Which reagent is used to determine chlorine residual in water?

The reagent used to determine chlorine residual in water is DPD, which stands for diethyl-p-phenylenediamine. This chemical is specifically designed to react with free chlorine in water samples, forming a colored compound that can be measured spectrophotometrically. The intensity of the color produced is proportional to the concentration of chlorine, allowing for accurate quantification of chlorine residuals.

In the context of water quality testing, accurately measuring chlorine levels is crucial, as it is a primary disinfectant used to ensure safe drinking water. The DPD method is favored because it provides a straightforward and reliable means to detect both free and total chlorine concentrations.

Other reagents mentioned in the options serve different purposes. Phenolphthalein is a pH indicator used primarily in acid-base titrations. Sodium thiosulfate is used in titrations for iodine, often in the context of measuring chlorine but does not directly measure chlorine residual itself. Bromothymol blue is another pH indicator, effective for determining the acidity or basicity of a solution but not suitable for measuring chlorine levels. Thus, DPD is the appropriate choice for its specific application in chlorine residual determination.