Help: ORP of water

ORP I. Overview ORP is the abbreviation for O*dation-Reduction Potential in English, and it represents the redox potential of a solution. The ORP value is a measure of the redox capacity of an aqueous solution, with the unit being mV. It consists of an ORP composite electrode and a millivoltmeter. An ORP electrode is a type of electrode that enables the absorption or release of electrons at the surface of its sensitive layer, which is an inert metal, typically made of platinum or gold. The reference electrode is a silver/silver chloride electrode, just like the pH electrode. 　　In natural water bodies, there are various valence ions and dissolved oxygen. When industrial wastewater is discharged into such waters, they contain large amounts of ions and organic substances. Due to the differences in the properties of these ions, redox reactions occur within the water body, leading to a state of equilibrium. As a result, natural water bodies are not composed of a single redox system, but rather of a mixed redox system. The potential measured by the electrode is also a mixed potential, which has significant experimental errors. Additionally, the pH value of the solution also affects the ORP value. Therefore, it is meaningless to emphasize the absolute potential of the solution during actual measurement. We can say that the ORP value of a solution indicates its state of reduction or oxidation at a certain value, or it reflects some property of the solution (such as its cleanliness level), but this value can vary significantly, and it is not possible to determine it quantitatively; this is a different concept from the accuracy in pH testing. Additionally, the temperature coefficient that affects the ORP value is also a variable that cannot be corrected; therefore, ORP meters generally do not have a temperature compensation function. 　　II. Scope of Application 1. Industrial wastewater treatment Used in redox systems for water treatment, primarily for the reduction of chromic acid and the oxidation of cyanides. Adding sodium disulfide or sulfur dioxide to wastewater can convert hexavalent chromium ions into trivalent chromium ions. Adding chlorine or sodium hypochlorite can be used to oxidize cyanides, followed by the hydrolysis of cyanogen chloride to form cyanates. This chemical reaction process is called an oxidation-reduction reaction system. The redox potential is a measure of electron activity, which is similar to the method used to measure hydrogen ion activity. 　　2. Disinfection and Application of Water Redox electrodes can measure the effectiveness of disinfection in swimming pool water, mineral water, and tap water. Since the sterilization effect of E. coli in water is affected by the redox potential, the redox potential is a reliable indicator of water quality. If the redox potential value in pool water and mineral water is equal to or higher than 650 mV, it indicates that the bacterial content in them is acceptable. 　　III. Installation and Inspection of Electrodes 1. Redox electrodes can be used with any pH/mV meter. 　　2. The ORP meter does not require calibration and can be used directly. Only when there are doubts about the quality of the ORP electrode or the test results, can an ORP standard solution be used to check whether the potential falls within the range of 200–275 mV, in order to determine the condition of the ORP electrode or the instrument. Instruction Manual for Redox Electrodes Instruction Manual for Redox Electrodes Instruction Manual for Redox Electrodes 3. The ORP measuring electrode (platinum or gold) should have a smooth surface; a rough or contaminated surface can affect the electrode’s potential in mv. It can be cleaned and activated using the following methods. 　　(1) For inorganic contamination, the electrode can be immersed in 0.1 mol/L dilute hydrochloric acid for 30 minutes, washed with pure water, then immersed in a 3.5 MOL/L potassium chloride solution for 6 hours before use. 　　(2) For organic oil and oil film contamination, the platinum or gold surface can be cleaned with detergent, followed by rinsing with pure water; after that, it should be immersed in a 3.5 MOL/L potassium chloride solution for 6 hours before use. 　　(3) If the surface of platinum is severely contaminated and an oxide film has formed, the surface can be polished using toothpaste, then cleaned with pure water, and after that immersed in a 3.5 MOL/L potassium chloride solution for 6 hours before use.

The pure water systems I’ve come across don’t seem to have this type of electrode; it’s more commonly found in systems for the anaerobic treatment of wastewater, where it is used in conjunction with dissolved oxygen sensors. I guess it should be installed after the activated carbon filter tank and before the cation exchanger; its main purpose is to monitor redox substances. If chlorine is used for disinfection, it is mainly to detect the residues of the oxidizing substance chlorine, as well as to check whether the activated carbon is effective in removing excess chlorine. Relevant information should be provided by the manufacturers of ion exchange resins. This is for reference only.