Oxidation-reduction potential (ORP) or redox is a measurement that indicates how oxidizing or reducing a liquid is. For example, water can be moderately oxidizing (like carbonated water), highly oxidizing (like chlorinated water or hydrogen peroxide solution), or reducing (like an environment where anaerobic microbes are active). In short, ORP is a measure of the cleanliness of water and its ability to break down impurities.
This measurement has a variety of applications, such as testing drinking water supplies for safe sanitation or monitoring 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 occur in the same reaction, which is why reactions involving oxidation and reduction are often referred to as redox reactions.
As an example, consider the reaction of oxygen gas with hydrogen gas to form water:
O2 + 2H2 — 2H2O
If we look more closely at the water molecule and write 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 have 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 get a charge of -2:
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 and received a charge of +1:
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.
When oxygen and hydrogen gas react to form water, the oxygen takes electrons from the hydrogen, so we can say that the hydrogen is oxidized by the oxygen. Similarly, we can also say that the oxygen is reduced by the hydrogen.
Some common oxidation processes include the decomposition of organic matter and the conversion of iron to rust (iron oxide).
Electrons and the redox scale
From the discussion above, you might guess where the word “oxidize” comes from. Oxygen gas is very good at accepting electrons from other atoms, and this is in fact the most common type of oxidation process that occurs in the environment. From this, one could also assume that an environment containing oxygen gas is an oxidizing environment. In such an environment, iron becomes rust and aerobic respiration can occur.
One could also assume that a reducing environment is one 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 donate electrons are called reducing agents.
The extent to which a liquid oxidizes or reduces (represented by redox) depends on the presence and strength of various oxidizing and reducing agents. ORP can also be viewed as a representation of the availability of electrons. Since reducing agents donate electrons, a reducing environment is one in which electrons are relatively available. In contrast, an oxidizing environment is one in which electrons are relatively unavailable.
Redox is expressed as an electric potential (voltage). Generally, 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 redox potential is the millivolt (mV), and most meters can display values between -1000 mV and +1000 mV. The more extreme the negative or positive value, the more the liquid is reduced or oxidized.
Different oxidation-reduction processes and conditions have different redox values, with aerobic conditions having higher redox values and anaerobic conditions having lower redox values.
Applications of redox measurement
One of the largest applications of redox is water disinfection. In municipal drinking water supplies, for example, strong oxidizers such as chlorine are used to kill bacteria and other microbes and prevent their growth in water supply pipes. Higher ORP values are associated with higher concentrations of disinfectant, so ORP is used to monitor and control disinfectant levels in water supplies. Disinfectants are used in swimming pools and spas to kill microbes that can transmit disease. Disinfectants are also used in outdoor pools and cooling towers to prevent algae growth.
ORP is also used to monitor and control many oxidation-reduction reactions in industrial processes. For example, redox is commonly used in automated industrial systems to maintain a small 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 occurring and to help operators manage the treatment system by promoting or preventing certain reactions. For example, ORP can be controlled in various parts of a system to digest organics, remove nitrate or phosphorus, and control odors.
Since low ORP values indicate anaerobic conditions, ORP can be used to detect anaerobic microbial activity in the environment such as in the water column or sediment. ORP can also be used to indicate soil saturation, making it useful for wetland mapping .
In other environmental applications, redox measurements can be considered an extension of the dissolved oxygen (DO) scale . Oxygen meters can cover the range of aerobic conditions, but they cannot indicate how severe an anaerobic environment is. The redox scale, on the other hand, covers a wide range of reducing conditions. For this reason, ORP can provide insights into the chemistry of anaerobic environments, such as the nature of microbial processes in sediments or reactions with contaminants in contaminated aquifers.
ORP can also be used in conjunction with membrane DO sensors to identify conditions where DO measurements may be erroneous . Under anaerobic conditions, membrane type DO sensors may give false readings if sulfides are present. If the ORP reading indicates anaerobic conditions, positive oxygen readings from these types of sensors should be considered suspect.
ORP is a rapid and inexpensive measurement of oxidizing and reducing conditions in an environment or system. This makes ORP measurement suitable for a wide range of industrial and environmental applications where oxidation and reduction conditions vary. ORP is particularly 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.