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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Free Chlorine Sensors and Accessories

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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.

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.

Common Applications:

  • 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


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