Analytical Instrumentation

What is Oxidation-Reduction Potential (ORP) ?

What is ORP (Oxidation – Reduction Potential) ?

ORP electrodes measure the voltage across a circuit formed by the measuring metal half cell and the reference half cell. When the ORP electrode is placed in the presence of oxidizing or reducing agents, electrons are constantly transferred back and forth on its measuring surface, generating a small amount of voltage.

ORP Measurement


Oxidation is the loss of electrons by an atom, molecule, or ion. It may or may not be accompanied by the addition of oxygen, which is the origin of the term.

Familiar examples are iron rusting and wood burning. When a substance has been oxidized, its oxidation state increases. Many substances can exist in a number of oxidation states. A good example is sulfur, which can exhibit oxidation states of

-2 (H2S); 0 (S); +4 (SO2); and +6 (SO4 -2).

Substances with multiple oxidation states can be sequentially oxidized from one
oxidation state to the next higher. Adjacent oxidation states of a particular substance are referred to as redox couples. In the case below, the redox couple is

Fe = Fe+2 + 2e-iron ferrous ion electrons

The chemical equation shown above is called the half-reaction for the oxidation, because, as will be seen, the electrons lost by the iron atom cannot exist in solution and have to be accepted by another substance in solution. So the complete reaction involving the oxidation of iron will have to include another substance, which will be reduced. The oxidation reaction shown for iron is, therefore, only half of the total reaction that takes place.


Reduction is the net gain of electrons by an atom, molecule, or ion.  When a chemical substance is reduced, its oxidation state is lowered. As was the case with oxidation, substances that can exhibit multiple oxidation states can also be sequentially reduced from one oxidation state to the next lower oxidation state. The chemical equation shown below is the half-reaction for the reduction of chlorine:

Cl2 + 2e- = 2Clchlorine

electrons chloride ion The redox couple in the above case is Cl2/Cl- (chlorine/
chloride).  Oxidation reactions are always accompanied by reduction reactions. The electrons lost in oxidation must have another substance as a destination, and the electrons gained in reduction reactions have to come from a source. When two half-reactions are combined to give the overall reaction, the electrons lost in the oxidation reaction must equal the electrons gained in the reduction reaction.

OXIDATION:  Fe = Fe+2 + 2 e-(Half- Reaction)
REDUCTION: Cl2 + 2 e- = 2 Cl- (Half- Reaction)

In the reaction above, iron (Fe) reduces chlorine (Cl2) and is called a reductantor reducing agent. Conversely, chlorine (Cl2) oxidizes iron (Fe) and is called an oxidantor oxidizing agent:

ORP Measurement:
ORP technology has been gaining recognition worldwide and is found to be a reliable indicator of bacteriological water quality for sanitation – determine free chlorine parameter. In swimming pool application, the ideal ORP value is approximately 700 mV where the Kill Time of E.Coli bacteria is the fastest to ensure good water quality. However ORP value also depends on the pH of pool water, which is typically between 7.2 and 7.6 pH. 
The pH of pool water has to be maintained at optimum level by dosing appropriate chemicals. If the pH of swimming pool is acceptable and ORP value is below 700 mV, then hypochlorite or other oxidizing chemicals need to be added.
Applicaions of ORP Measurement 
Cyanide Oxidation
Aquarium Monitoring
Chromate Reduction
Drinking Water
Swimming Pool Water
Pulp Bleaching
Cooling Tower
Ozone monitoring
Water pollution monitoring

Troubleshooting Tips for ORP measurement.

These are the problems may occur during the ORP measurement. Slow Response,Noisy Response,Measurement Drift,Incorrect Measurement.
Solution for this problems are given in the table.
ORP measurement - Troubleshooting tips

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