Which method is used to test for residual chlorine?

The DPD (N,N-diethyl-p-phenylenediamine) method is widely used for testing residual chlorine in water. This method is popular due to its sensitivity and accuracy in detecting chlorine levels, which is crucial for ensuring water quality and safety in treatment processes. In the DPD method, when chlorine is present in the water sample, it reacts with the DPD reagent to form a pink color. The intensity of this color correlates directly with the concentration of chlorine in the sample, allowing for quantitative analysis using either a colorimetric comparator or an automated spectrophotometer.

Other methods, like phenolphthalein and litmus paper, are more suited to measuring pH levels rather than specific chlorine concentrations. Colorimetric analysis is a broader category that can include several different reagents and procedures, but in the context of testing specifically for residual chlorine, the DPD method stands out due to its established reliability and ease of use. This makes DPD a preferred choice for many water treatment facilities.