In this article we’ll look at breakpoint chlorination.
Water dissolves chlorine, forming free chlorine in the form of hypochlorous acid and hypochlorite ion. Chlorine donors, such as calcium hypochlorite or sodium hypochlorite, are used to add chlorine to the water. As a potent oxidizing agent, chlorine is capable of breaking down pollutants in the water. Hypochlorous acid is the primary disinfecting component of chlorine disinfectants, and hypochlorite ion serves as the oxidizing portion of chlorine.
When ammonia is present in the water, it reacts with part of the free chlorine to form monochloramine. This is the first component of combined chlorine. This is acceptable and will not cause any issues for swimmers, as it is not an irritant. Monochloramine reacts with additional free chlorine to form dichloramine. This can cause mild irritation to the eyes and nose of swimmers. Proper pool maintenance is critical, as dichloramine can continue to react and produce trichloramine, which is more problematic.
To prevent this, it is crucial to maintain a higher level of free chlorine in the water (relative to combined chlorine), which is achieved through continual dosing and adequate fresh water dilution, along with a good pre-swim showering regime. The goal is to achieve breakpoint chlorination, where the level of combined chlorine is less than half that of free chlorine. When chlorine is added, more of it will remain as free chlorine since there is no dichloramine left to react with. Zero combined chlorine is even better. Proper pool maintenance can help ensure that dichloramine decomposes, avoiding trichloramine formation.