At a pH value of 5-6 chlorine dioxide reacts with N,N-diethyl-1,4-phenylene diamine (DPD) to form a red-violet dye.

“Chlorine dioxide and other oxidizing reagents (indicated as chlorine)” according to DIN 38408-G 5, analogous to Standard Methods, 18 ed., 4500-ClO2 D.

Chlorine dioxide has a significantly higher oxidizing power than chlorine. Thus, is can still attack such bacteria, spores and viruses that chlorine cannot affect anymore. Additionally, as the effect of chlorine dioxide is independent of water pH value, it leads to biofilm-free systems. Using chlorine may lead to the formation of undesirable or toxic trihalomethanes (THMs), which are suspected of being carcinogenic, while formation of THMs is avoided when using chlorine dioxide. In contrast to chlorine, chlorine dioxide doesn’t react with ammonium and ammonium compounds to chloramines, which negatively affect water disinfection. Similarly, strong smelling chlorphenoles which form during chlorination of water are prevented when using chlorine dioxide. Hence, chlorine dioxide is used more and more as a universal disinfecting agent, replacing chlorine especially in drinking and swimming pool water processing.