Chlorine T&O is familiar to everyone, but there are important differences. All odorous substances are, by definition, volatile molecules, and that means elemental chlorine gas, or CI2 when we smell Free Available Chlorine or FAC. That includes the hypochlorous acid molecule, HOCI, which is part of the equilibrium. These have a rather “pure” smell when compared with the heavy, duII smell of monochloramine, which is a form of chlorine intentionally produced by many water works trying to limit the production of THMs. And since monochloramine is much weaker as a disinfectant than FAC, it is often used at much higher concentrations, making the T&O even worse. “It is produced by adding ammonia (NH3) to water that aJready has a residual of FAC, and if the wrong proportions are used or the pH is off, dichloramine may also be produced. Trichloramine or nitrogen trichloride exists only at very low pH, so it can be ignored. These chloramines can be recognized as the smell of a poorly managed swimming pool, which has a steady input of ammonia from the – perspiration and urine of swimmers. If the chlorine level does not keep pace with the ammonia influx, the pool area will stink of chloramines, which is much worse than the smell of ordinary free chlorine.

The same is true of drinking water, where chloramines may also be unintended. When raw water is first chlorinated, ammonia and several naturally-occurring amines react first; if there isn’t enough chlorine to oxidize them completely, disinfection suffers. As more chlorine is added, the oxidation of the nitrogen in the amines progresses from mono- to dichloro- to trichloramine, and finally to nitrogen gas, which bubbles away and is lost. The last reaction is known as the breakpoint reaction because that is the one that destroys the last of the chlorine demand and permits FAC to persist so that actual disinfection can begin. The chemical reactions for these chlorine species are all given below:

Free Available Chlorine

CI2 + H2O <=> HOCI + H+ + CI <=> OCI + 2 H+ + CI
chlorine hypochlorous
acid
hypochlorite
ion

Combined Chlorine

CI2 + NH3 <=> NH2CI (sometimes intentionally produced, sometimes not)
Chlorine ammonia monochloramine
+ CI2 <=> NCI3 (only at pH < 3)
Nitrogen
trichloride
+ CI2 <=> N2 + N2O + HCI
Nitrogen gas Nitrous
Oxide
(the “Breakpoint Reaction”)