Quick Definition:

Fading endpoint is a situation occurring during testing of calcium hardness where the tested water sample turns purple instead of the expected blue color.

What is Fading Endpoint in swimming pool terms?

Fading endpoint is a phenomenon that can occur when testing for calcium hardness.  When testing using a clear comparator tube and after adding the necessary reagent, the water in the tube becomes purple instead of the expected blue color.  This occurs when traces of iron or copper are present in the reagent, which also means there are metal ions also present in the sample.  The metal ions react with the reagent and interfere with the color test result.

To correct for this, a new swimming pool water sample is taken, and a hardness reagent is added prior to adding the buffer and indicator reagents.  The fading endpoint should be avoided to ensure the pool water calcium hardness level is determined, so that balanced swimming pool water is maintained.


  • Source: “Pool & Spa Water Chemistry, A Testing and Treatment Guide, Waterproof Edition, 2005. Taylor”

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