ORP Oxidation Reduction Potential Redox

Understanding Oxidation-Reduction Potential (ORP) in Water

Oxidation-reduction potential (ORP)—sometimes referred to as redox—is a measurement that determines water’s oxidizing or reducing potential, quantifying a solution’s electron transfer capacity. ORP is valuable for assessing the presence of oxidizing or reducing agents, such as oxygen, chlorine, or hydrogen sulfide. It is measured in millivolts (mV), where positive values indicate oxidizing conditions and negative values indicate reducing conditions.

ORP is a nonspecific measurement—the measured potential reflects a combination of the effects of all the dissolved species in the medium. Therefore, the measurement of ORP in relatively clean environmental water (e.g., ground, surface, estuarine, and marine) has only limited value unless a predominant redox-active species is known to be present.

The value of ORP in determining environmental water content is greatly enhanced if the researcher has some knowledge or history of the site and/or knows that one sample component is primarily responsible for the observed value. ORP data can become more valuable if used as an indicator over time and/or with other common parameters (e.g., pH, dissolved oxygen, turbidity, conductivity) to help develop a complete picture of the water quality being tested.

Oxidation and Reduction Processes

In a water sample, oxidation involves the loss of electrons, while reduction refers to the gain of electrons. Oxidizing agents, such as chlorine, increase ORP values, while reducing agents, like decaying organic matter or hydrogen sulfide, decrease it. Monitoring ORP helps determine the overall redox state of water, which is critical for understanding its ability to support or break down certain chemical compounds.

ORP values can be positive or negative. As more electrons become available, the ORP value of a solution becomes more negative. It’s important to note there are no free electrons in solution. Instead, ORP is a consideration of chemical species and their ability to accept or donate electrons.

A redox tower is a graphical representation of common redox reactions, ranking electron donors and acceptors based on their reduction potential. The higher a substance is on the tower, the more likely it is to donate electrons (reduced), while substances lower on the tower are better electron acceptors (oxidized).

In water treatment, the redox tower helps identify which chemical reactions are occurring, such as oxygen serving as a strong electron acceptor in aerobic conditions or nitrate in anoxic conditions. Understanding the redox tower is crucial for optimizing processes like nitrification, denitrification, and organic matter breakdown in various water systems.

Why Measure ORP in Surface Water?

In surface water environments, ORP indicates the water’s ability to break down contaminants or support biological processes. A high ORP value indicates the presence of strong oxidizing agents like dissolved oxygen (O2). Oxygen is a key driver of oxidative reactions, helping break down pollutants, support fish respiration, and prevent the growth of harmful anaerobic bacteria. Surface water with consistently high ORP values tends to be well-aerated, supporting a healthy, balanced ecosystem.

Conversely, low ORP values in surface water can signal potential issues, such as eutrophication or pollution. Eutrophication occurs when excessive nutrients, often from agricultural runoff, enter a water body, promoting the overgrowth of algae. As algae die and decompose, the oxygen in the water is consumed, lowering the ORP and leading to hypoxic (low oxygen) or even anoxic (no oxygen) conditions. These conditions can severely impact fish and other aquatic organisms that rely on oxygen for survival. In such cases, ORP serves as an early warning system, allowing water managers to take corrective actions before a full-scale ecological imbalance occurs

ORP is also valuable in assessing the presence of contaminants, such as heavy metals or industrial pollutants, which can significantly affect the water’s redox potential. High ORP values indicate that the water can neutralize these contaminants, while low values suggest the water may not have enough oxidizing power to break them down, allowing harmful substances to persist.

Why Measure ORP in Aquaculture?

Maintaining optimal ORP levels in aquaculture is critical for creating a safe and productive environment for fish and other aquatic organisms. ORP is used to monitor the balance of oxygen and organic waste in fish farms, hatcheries, and ponds, ensuring that conditions remain suitable for the health and growth of aquatic species.

