Free chlorine reacts with N,N-diethyl-1,4-phenylene diamine (DPD) to form a red-violet dye. The pH value is determined with phenol red as indicator.

For test kits for the determination of chlorine alone see VISOCOLOR® Chlorine.

Chlorination of water in swimming pools with chlorine or chlorine compounds takes place within certain limits. One distinguishes between free chlorine and bound chlorine (chloroamines); the sum of both is called total chlorine. The content of free chlorine should be between 0.3 and 0.6 mg/L. Chlorination alters the pH value of the swimming pool water. The ideal pH value is pH 7.4. This prevents formation of malodorous pollutants and irritants to the mucous membrane and prevents damage to the water treatment system.