Quick Definition:

Phenol red, often referred to as PSP, is a liquid reagent used during the testing of pH in swimming pools.

What is Phenol Red for swimming pools?

Phenol red is a water soluble dye, named phenolsulfonphthalein, though abbreviated as PSP.  It is used as a color indicator in pH tests, showing the degree of acid or base in a solution.  Phenol red is added as a reagent to water-based solutions, changing color from yellow to red as the pH changes from 6.8-8.2.  At levels over pH 8.2, phenol red will turn bright pink.

In swimming pool management, the phenol red is used to test the pool pH level to help monitor and maintain the pool water at an acceptable pH range.  The phenol red reagent is usually part of a swimming pool test kit, used to test a variety of pool water balance tests.


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