This part of 7393 specifies a method for the determination of free chlorine and total chlorine in water, readily applicable to field testing; it is based on measurement of the colour with a scale of standards which is regularly calibrated. Sea water and waters containing bromides and iodides comprise a group for which special procedures are required. This method is applicable to concentrations, in terms of chlorine (Cl2), from 0,000 4 to 0,07 mmol/l (0,03 to 5 mg/l) total chlorine. For higher concentrations the test portion must be diluted. If the speed of operation and the compactness of the equipment are not overriding requirements, spectrometic measurement is described as an alternative procedure. In annex A a procedure is presented 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. Interferences are noted in clauses 7 and 9.