scope:

This document specifies a method for the determination of free chlorine and total chlorine in water, readily applicable to lab‑ and field‑testing; it is based on measurement of the absorption the red DPD colour complex in a photometer or the colour intensity by visual comparison of the colour with a scale of standards that is regularly calibrated. Seawater and waters containing bromides and iodides comprise a group for which special procedures are required. This method is in practice applicable to concentrations, in terms of chlorine (Cl2), from e.g. 0,000 4 mmol/l to 0,07 mmol/l (e.g. 0,03 mg/l to 5 mg/l) total chlorine. For higher concentrations, the test portion must be diluted. Commonly, the method is applied as a field method with mobile photometers and commercially available ready‑for‑use reagents (liquid reagents, powders and pellets). It is essential that those reagents fully comply with minimum requirements and contain the essential reagents and a buffer system suitable to adjust the measurement solution to a pH range of 6,2 to 6,5. If there is doubt that water samples have uncommon pH values and/or buffer capacities the user has to check and if necessary to adjust the sample pH to the required range. The pH of the sample shall be within the range of pH 4 and 8. Adjust, if necessary, with sodium hydroxide solution or sulfuric acid before the test. A procedure for the differentiation of combined chlorine of the monochloramine type, combined chlorine of the dichloramine type and combined chlorine in the form of nitrogen trichloride is presented in Annex A. In Annex C, a procedure is presented for the determination of free and total chlorine in drinking and other low polluted waters, for disposable planar reagent‑filled cuvettes using a mesofluidic channel pump/colorimeter.