For determination of free chlorine, the sample is first adjusted to a pH near 6.3. When the DPD reagent is added to a sample containing chlorine, the reagent is immediately oxidized. The reaction between chlorine and the reagent causes a red colored complex to form; the greater the concentration, the deeper the color. A color comparator cube is included for an accurate, side-by-side, visual analysis to determine free chlorine concentration. These reagents are designed to be used with samples that have an expected free chlorine range of 0.0 to 2.5 mg/L (ppm) Cl₂.