Because of its strong oxidizing properties, chlorine in aqueous solution is an excellent biocide used to treat potable waters, municipal wastes and swimming pools. When used to treat potable water, chlorine not only acts as a biocide but also helps alleviate the adverse effects of iron, manganese, ammonia and sulfide.  The Maximum Residual Disinfectant Level for chlorine is 4 mg/L in drinking water.

The Colorimetric Methods. CHEMetrics offers two methods for the determination of free and total chlorine. In the USEPA-approved DPD methodology free chlorine reacts with DPD to form a red product. When ammonia or amines are present, some of the chlorine may exist as "combined chlorine". Combined chlorine will not interfere with the free chlorine results, provided the readings are taken at 1 minute. To determine total chlorine (the sum of free and combined), use the A-2500 activator solution (potassium iodide) supplied in the kit. Results are expressed as ppm (mg/L) Cl₂.

The DDPD method is a derivative of the DPD method. Test kits based on this reagent are well suited for use where biocides and chromate corrosion inhibitors are used simultaneously. The DDPD compound reacts with chlorine to form a purple product. When ammonia or amines are present in the sample, some of the chlorine may exist as "combined chlorine". Combined chlorine will not interfere with the free chlorine result, provided the reading is taken at 1 minute. To determine total chlorine (the sum of free and combined), use the A-2500 activator solution (potassium iodide) which is supplied in the kit. Results are expressed as ppm (mg/L) Cl₂.