Free chlorine encompasses hypochlorous acid (HOCl) and its conjugate base, the hypochlorite ion (OCl^–), while combined chlorine (chloramines) includes the compounds formed when ammonia or organic nitrogen is present. Total chlorine is the sum of both free and combined chlorine compounds.

Monitoring water streams containing chlorine is important for reporting to regulatory bodies as well as determining suitability for use in other sectors. Total chlorine content of treated wastewater is analyzed to ensure minimal levels are present before discharge into the environment, otherwise marine life may be harmed. Additionally, the amount of free chlorine is important to measure in drinking water facilities to safeguard against contaminants as well as to take care that the central dosing system is working correctly.

In the method concerned, the sample is mixed with reagents in a fully automated procedure, resulting in a soluble product which can be measured photometrically at a wavelength of 505 nm. In the case of total chlorine, an additional iodide reagent is added, which is oxidized to iodine in the presence of chloramines. The iodine and free chlorine react with DPD (N,N-diethyl-p-phenylenediamine).