What is a common method for measuring chlorine residual in water?

• A. Ferrous iron titration
• B. DPD
• C. Potassium iodide test
• D. Chlorine colorimetric test

Answer: (B)

Measuring chlorine residual in water is best done with a colorimetric method that directly responds to chlorine concentration. The DPD method uses N,N-diethyl-p-phenylenediamine; when chlorine is present, it oxidizes the DPD reagent to form a pink color, and the shade is proportional to how much chlorine is in the sample. Reading that color with a simple color comparator or a small spectrophotometer gives a quick, reliable measure of chlorine levels. This approach is favored because it’s specific to chlorine, works for both free and total chlorine with straightforward reagents, and is easy to perform in the field. Other methods either measure broader oxidizing power rather than chlorine specifically or can be affected by interferences, making them less practical for routine chlorine residual determination.