When DPD reacts with chlorine, what color indicates a positive response?

When DPD (N,N-diethyl-p-phenylenediamine) is used in chlorine testing, it reacts with free chlorine in the water sample. A positive response indicative of the presence of chlorine is shown by a distinct red color. This reaction occurs as DPD forms a colored complex with free chlorine, and the intensity of the red color can correlate to the concentration of chlorine in the sample.

In the context of water quality testing, recognizing this color change is essential for operators to ensure that chlorine levels are within safe and effective ranges for disinfection purposes. Understanding this response is critical for maintaining water safety and compliance with regulations, as it directly impacts public health.

The other options do not indicate a positive reaction for chlorine presence when using DPD. Blue, yellow, and green would not be associated with the DPD-chlorine reaction, thus confirming that red is the only correct answer when testing with DPD.