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A reagent is a substance that creates a chemical reaction when it is added to another substance. Reagents are used when testing pool water chemistry. For example, phenol red is added to a sample of water to determine the water’s pH.

Reagents typically come with a test kit. They can help measure a variety of situations, from cyanuric acid to water hardness. Reagents should be stored according to the manufacturer’s instructions. Replacement supplies are sold separately from the kits.

When a reagent is added to the pool’s water, it may change the color of the water, depending on the test being done. Next the color of the new solution is compared to a color chart that comes with the kit. In some applications, a litmus strip is dipped into the solution.

Photometers are available that can digitally output the results of a particular water chemistry test. These devices are merely digitally comparing the actual color with a color chart, so reagents — along with vials and/or litmus papers — must still be used.

Regardless of the method, the test is only as good as the steps taken using the reagent. Follow the manufacturer’s instructions carefully, counting the number of drops added and waiting the prescribed amount of time, in order to get the most accurate results.