What is redox?

26/10/2015

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There are many technical terms surrounding the world of water treatment for swimming pools and spas. Let's start with the basics; what's redox?

oxydo-reduction

Redox: General Information

A redox reaction, or oxidation-reduction reaction, is a chemical reaction during which an electron is exchanged. The term “redox” is a contraction of “reduction” and “oxidation” as, during this reaction, reducing agents transfer electrons to oxidising agents.

Each chemical element has electrons and, depending on the reaction conditions, will be able either to give them or take them, thereby forming a redox couple. These exchanges of electrons will generate a potential, called the “redox potential”, which is measured in mV.

When there is a chemical oxidation-reduction reaction between two elements, as their redox potentials are different, this will create a measurable voltage (potential difference).

Application in Swimming Pools

In swimming pools, most of the disinfectants used are oxidising agents: chlorine, bromine, active oxygen, etc.

These will react with the reducing agents in the pool: nitrogenous content, metals, organic compounds, etc.

Therefore, oxidation-reduction couples will be created that will generate an overall difference in potentials that is called the ORP (Oxidation-Reduction Potential).

Measuring this ORP will indicate to us whether there are oxidation-reduction reactions in the pool.

Chlorine for Disinfecting Swimming Pools

Regardless of the type of chlorine used to treat the water in swimming pools (tablets, granules, powders, liquids or electrolyser), depending on the water’s pH, this will create the chemical ClOH.

ClOH is a powerful disinfectant (it destroys bacteria and viruses) but it is also an oxidising agent.

Thus, it will react with all of the reducing agents in the water, and its ORP will be measurable.

Depending on the pH, the water temperature and the chlorine content, this ORP will vary. By way of an example, here is a table showing the ORP value depending on the chlorine rate and the pH for water at a temperature of 25°C.

Conclusion: RedOx in Swimming Pools

Redox (or ORP) measurements in a swimming pool are linked to the activity of chlorine, which will react with the other chemical elements in the water (nitrogenous content, metals, organic materials, etc.).

This value will enable you to have an overview of all of the oxidation-reduction reactions linked to ClOH.

As each swimming pool is different (in terms of the water it is filled with, its equipment, its pollution, etc.), the ORP may differ depending on the pool’s chlorine needs.

It is therefore recommended that you balance the water, manually adjust the chlorine rate to the desired content level, and then measure the ORP and take this value as the set point for regulation purposes.

That way, the chlorine content will always be ideal and suited to the pool’s needs.

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