Which method is typically used for testing residual chlorine levels in water?

The DPD (N,N-diethyl-p-phenylenediamine) method is traditionally utilized for testing residual chlorine levels in water due to its reliability and accuracy. This method operates on a colorimetric principle, where residual chlorine reacts with DPD to produce a colored compound. The intensity of the color formed is directly proportional to the concentration of chlorine present in the water sample. By measuring the absorbance of the colored solution with a spectrophotometer, water quality professionals can determine the exact levels of residual chlorine.

While other methods like ion chromatography are effective for various water quality analyses, they are not primarily focused on measuring free chlorine. The O-Toluidine method also has applications in measuring chlorine but is less commonly used compared to the DPD method. Colorimetry, while a broad category that the DPD method falls into, does not specify the particular reagent or procedure most suited for residual chlorine detection, making it less precise for this particular application. Thus, the DPD method stands out as the preferred choice for accurately assessing residual chlorine levels in water.