At a pH value of 5 – 6 free chlorine reacts with N,N-diethyl-1,4-phenylene diamine (DPD) to form a red-violet dye. In the presence of iodide ions the content of total chlorine can be determined, too. (Reaction principle is analogous to APHA 4500CL G & DIN EN ISO 9963-1)

The addition of chlorine to swimming pools, water reservoirs and water mains is an approved procedure to rid the water of germs. With the correct dose harmful microorganisms are destroyed, many impurities removed and the growth of algae is prevented. However it is imperative that the chlorine content is regularly checked, since excessive chlorine not only impairs the smell and taste of the water but can be hazardous.

One distinguishes between free chlorine and bound chlorine (chloroamines); the sum of both is called total chlorine.