the ORP value provides a clue about the ratio between the oxidizing substances and reducing substances. For example, iron ions are often the dominant pair of oxidizing and reducing ions in groundwater. In natural water, many redox-sensitive species contribute to the reading. The ORP of solutions can be used to infer the form of the multivalent elements that dominate aquatic systems, such as ferrous ion (Fe2 +) in reducing conditions vs ferric ion (Fe3 +) in oxidizing conditions (Hem and Cropper, 1959). Reduction–oxidation (redox) processes affect the chemical quality of groundwater in all aquifer systems (McMahon et al., 2011). Beginning in the 1970s, continuous measurements of ORP in groundwater became routine with concurrent water-quality sampling as an indicator of the overall or governing geochemistry of a system (Wood, 1976).