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Oxidation Reduction Potential of water
- What is the ORP of water?
- The Importance of Determining ORP
- ORP determination and measuring instruments
What is the ORP?
Oxidation Reduction Potential potential (RedOx potential) is a measure of the activity of electrons in a substance (in this case, in water). Oxidation is the process of losing electrons, and reduction is the process of gaining them. The total strength of this electronic exchange between substances in water directly affects the ORP level. This parameter is expressed in millivolts (mV) and is measured with an ORP meter. Negative values indicate the reducing properties of water, while positive values indicate oxidizing properties.
ORP is used as a measure of the oxidation state of water in swimming pools, water treatment systems and wastewater treatment systems. Redox potential is sometimes referred to as a measure of the effectiveness of a disinfectant in water; the higher the oxidation potential, the more effective the disinfectant.
ORP can be a valuable measurement if the user knows the specific component in the sample that is primarily responsible for the readings obtained. For example, excess chlorine in wastewater will lead to a large positive ORP value, and the presence of hydrogen sulfide will lead to a large negative value.
Since the ORP is temperature dependent, it must be recorded with each measurement so that the values obtained can be compared. In addition to temperature, ORP measurements often depend on the pH of the solution being analyzed. For example, chlorine in liquids exists as hypochlorous acid (HOCl), and depending on the pH, HOCl can produce more or less free chlorine. At lower pH values, more chlorine is formed.
The importance of ORP measurement
ORP depends on any substance that exchanges electrons. Studies have shown that the lifespan of bacteria in water is highly dependent on the value of the redox potential.
Organisms that break down unwanted substances in water require dissolved oxygen. Therefore, the lower the ORP level, the less oxygen or similar oxidizing agents, the more polluted the water source.
Conversely, a higher ORP value will mean more sanitized water. Drinking water will be properly disinfected at around 650 mV on the ORP scale, while swimming pool water will have a higher level (due to the presence of chlorine acting as an oxidizing agent) between 700 and 750 mV.
Thus, it is important to have a basic understanding of ORP levels in water for any purpose. In most applications, ORP is included in the list of water analysis parameters, as it indicates the degree of purity of water depending on its oxidizing and reducing properties. Too high and too low ORP values may indicate that the water is oversaturated with certain substances, making it unusable. For example, high ORP values indicate the saturation of water with chlorides and ferric ions. Low ORP values can be caused by water contamination with sulfides and organic degradation products. By regularly analyzing ORP levels, you can monitor the effectiveness of the disinfectant and adjust your water treatment plan accordingly.
Determination of ORP
Measuring the ORP of water takes three simple steps:
- Calibration: take a calibration solution with known ORP in mV. Immerse the sensor in the calibration solution. Once the ORP values are established, run the calibration command. You can find out more about the calibration of the ORP meter in the article.
- Measurement: immerse the ORP electrode in water and the ORP meter will display the mV value.
- Calibration repeat: every time the analyzed sample is changed.
Measuring instruments
The basis of the meter, as in pH meters, is an electrode - a sensor needed to measure the ability of water to carry electrons. The ORP electrode works similarly to a pH sensor, but instead of measuring hydrogen ions, it measures the activity of electrons. The electrode transmits the result to the ORP meter / controller.
ORP sensors work by measuring the electrical potential (voltage) between two electrodes in contact with water. One electrode is called an indicator electrode and is usually made of platinum. The other electrode is called a reference electrode; usually made from silver and silver chloride (Ag / AgCl). This electrode contains a filling solution containing potassium chloride (KCl). The electrons in the analyzed solution interact with both electrodes, creating a voltage between them, which is measured by the ORP meter.
If the contaminating components in water are oxidizing, then the redox electrode acts as a donor; to reducers, the electrode acts as an acceptor.
Although the electrode design and reference system is similar to a traditional pH electrode, ORP measurements cannot be described as being the same. While the pH electrode is selective for the concentration of hydrogen ions in solution, the ORP sensor provides a response according to the sum of redox reactions occurring in the sample; that is, the redox potential is a composite parameter of all ions and molecules involved in the redox reaction. The ORP sensor is a non-selective electrode and is not specific to any one element or chemical.
ORP meters and electrodes Hach
By the nature of use, ORP meters are divided into laboratory, portable and pocket ones. The choice of sensor depends on the sample to be analyzed and the measurement frequency. Use laboratory electrodes for standard measurements; for continuous monitoring in more challenging environments, choose industrial.
Measurement accuracy is also an important factor. For example, electrodes from Hach Lange have a measurement error of only ± 0.02 mV. You can find more information about Hach ORP measuring instruments and sensors by following the links below:
- Pocket ORP meters PocketPro
- HQ Series Portable ORP Meter
- Laboratory ORP Meter
- Laboratory combined ORP electrode
- Laboratory refillable ORP electrode
- Field ORP electrode
- Industrial ORP electrode
In addition to measurement accuracy, the advantage of benchtop and portable Hach Lange meters is the ability to measure not only ORP, but in parallel such water quality parameters as: pH, conductivity, dissolved oxygen, concentration of ammonium ions, chloride, etc. to learn more about HQ meters follow the link ->
The accuracy of the readings is influenced by a number of factors, the main of which are calibration and proper cleaning of the electrode. Learn more in our article on calibrating and cleaning ORP sensors.
AquaAnalytics® is the official representative of Hach in Russia and the CIS countries. For more information and advice on which ORP meter is suitable specifically for your application, please contact us in a convenient for you way.
Representative office in Russia: +7 (495) 201-53-02 info@aquaanalytics.ru
Representative office in Uzbekistan: +998 (90) 174-57-84 info@aquaanalytics-tekhnika.ru
