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What is a DPD Test for swimming pools?

A DPD test is a water test designed to test oxidizer levels such as free chlorine, total chlorine, chlorine dioxide, bromine and ozone.  DPD, also known as DPD-1 and known chemically as N,N Diethyl-1,4 Phenylenediamine Sulfate.  For a DPD test to be accurate the water pH level needs to be balanced, or the test results can become errored.

DPD is the chemical responsible for indicating the presence of an oxidizer in the pool water, by showing a range of colors and can be compared to a color chart or used with a comparator tube with a color range molded to it.  A slight pink hue is shown for the presence of light concentrations of an oxidizer.  A deeper pink is shown when medium levels of an oxidizer are present, and a deep red-orange color will appear when high concentrations are present.  The water sample may even become clear in extremely high levels of chlorine, as the water sample color will become bleached.

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For swimming pools, a DPD test can be performed with a test kit, test strips, and electronic testing devices.  Pool testers can also use DPD tablets and powder pillows, dropping the substance into a water sample and comparing the resulting color for the presence of oxidizer.  The DPD test can be used to test free chlorine and total chlorine, the result of which combined chlorine can be measured.  Bromine and ozone are also commonly tested if these levels are to be measured, such as bromine-based pools or pools with ozone generators installed.

A variety of the DPD test an be accomplished by titration.  Instead of using the DPD reagent resulting color as the determining factor to calculate the chlorine level in pool water, the titrant serves as the reducing agent in the process which converts that pink/red color to a colorless mixture.  Based on the amount of titrant, usually counted by the drops added, the chlorine level is estimated.

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