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Tips and Information on Measuring Free Chlorine

April 18, 2014

Operators with water systems that provide chlorine disinfection must test for free chlorine to ensure effective treatment. The U.S. Environmental Protection Agency (EPA) has approved test methods for this purpose. Although these methods are simple and reliable, operators can get poor results unless they use proper techniques. This post offers some background information and tips on properly measuring free chlorine.

Small water systems usually use the DPD colorimetric and ITS test strip methods. These test methods use an indicator chemical that develops a color when added to chlorinated water. The color turns darker with higher chlorine residuals. Users compare this color to a color scale to determine the chlorine residual in milligrams per liter (mg/L). If there is no color change, there is no chlorine in the water. If you have color blindness, you may have difficulty reading the true color.

Free chlorine is a stronger disinfectant than other types of chlorine. Systems that disinfect must maintain detectable chlorine residual in the distribution system. A free chlorine residual of 0.2 mg/L meets this requirement. Systems with a contact time (CT) requirement must also maintain a free chlorine residual at the point of entry and/or in the distribution system.

ITS Test Strip Method

 ITS Test Strip Method

DPD Colorimetric Method

DPD Colorimetric Method

For any test field kit, be sure to read and follow the manufacturer’s instructions. Check the expiration date prior to use. If it’s expired, don’t use it.

Tips for Sample Collection

  • Keep the sample containers clean and scratch-free. Replace discolored or damaged containers.
  • Take samples only from cold-water taps.
  • Collect samples in the distribution system from actively used connections.
  • Let the water run for a while before collecting the water sample (~1 min.)
  • Fill the sample container directly from the sample tap.

 Test Method Table


Washington State Department of Health, Division of Environmental Health Office of Drinking Water. 2010. Measuring Free Chlorine. DOH331-442. Updated October 2010. Accessed April 3, 2014.

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