By tracking ORP, operators can adjust the use of disinfectants to ensure they are effective without causing harm to the fish or other aquatic organisms. For instance, ozone is a common disinfectant added to recirculating aquaculture systems (RAS)—where water is continuously filtered and reused—resulting in the oxidation of organic and inorganic compounds. ORP is monitored to ensure ozone is completely consumed in the treatment area of the system before the water is returned to the rest of the system, as ozone is toxic to aquatic species¹.

Why Measure ORP in Groundwater?

Redox reactions play a critical role in the chemistry of our groundwater sources. They determine how a chemical compound—such as a contaminant—moves throughout the aquifer, how it interacts with other substances, and how it may degrade into other chemical species².

Natural factors in groundwater redox conditions include aquifer rock type and groundwater age. Rock types such as unconsolidated sand and gravel are typically more oxic (i.e., oxygen is present and is the preferred electron acceptor), while sandstone aquifers are among the principal aquifer rock types considered less oxic².

Regarding groundwater age, recently recharged groundwater is more likely to be toxic, while much ‘older’ groundwater that recharged many years ago—millions of years ago in some instances—is more likely to be anoxic. It should be noted that redox conditions can vary across short distances for various reasons²

ORP measurements can yield insights into the types of contaminants that might be present in groundwater. For example, anoxic groundwater—ORP would have a negative value in this environment—supports the reduction of nitrate to nitrogen gas. Arsenic concentrations are also more likely to be elevated in anoxic conditions, as this toxic compound is released from rock and sediments when oxygen is not present. In contrast, concentrations of nitrate and uranium would be elevated in oxic conditions².

Redox conditions play a prominent role in the natural attenuation of contaminated groundwater. Microbes can degrade oil that has leaked into an aquifer, and the degradation process changes redox conditions in the subsurface. Like other organisms, microbes need to respire (i.e., breathe). Respiration requires an electron acceptor, and since oxygen is preferred, DO is quickly depleted where contamination is present. Therefore, DO can only be found outside a plume of contaminated groundwater⁴.

Check out our blog on How ORP Sampling Helped Determine Arsenic in Drinking Water Wells to see a specific example of ORP measurement in groundwater.

Why Measure ORP in Wastewater?

ORP is essential for optimizing biological processes in wastewater treatment, particularly in systems like Sequencing Batch Reactors (SBRs) and continuous flow systems. ORP indicates whether water is in an oxidizing or reducing state by measuring its electron transfer capacity, helping operators control treatment processes such as aeration and nutrient removal.

By monitoring the ORP of wastewater, an operator can determine what biological reaction is occurring and if operational conditions should be changed to promote or prevent that reaction.

In nitrification, oxygen acts as a strong electron acceptor, generating positive ORP values, typically between +100 and +350 mV. This process helps convert ammonia (NH₄⁺) into nitrate (NO₃^-), indicating that the system is in an oxidizing state. In denitrification, nitrate is the electron acceptor, with ORP values ranging from -100 to +100 mV in an anoxic environment, where oxygen is depleted.

ORP provides a broader range of information than dissolved oxygen (DO) alone. For instance, while DO can indicate the presence of oxygen, ORP gives deeper insights into the status of biological reactions, such as nitrification and denitrification. ORP can also reveal if other electron acceptors, like nitrate or sulfate, are present, offering more detailed control over the treatment process.

Monitoring ORP allows operators to determine whether the wastewater is in the oxic, anoxic, or anaerobic zones, critical for biological nutrient removal. High ORP values in the oxic zone suggest active oxidation, while lower ORP values in the anoxic or anaerobic zones indicate different processes, like denitrification or fermentation. Adjusting treatment strategies based on these values ensures that the desired reactions are taking place efficiently.

ORP also plays a role in phosphorus removal, though phosphorus is not directly involved in redox reactions. Low ORP values in the anaerobic zone promote phosphorus release, while higher values in the aerobic zone support phosphorus uptake, allowing operators to control these processes effectively.

In short, ORP is a critical tool in wastewater treatment, providing insight into nitrification, denitrification, and other biological reactions—read ORP Management in Wastewater as an Indicator of Process Efficiency to learn more about these reactions and typical ORP values.

