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Water Quality 101: What Is ORP in Water Testing?


ORP | Jenco Instruments

You may have heard of ORP, but what does it mean and what role does it play in water-quality testing? We’ve laid out the basics of ORP to help you get up to speed.

What Is Oxidation Reduction Potential (ORP)?

Oxidation reduction potential, or ORP, is a measure of a substance’s ability to either oxidize or reduce another substance. It is measured by the electrodes of an ORP meter. A positive reading on an ORP meter means that the substance is an oxidizing agent; a negative reading indicates that the substance is a reducing agent.

Oxidation and Reduction Basics

If a substance doesn’t have enough electrons, it will actively attempt to acquire them through the process of oxidation—a chemical reaction in which one agent is oxidized (i.e., loses electrons to another agent). Because substances with positive ORP readings are looking to acquire electrons, they are considered oxidizing agents.

On the other hand, electrons that have a surplus of ions can afford to lose ions to oxidizing agents without becoming destabilized themselves. For this reason, they are referred to as antioxidizing agents, or reducing agents. The higher or lower an ORP reading (positive or negative), the more oxidizing or antioxidizing a substance is, respectively.

Why ORP Readings Are Important to Water Testing

ORP is integral to water testing because it indicates how sanitized or contaminated water is based on its oxidation and reduction properties. In order to be safe to consume, recycle, or come in contact with our skin, water is treated with a sanitizing agent to have a higher oxidation potential, producing a higher ORP reading. By routinely monitoring ORP levels, it’s possible to monitor the effectiveness of a sanitizing agent and adjust your water treatment plan accordingly.

Common Uses for ORP Meters

There are many reasons why someone would be interested in knowing the ORP of a substance. Perhaps the most common application is water-quality testing of chlorinated swimming pools. Because a substance’s ORP is affected by all agents that are present in the substance, an ORP measurement is considered more informative than a  pH reading alone, which only recognizes acids (hydrogen ions) and bases (hydroxide ions). Larger pools will often have inline ORP sensors, whereas backyard pools that lack built-in monitors may require testing with a portable ORP monitor.

When you add chlorine to a pool, the chlorine gives up its own electron surplus to neutralize or oxidize potentially harmful bacteria. Active chlorine electrons increase the ORP level of the water, thus neutralizing contaminants. For chlorinated pools and spas, a safe ORP reading is usually between 650 millivolts (mV) and 750 mV. As time passes and chlorine breaks down and loses its oxidation potential, the overall ORP reading for the water gradually decreases. Although ORP levels typically correlate with the ratio of sanitizer in the water, ORP does not measure sanitizer levels; rather, it measures the net oxidation and reduction properties of all agents present.

In addition to being used to monitor water quality in chlorinated pools, ORP meters are widely used in the food processing and water treatment industries to ensure that water is free of contaminants and safe to recycle or consume. Bottled water or tap water that is contaminant-free will have a positive ORP value.

Adjusting ORP Levels

When it comes to swimming pools, ORP levels can usually be adjusted by adding more active chlorine to the water to boost its antioxidizing potential.  More sophisticated water purification systems may involve a more complex balancing of total dissolved solids (TDS) in water to achieve a more favorable oxidation reduction potential.

For more information about how to interpret ORP readings and use ORP meters, research your chosen ORP meters and its unique features.

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