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Free Chlorine (FCL)

What is Free Chlorine (FCI)?

Free chlorine refers to chlorine that is present as hypochlorous acid (HOCl) or the hypochlorite (OCl–) ion. When the chlorine demand of water is satisfied, the remaining free chlorine is available to oxidize contaminants, which is measured by a Free Chlorine sensor.

Like all halogens, chlorine is a highly reactive element. In solution, chlorine can be found in many different molecular forms. When chlorine is added to water, it reacts with organic materials and metals, forming combined chlorine. Because combined chlorine is not available for disinfection, this effect is called the chlorine demand of the water.

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Why is Free Chlorine Measurement Necessary?

Free chlorine measurements are one of the most important measurements in the water industry today. These measurements ensure an adequate amount of chlorine is available for disinfection purposes, and in some cases that the chlorine has been removed.

Drinking water safety

Vegetable washing and disinfection

Sanitizing production lines

Swimming pool disinfection control

Ballast water treatment and post treatment

Cooling tower disinfection

Waste water treatment

How is Free Chlorine Measured?

Many people are familiar with DPD test kits for measuring chlorine. DPD is a reagent that reacts with chlorine and produces a red color. While DPD test kits may suffice for simple applications, this methodology does not distinguish between free chlorine and combined chlorine in solution. For applications where the free chlorine concentration is critical for disinfection, the most robust measurement technique is the use of an amperometric free chlorine sensor.

An amperometric sensor design consists of two electrodes (anode and cathode), a membrane and fill solution. These sensors measure a change in current caused by the chemical reduction of hypocholorous acid (HOCl) at the cathode. The current flow of this reduction is proportional to the chlorine available. A membrane and electrolyte help to control the reaction. Constant flow rate and pressure are controlled to ensure an accurate measurement.

In some applications, oxidation reduction potential (ORP) measurement can be used as an indicator of free chlorine concentration. Because free chlorine is an oxidizer, ORP is a measure of free chlorine sanitization activity. Measuring ORP as an indicator of free chlorine is especially popular in swimming pool monitoring applications.

Free Chlorine

Our dependable free chlorine sensor features amperometric measurement technology. Add our flow cell, flow regulator, and display for a complete system, or use your own.

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Popular ProductsSensorex Free Chlorine Best Sellers

FC72 – Flow Cell for Free Chlorine (FCL) and Chlorine Dioxide (CLD) Sensors

Our FC72 flow cell facilitates high-accuracy measurements of free chlorine and chlorine dioxide by preventing air bubble formation on the…

Add to Cart | $418.60

FCL – Free Chlorine Sensor, Amperometric

Chlorine exists in different chemical forms in solution, including free chlorine (hypochlorite ion and hypochlorous acid) and combined chlorine. Free…

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CLD – Chlorine Dioxide Sensor, Amperometric

Our dependable chlorine dioxide sensor features amperometric measurement technology. Designed for use in disinfection applications. Sensor directly outputs a 4-20mA…

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FM001 – Flow Regulator (0.1-1.0 gpm)

Our FM001 Water Flow Regulator allows for simple flow control. It is recommended for usage with the FCL Series Free…

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FCLA-5018 – Maintenance Kit, Free Chlorine

Keep your free chlorine amperometric sensors running for extended lifetime and improved accuracy using our FCLA-5018 free chlorine maintenance kit.…

Add to Cart | $207.70

CLDA-5018 – Maintenance Kit, Chlorine Dioxide

Keep your chlorine dioxide amperometric sensors running for extended lifetime and improved accuracy using our CLDA-5018 chlorine dioxide maintenance kit.…

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LPM100 – Display, Loop-Powered 4-20mA

The LPM100 is a durable, loop-powered 4-20mA display for use in process monitoring applications. Compatible with any sensor with a 4-20mA output,…

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