Texas Commission on Environmental Quality (TCEQ). Class C Water License Exam

What color does the sample turn when DPD is added to water containing chlorine?

1. Blue

2. Yellow

3. Green

4. Red

The correct answer is: Red

When DPD (N,N-diethyl-p-phenylenediamine) is added to a water sample containing chlorine, the resulting color change indicates the presence of chlorine. Specifically, the chlorine reacts with DPD, producing a pink or red color. This color change is directly proportional to the concentration of chlorine in the water, which is essential for water quality testing. The presence of red indicates that chlorine is present and can be quantified by comparing the intensity of the color to a standard chart. The formation of the pink or red color distinguishes it from other possible colors that may occur with various reagents or tests. Understanding this reaction and the subsequent visual results is crucial for monitoring chlorine levels in water systems, ensuring safe drinking water standards are maintained.