An operator uses which reagent to test for residual chlorine?

An operator uses which reagent to test for residual chlorine?

• A. Cresol Red
• B. DPD
• C. Methyl Orange
• D. Sulfuric Acid

Answer: (B)

Testing residual chlorine relies on a colorimetric reagent that reacts specifically with chlorine to produce a color you can compare to a chart. The reagent used is DPD, which stands for N,N-diethyl-p-phenylenediamine. When DPD is added to water containing chlorine, it forms a pink to red color whose intensity is proportional to the amount of chlorine present. This simple color change lets you determine the chlorine residual quickly in the field or lab, using a comparator or a small photometer. Cresol Red and Methyl Orange are pH indicators; they change color based on acidity or basicity, not chlorine concentration. Sulfuric acid is a strong acid used to adjust sample conditions in some tests, but it’s not the reagent that measures residual chlorine.

Testing residual chlorine relies on a colorimetric reagent that reacts specifically with chlorine to produce a color you can compare to a chart. The reagent used is DPD, which stands for N,N-diethyl-p-phenylenediamine. When DPD is added to water containing chlorine, it forms a pink to red color whose intensity is proportional to the amount of chlorine present. This simple color change lets you determine the chlorine residual quickly in the field or lab, using a comparator or a small photometer.

Cresol Red and Methyl Orange are pH indicators; they change color based on acidity or basicity, not chlorine concentration. Sulfuric acid is a strong acid used to adjust sample conditions in some tests, but it’s not the reagent that measures residual chlorine.