Oxidation-reduction potential (ORP) or redox is a measurement that indicates how oxidizing or reducing a liquid is. For example, water may be moderately oxidizing (such as aerated water), strongly oxidizing (such as chlorinated water or hydrogen peroxide solution), or reducing (such as an environment where anaerobic microbes are active). In short, ORP is a measure of the cleanliness of the water and its ability to break down contaminants. This measurement has a variety of applications, such as checking for safe sanitation of drinking water or monitoring fluid for the suitability for anaerobic microbial processes.
What are oxidation and reduction?
Oxidation and reduction are related chemical processes that refer to the exchange of electrons in a reaction. Oxidation refers to when a chemical loses electrons. Reduction refers to when a chemical gains electrons, so reduction is the opposite of oxidation. Both oxidation and reduction can happen in the same reaction, which is why reactions involving oxidation and reduction are often called redox reactions.
As an example, let’s look at the reaction of oxygen gas with hydrogen gas to form water:
O2 + 2H2 -- 2H2O
If we look closer at the water molecule, writing it as (H+)2(O-2), it can be viewed as a combination of two ions, O-2 and H+, that have electrical charges because they gained or lost electrons:
2H+ + O-2 -- (H+)2(O-2)
Electrons have a negative charge, so the oxygen atom in the water molecule gained two electrons to end up with a −2 charge:
O + 2e- -- O-2
In the above reaction, the oxygen atom was reduced because it gained electrons.
Each of the two hydrogen atoms in the water molecule lost an electron to end up with a +1 charge:
H2 -- 2H+ + 2e-
In this reaction, the hydrogen atoms were oxidized because they each lost an electron.
Oxidation or Reduction?
O + 2e- -- O-2
The oxygen atom gains electrons.
The oxygen atom is reduced.
H -- H+ + e-
The hydrogen atom loses an electron.
The hydrogen atom is oxidized.
O2 + 2H2 --2H2O
The oxygen atoms are reduced.
The hydrogen atoms are oxidized.
In the reaction of oxygen and hydrogen gas to form water, the oxygen accepts electrons from the hydrogen, so we can say that the hydrogen is oxidized by the oxygen. Likewise, we canalso say that the oxygen is reduced by the hydrogen.
Some common oxidation processes include decomposition of organic matter and conversion of iron to rust (iron oxide).
Electrons and the ORP scale
From the above discussion, one might guess where the word “oxidize” comes from. Oxygen gas is very good at accepting electrons from other atoms, and this is indeed the most common type of oxidation process that occurs in the environment. From this, we might also suppose that an environment that contains oxygen gas is an oxidizing environment. In such an environment, iron will turn to rust, and aerobic respiration can occur.
One might also guess that a reducing environment is an environment without oxygen gas. Such an environment often includes dissolved gases that are products of anaerobic activity, such as methane, hydrogen sulfide, and hydrogen.
Chemicals (such as oxygen) that accept electrons from other compounds are called oxidizing agents, and substances (such as methane or hydrogen) that give up electrons are called reducing agents.
The degree to which a fluid is oxidizing or reducing (represented by ORP) depends on the presence and strength of various oxidizing and reducing agents. ORP can also be thought of as representing the availability of electrons. Because reducing agents give up electrons, a reducing environment is one where electrons are relatively available. In contrast, an oxidizing environment is one where electrons are relatively unavailable.
ORP is expressed as an electrical potential (a voltage). Generally speaking, a reducing environment is indicated by a negative reading, and an oxidizing environment is indicated by a positive reading. The most common unit for expressing ORP is the millivolt (mV), and most meters can read values ranging from -1000 mV to +1000 mV. The more extreme the negative or positive value, the more reducing or oxidizing the fluid is.
Different oxidation-reduction processes and conditions have different ORP values, with aerobic conditions having higher ORP values and anaerobic conditions having lower ORP values. \
Applications of ORP measurement
One of the biggest applications of ORP is in water disinfection. Municipal drinking water supplies, for example, use strong oxidizers such as chlorine to kill bacteria and other microbes and to prevent their growth in water supply lines. Higher ORP values are associated with higher concentrations of the disinfectant, so ORP is used to monitor and control disinfectant levels in water supplies. In swimming pools and spas, disinfectants are used to kill microbes that may transmit diseases. In outdoor swimming pools and cooling towers, disinfectants are also used to prevent the growth of algae.
ORP is also used for monitoring and control of many oxidation-reduction reactions in industrial processes. For example, in automated industrial systems, ORP is often used to maintain a slight excess of oxidizing chemicals such as chlorine, hydrogen peroxide and ozone, or reducing chemicals such as sulfur dioxide and sodium sulfite.
In wastewater treatment, ORP is used to determine the types of microbial processes that are occurring and to help operators manage the treatment system by promoting or preventing certain reactions. For example, ORP may be controlled in various parts of a system to digest organic matter, remove nitrate or phosphorus, and control odors.
Because low values of ORP indicate anaerobic conditions, ORP can be used to detect anaerobic microbial activity in the environment, such as in the water column or in sediment. ORP can also be used to indicate soil saturation, which makes it useful for mapping wetlands.
In other environmental applications, ORP measurements can be viewed as an extension of the dissolved oxygen (DO) scale. DO meters can cover the range of aerobic conditions, but they cannot indicate how reducing an anaerobic environment is. The ORP scale, on the other hand, covers a wide range of reducing conditions. Because of this, ORP can provide insight into the chemistry of anaerobic environments, such as the types of microbial processes in sediments or reactions involving pollutants in contaminated aquifers.
ORP can also be used in conjunction with membrane DO sensors to identify conditions where the DO measurements may be faulty. Under anaerobic conditions, membrane-type DO sensors may give false readings if sulfides are present. If the ORP measurement indicates anaerobic conditions, then positive DO measurements taken from these types of sensors should be considered suspect.
ORP is a fast and inexpensive measurement of the oxidizing and reducing conditions in an environment or system. This makes ORP measurement suitable for a wide range of industrial and environmental applications where oxidizing and reducing conditions vary. ORP is especially useful for routine or continuous monitoring situations where slower and more expensive chemical tests would not be as practical.
 U.S. Environmental Protection Agency (2017) Field measurement of oxidation reduction potential (ORP). SESDPROC-113-R2.