The test ampoule is placed in the center of the comparator and compared with the eight color standards around it. Above 1 ppm. The flat comparator is usually used for these concentrations. The filled ampoule is placed between standards until a color match is found. Test results are determined by direct Nesslerization.
Concentration is directly proportional to the intensity of the yellow color formed.
A stabilizer solution is added if the test water contains sufficient concentrations of calcium and magnesium to cloud the reagent. The test has a shelf-life of 5 months. Refrigeration will double the shelf-life of the kits. Free chlorine reacts with DPD to form a red color. The intensity of the red color is directly proportional to the concentration of chlorine in the sample.