Testing potable water for free chlorine

Chlorine Residual Testing:

A document from CDC/SWS (Safe Water System) describing how and why to test for residual (free) chlorine in drinking water.  Includes pros & cons of various methods. Quoting…

“The presence of chlorine residual in drinking water indicates that: 1) a sufficient amount of chlorine was added initially to the water to inactivate the bacteria and some viruses that cause diarrheal disease; and, 2) the water is protected from recontamination during storage. The presence of free residual chlorine in drinking water is correlated with the absence of disease-causing organisms, and thus is a measure of the potability of water.

This Fact Sheet describes:

  1. The processes that occur when chlorine is added to water, and the definitions involved with these processes;
  2. How and why the Safe Water System project recommends testing of free chlorine;
  3. Methods to test free chlorine in the field in developing countries.


When chlorine is added to drinking water, it proceeds through a series of reactions described below and depicted graphically on the next page above. When chlorine is added to water, some of the chlorine reacts first 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 chlorine concentration after the chlorine demand is accounted for is called total chlorine. Total chlorine is further divided into: 1) the amount of chlorine that has reacted with nitrates and is unavailable for disinfection which is called combined chlorine and, 2) the free chlorine, which is the chlorine available to inactivate disease-causing organisms, and thus a measure to determine the potability of water.

Methods include pool test kits (low cost but inaccurate — essentially worthless for the intended use), color wheel test kit (low cost, accurate enough only if properly used & understood), and colorimeter (expensive but accurate). Here is a

Hanna Instruments HI701 Checker HC Handheld Colorimeter for Free Chlorine

for $50 at Amazon. It seems to combine the lower cost of color wheel test kit with accuracy and ease of use of digital colorimeter.

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1 Response to Testing potable water for free chlorine

  1. Pingback: A Simple Low Cost DIY Chlorinator | Osa Mountain Village

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