DPD No. 4R tablet contains reagents needed for one-step reading of total residual chlorine. Drop a single tablet in the test sample and compare the resulting color reaction against the permanent color standards in the comparator. Color standard values are 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0 and 3.0 ppm chlorine.