A helpful guide to ORP measurement

Jon Penn 15th June 2021
 Jon Penn, 
 Global Product Line Manager - CWA
 Measurement & Analytics  

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If you’ve ever wondered what ORP measurement is and how it’s different to pH, you’re not alone. When it comes to industrial measurement, ORP is very definitely the silent partner in the pH/ORP relationship, so much so that ORP instruments are outsold by pH instruments by a ratio of 20 to 1.


To stop you missing out on the ‘potential’ benefits that ORP measurement can bring, we’ve put together the following guide to help you, explaining what it is, why it matters and how to measure it. 

What does ORP stand for?

ORP stands for Oxidation Reduction Potential. As such, it measures the rate of both oxidation and reduction in a reaction between two substances.

What is oxidation and reduction?

Fundamentally, oxidation is a gain of oxygen, while reduction is a loss of oxygen.

An oxidizing agent is a substance that gives oxygen to another substance. A reducing agent removes oxygen from another substance.

Perhaps the best way to describe the fundamentals of oxidation and reduction is to imagine what happens to a piece of iron or steel when exposed to the elements. In the presence of oxygen from the air and of water from rain and snow, the metal will steadily corrode, causing it to eventually dissolve. Corrosion is an electrochemical process where electrons are transferred from one substance to another by the creation of an electrical charge
The metal itself acts as both an anode – a substance which gives up electrons, and a cathode, a substance which accepts electrons. The water from the rain and snow acts as an electrolyte, a liquid that enables electrons to move freely.
The process begins when rain water, combining with carbon dioxide in the air, forms a mild carbonic acid solution, creating a stronger electrolyte. This acid attacks the metal, which acts as the anode - it releases electrons which flow through the water to another less reactive part of the metal, which acts as the cathode.

This example highlights the concepts of oxidation and reduction. The iron is the reducing agent (gives up electrons), while the oxygen is the oxidising agent (gains electrons).

The air removes oxygen from the rusting metal, which is oxidised as electrons are lost, forming iron (II) ions. These electrons then move to the edge of the water droplet, where there is a greater concentration of oxygen, leading to the formation of hydroxide ions, which in turn combine with the iron (II) ions to form iron (II) hydroxide. In the final stage, this iron (II) hydroxide reacts with oxygen and water to form hydrated iron (III) oxide, or rust.

Because reduction and oxidation are going on side-by-side, this complete process is referred to as a redox reaction.

What does ORP actually measure?

ORP refers to the electrical potential of a redox reaction. It is also a measure of how much oxidation and reduction takes place under a particular set of conditions. The two processes always occur together.

This potential, which can be either negative or positive, is a measure of the power of a substance to gain electrons in solution. A strong oxidizing agent will have a high positive redox potential, while a strong reducing agent will have a high negative redox potential.

ORP is measured in millivolts (mV). For example, a substance with an ORP reading of +400mV is four times more oxidizing than a substance with an ORP of +100mV. Similarly, a substance with an ORP reading of -400mV is four times more reducing than one with an ORP of -100mV.

How does ORP differ from pH?

An ORP measurement is a measure of the level of activity in the a gas or liquid. This differs from a pH test which is a measure of concentration.

As we all know from our school days, the pH value of a solution is a measurement of how acidic or alkali a substance is. Values are given on a scale of 0 to 14. Seven is neutral, less than seven is acidic and greater than seven is alkali.

When would you use an ORP measurement?

ORP measurements are a major tool in water quality management when processes contain high concentrations of redox-active species such as metals salts and strong oxidising and reducing agents such as chlorine or sulphite. The ORP level of a water sample is a good indicator of treatment process efficiency. It shows the activity of a chemical disinfectant, rather than being a measure of the dose applied. For example, in water with an ORP greater than +665mV, the bacterium that causes listeria will survive for less than 30 seconds.

What equipment can you use to measure it?

ORP instruments are small handheld electrical devices, very similar to pH measuring instruments.
Absolute potentials are difficult to measure accurately, so reduction potentials are defined in relation to a reference electrode. The method uses an inert sensing electrode, made of platinum, or sometimes gold or graphite, which is put in contact with the solution. The low resistance of the electrode means it will either surrender electrons to an oxidant or acquire them from a reductant. This will continue until the electrode builds up a potential equal to the ORP of the solution being measured.

The resulting voltage is compared to a stable reference electrode connected to the solution via a silver salt bridge. This electrode is typically the same silver chloride electrode as used for pH measurements.

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