What is Free Chlorine (FCI)?

Free chlorine refers to both hypochlorous acid (HOCl) and the hypochlorite (OCl) ion. When chlorine is added to water, some of the chlorine reacts with organic materials and metals in the water and is not available for disinfection (this is called the chlorine demand of the water). The remaining free chlorine is available to disinfect disease-causing organisms.

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


Browse Our Products

Check out our latest water quality analysis innovations and browse our product catalog.


Find a Local Distributor

Use our distributor locator tool to find your nearest distributor of Sensorex water analysis sensors and instrumentation.


Contact Us

Need assistance? Our application engineers and customer support specialists are ready to help!