For determination of free chlorine, the DPD powder reagent is added to a sample containing chlorine. The reaction between chlorine and the reagent causes a pink colored complex to form; the greater the concentration, the deeper the color. A Checker Disc 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 3.5 mg/L (ppm) Cl₂.