What Is ORP?


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Oxidation-reduction potential (ORP), also known as redox, is the electron exchange potential that measures if a substance (usually liquid) is either oxidized or reduced. ORP is commonly used to measure cleanliness in water systems and the breakdown of waste products, detritus, and contaminants.

ORP tells us if water quality is high or low in any application. ORP meters use electrodes to measure ORP levels. ORP is often conveyed as an electrical potential – if readings are positive, the substance is an oxidizing agent, and if negative, it is a reducing agent.  

ORP plays a vital role in water sanitation applications, ensuring that water is safe for human consumption and skin contact. When water is treated with sanitizing agents, ORP levels are closely monitored. 

Oxidation & Reduction

To understand ORP, we need to know what oxidation and reduction are. Oxidation and reduction are chemical processes that relate to electrons being exchanged in a reaction. 

Substances go through an oxidation process when there are not enough electrons, in the hope to acquire some more. When substances are positive, they search for electrons so they are considered oxidizing agents. 

When substances contain electrons with too many ions, they can give them away without destabilizing themselves – this creates antioxidizing or reducing agents. 

Oxidation and reduction reactions can become very complicated so just remember that oxidation is the loss of electrons, and reduction is the net gain of electrons. It is also important to note that a redox reaction is an oxidation reaction paired with a reduction.

Substances containing dissolved oxygen (DO) and chlorine will contribute to higher oxidation, thus higher ORP readings. These substances have higher electronegativity (oxidizing agents) and will normally steal electrons from other substances like unwanted contaminants and other pollutants. 

How to Measure ORP

ORP is measured in millivolts (mV) by using ORP probes and sensors. ORP is measured by inserting an ORP probe/sensor into the water; this can be either handheld or built into an application. 

ORP probes and sensors use electrodes to measure the electrical potential (voltage), and are widely used in different applications due to their fast and inexpensive measurements. Higher readings correspond to a more sanitary water system. 

ORP probes are very safe and easy to use, especially when you use an ORP kit that has all the components you need to get accurate and interference-free readings. ORP probes measure the relative amount of electron activity with each probe containing dual electrodes; one for reference and one for measuring the sample. Before use, an ORP probe must be calibrated.

Applications That Use ORP 

ORP is a measurement suitable for testing oxidizing and reducing conditions for a range of different industrial and environmental applications. ORP comes in handy when slower and more costly chemical tests are not constructive, especially when frequent monitoring is required. 

The most common use of ORP is in industrial applications like wastewater systems to oxidize or reduce components before discharge. Chromate is used in electroplating metals to alter chemical compositions. This is toxic, so it must be removed before environmental release. A substantial decrease in ORP indicates a complete reduction of chromate. 

Additionally, ORP is measured to control disinfectant levels and bacterial growth in water systems such as swimming pools and spas. When maintaining a swimming pool or spa, measuring ORP levels determines if it is a safe environment. ORP-positive mV readings that range between 650 mV and 750 mV are considered ideal for water sanitation. 

Disinfectants are used to treat swimming pools and spas to kill any disease-transmitting microbes that may be present in the water. In outdoor swimming pools and spas, algae growth is likely due to surrounding environmental conditions; disinfectants are used to prevent algal growth. 

ORP is often used in conjunction with DO for biological phosphate and nitrogen removal using bacteria. Measuring ORP levels helps to monitor bacterial metabolic processes, eliminating microorganisms that exist in different conditions (aerobic, anoxic, & anaerobic).

ORP probes/sensors are also widely used to measure biological growth in the food processing industry to ensure the water is free from harmful bacteria and contaminants. Contaminant-free drinking water should have a positive ORP reading of at least 650 mV to ensure suitable sanitation. 

ORP can be used to specify water content in soil (soil saturation); this is useful to understand nutrient availability when mapping wetlands. 

ORP is also useful in the winery industry helping to control aeration during the yeast fermentation process. ORP levels help maintain an aerobic environment which allows volatile sulfur aromas and hydrogen sulfides to form which is crucial for wine products. 

How to Adjust ORP Levels?

In water systems such as swimming pools, ORP levels can be adjusted by adding chlorine which enhances the antioxidizing/reduction potential. 

Complex water purification systems may require balancing total dissolved solids to achieve the desired water quality and ORP levels. 

Factors That Can Affect ORP Measurements

Even though measuring ORP levels is easy, there are 4 factors that can affect readings:

  • Temperature
  • Pressure
  • Salinity
  • pH


As temperature levels increase, DO levels decrease, lowering the ORP level. You must take into consideration the time of day and seasonal weather when measuring ORP. 


When the pressure decreases, so does DO, affecting ORP levels. 


As salinity increases, DO decreases, affecting ORP levels. 

pH levels

Increased pH levels can decrease ORP levels due to a higher proportion of Hypochlorous Acid (HOCl) to Hypochlorite Ion (OCl-). Chlorine is a large benefactor to high ORP levels; when pH levels increase, chlorine levels decrease greatly reducing the ORP level. 


ORP is fantastic in measuring how a substance is either oxidized or reduced. ORP testing is used in a variety of industries, but most commonly ORP is used to measure cleanliness in water systems and the breakdown of waste products, detritus, and other contaminants.

ORP meters use electrodes to measure ORP levels; if readings are positive, the substance is an oxidizing agent, and if it is negative, it is a reducing agent.  Positive ORP readings in the 650-750 mV range are ideal for sanitation and safe for human consumption.

If you are unsure exactly which ORP device will best suit your needs, or you would like to learn more about other water measurements, do not hesitate to reach out to the world-class team at Atlas Scientific.

ORP Probes & Sensors

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