What is Free Chlorine (FCI)?

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.

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.

free chlorine sensor

How is Free Chlorine Measured?

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.

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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