To learn more about ORP measurement in wastewater, check out our webinar on Monitoring Oxidation Reduction Potential in Biological Nutrient Removal or our blog post on ORP, a Versatile and Misunderstood Wastewater Treatment Parameter.

How to Measure ORP in Water

How Does an ORP Sensor Work?

An ORP sensor consists of two separate electrodes: 1.) a reference electrode immersed in electrolyte and 2.) a measuring electrode that contacts the sample. The reference electrode measures a constant potential that doesn’t change with the sample, while the measuring electrode’s potential does. The difference in electrical potentials between these two electrodes produces a mV value—reflecting the redox state of the sample—and is displayed by the instrument.

The measuring electrode is often a platinum band, as platinum can both donate and accept electrons without participating in the reaction. Gold is another common electrode for ORP measurement.

To learn more about the use of ORP sensors, check out our blog post on ORP Measurement: Tips, Cautions, and Limitations.

What is the Difference Between pH and ORP Sensors?

pH measures the concentration of hydrogen ions, indicating whether water is acidic or basic. pH data are more absolute; a specific pH value equals a particular concentration of hydrogen ions.

In contrast, ORP measures the ability of water to accept or donate electrons, giving insight into its oxidizing or reducing properties. It is a relative measurement—an ORP value doesn’t necessarily correspond to a specific concentration of any particular ion or chemical species. Typically, a single ORP data point is of limited value. Instead, seeing data trends over time is when an ORP sensor is the most valuable.

The internal components of pH and ORP sensors are essentially the same; they both feature a reference electrode, junction, electrolyte, and measuring electrode. In fact, some pH and ORP sensors are combined into a single sensor body, each sharing the same reference system. If you have a sensor like this, ORP is the smaller bulb, pH is the larger.

Unlike a pH electrode, a silver/silver chloride wire inside the ORP sensor (see diagram below) makes direct contact with the bulb, allowing electrons to flow into and out of the bulb, generating a potential that is then read by the instrument.

To learn more about pH and ORP sensors—including their differences—check out our webinar on How pH and ORP Sensors Work.

How to Select the Right ORP Instrument

The ideal ORP measurement system will depend on a few factors:

• Will measurements be in the field or the lab?
  • Field ORP instruments (e.g., YSI EXO, ProDSS, and ProQuatro) are much more rugged than lab instruments (e.g., the MultiLab 4010-3W or TruLab 1320). However, lab ORP systems also have their advantages. For example, glass ORP electrodes for the lab can be refilled with reference electrolytes—unlike field sensors—resulting in a longer life and faster response time, among other advantages.
• Do other parameters need to be measured?
  • Most YSI field and lab instruments measure more than ORP—such as those listed in the first bullet above—but there are some options for those that only need to measure ORP. For the field, the Pro10 is a great single-parameter instrument that measures ORP. In the lab, the TruLab 1110 or pH1000A are good options. A more economical option for the field or lab is the ORP15A pen-style tester.
• Does data need to be continuously collected?
  • Continuous ORP monitoring, especially alongside other parameters, is ideal. When ORP data are collected over time and in conjunction with other measurements, it offers a comprehensive view of water quality. The EXO is the best option for monitoring surface water bodies, while the IQ SensorNet system is specifically designed for wastewater, drinking water, and aquaculture applications.
  • For groundwater, the diameter of the well may dictate if continuous long-term monitoring is possible. Standard 2-inch wells can accommodate handheld instruments for collecting discrete measurements, such as the ProDSS or ProQuatro, as well as the EXO1 for continuous monitoring. Considering that EXO1 cannot be equipped with a Central Wiper for antifouling, it should only deployed in low-fouling conditions. The EXO2 and EXO3, which are compatible with the Central Wiper, are ideal for continuous groundwater monitoring in larger-diameter wells to provide reliable and stable deployment data.

Do you have questions about ORP or need help selecting an ORP measurement solution? Ask our experts or schedule a free virtual consultation today!

Sources

1. Lindholm-Lehto Water quality monitoring in recirculating aquaculture systems
2. USGS Oxidation/Reduction (Redox)
3. USGS Redox conditions by principal aquifer rock type
4. Parsons Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents