An operator uses which reagent to test for residual chlorine?

The correct choice for testing residual chlorine is DPD, which stands for N,N-diethyl-p-phenylenediamine. DPD is a colorimetric reagent that reacts with free chlorine in water, forming a pink color proportional to the concentration of chlorine present. This property makes DPD ideal for monitoring chlorine levels in water treatment processes, ensuring that sufficient residual chlorine is maintained for disinfection purposes.

The effectiveness of DPD in detecting chlorine lies in its specificity for the oxidizing properties of free chlorine, making it a standard method used in many water testing protocols. As water quality and safety are critical, the use of DPD is widely adopted in laboratories and field tests to provide accurate and reliable results for residual chlorine concentrations.