What chemical is commonly used to detect chlorine residuals in water?

DPD, or N,N-diethyl-p-phenylenediamine, is commonly used as a reagent to detect chlorine residuals in water. When DPD is added to a water sample containing chlorine, a color change occurs, which can be quantitatively measured to determine the concentration of chlorine present. This method is widely used due to its reliability and sensitivity, making it a standard practice in water quality testing.

In this context, chlorine dioxide, hypochlorite, and ammonia are not used for the direct detection of chlorine residuals. Chlorine dioxide is an alternative disinfectant, hypochlorite is a compound used in disinfection processes, and ammonia can affect chlorine residuals but is not a reagent for measuring them. Therefore, DPD stands out as the correct choice for detecting chlorine in water samples.