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Chlorine Residual Testing

There are 3 primary types of chlorine residual:

  • Free Residual - Strong disinfecting power, relatively unstable
  • Combined Residual - Weak disinfecting power, relatively stable
  • Total Residual = Free Residual + Combined Residual

Typically, well water will only create a free residual. Surface water may have free and/or some combined residuals. Wastewater will, typically, have combined and free residuals, depending on the treatment process.

The most common field test method for determining chlorine residual concentration is the DPD test. This is a colorimetric test that relies on comparison of a developed color in a water sample against a color standard.

Color standards that are made of plastic, liquid filled glass, or dye colored glass will fade with time. If the standard fades, then your test will not be valid. Only glass standards made with rare earth elements will not fade with time. Plastic color standards should be replaced every 6 -12 months, even if kept out of the light. Using a faded color standard will cause you to under-chlorinate and expose you to the liability of under-chlorination. The current technology offers a digital tester, also using the DPD test.

There are three common DPD reagents:

  • DPD #1 is for free residual testing
  • DPD #3 is for combined residual testing
  • DPD #4 is for total residual testing (DPD #1 + DPD #3 = DPD #4)

There are four forms of DPD reagents available on the market:

  • DPD Tablets, plastic/foil wrapped:
    The DPD tablets have a 3 year shelf life guarantee. If a DPD tablet is bad it will either change color or crumble, to indicate that it is bad. The plastic/foil wrapper can be opened with your fingers for ease of use.
  • DPD Powder Pillows:
    The powder pillow has no shelf life guarantee and it is not possible to know if the powder has gone bad. The plastic pillow will pass moisture which destroys the DPD powder. The pillow must be cut open with a cutting instrument. The pillows can not be opened with your fingers.
  • DPD Liquid, in a dropper bottle:
    The liquid has no shelf life guarantee and it is not possible to know if the liquid has gone bad. Also, the size of the drops can vary and affect the accuracy of the test.
  • DPD Powder Pop Dispenser:
    The powder pop dispenser delivers one dose of reagent directly into the tester. It typically contains 500 doses. The dispenser should be used completely within one year. It is not possible to know if the powder has gone bad.

If the reagent that you are using for your chlorine residual test is no good, then your chlorine residual test is no good. Make sure that you are using fresh reagents, and your color standard is accurate, to ensure that your tests are valid.

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