Chloramination, a process often used for disinfection of drinking water and wastewater, involves mixing chlorine and ammonia to form chloramines. The relative concentrations of both chlorine and ammonia are essential for optimum disinfection. Some reactions between ammonia and chlorine are still being investigated, but in general, if chlorine is added to ammonia at a relatively neutral pH (similar to most municipal applications) a predictable series of reactions will occur.
First, the chlorine will react with free ammonia to form monochloramine. As more chlorine is added, additional monochloramine is made until all the free ammonia is consumed. When all the free ammonia is gone, the chlorine will begin to react with the monochloramine to form dichloramine. This reaction will continue until all the monochloramine is gone. If more chlorine is added, the chlorine will react with the dichloramine to form trichloramine (also called nitrogen trichloride) until all dichloramine has been reacted. This is called breakpoint and no more reactions are possible between the ammonia and chlorine. If more chlorine is added, it will remain as free chlorine in solution.
Each chloramine has a different effect on disinfection and the key to chloramination is to produce the correct chloramine species. Monochloramine is an effective disinfectant, is relatively stable in solution, and is often the preferred disinfectant in drinking water. Dichloramine is also a strong disinfectant but often gives an off taste and odor in drinking water. Trichloramine is unstable in solution and decomposes quickly.