When using measuring equipment we often need to add a chemical substance to the water in order to be able to detect the substance we are interested in. As an example, the common reagent used for testing chlorine levels is called diethyl-p-phenylene diamine (DPD), which changes its colour when reacting with chlorine. It is important that the correct reagents are used in the procedure and that it is ensured the reagents have not expired as they have a limited shelf life.
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