Breakpoint Chlorination Explained
Breakpoint chlorination is a chemical point where enough chlorine has been added to the pool to eliminate the bonds that are combining chlorine to contaminants. So that’s the definition, but let’s back up a bit and provide some context
When swimmers complain that they can smell the “chlorine” in the pool, what they are actually referring to is the smell of combined chlorine or chloramines. Chloramines are formed when chlorine combines with contaminants in the pool water such as sweat, debris, algae, bacteria and skin cells. Chloramines still have disinfectant properties but perform far less well than freely available (or uncombined chlorine). The smell is a sign that you need to to “shock the pool” in order to increase the free chlorine levels.
The actual process of getting rid of the choloramines involves reaching a chemical point called “breakpoint chlorination.” This occurs when enough chlorine is added as part of a pool shock treatment to eliminate the molecular bonds that are combining chlorine to the contaminants. This will initially have the effect of lowering the chlorine levels in the pool. More chlorine is added to reach the breakpoint which is when chlorine has reacted with all chemical agents in the water, and then past this point when available levels of chlorine start to rise once more.
Breakpoint chlorination graph
(adapted from a Factsheet by the New South Wales Department of Health in Australia)
This may sound like a complicated process but in practice all you need to do is keep adding chlorine until more than 85% of the total chlorine in the pool is available as free chlorine. Pool shocking is an all or nothing process. Not adding enough chlorine to reach breakpoint will result in more chloramines and a lower chlorine residual. There is no point continuing to shock the pool if you never achieve chlorine breakpoint. On the other hand, if too much chlorine is added, swimmers should wait until the chlorine residual drops to a safe level before getting in the water.
In working out how much chlorine is needed as part of the shock treatment the first step is to work out the level of the combined chlorine in the pool. Most tests measure total chlorine and free chlorine with combined chlorine being the difference between these two measures, Then around 10 times the amount of combined chlorine is needed for a shock treatment. So for example, if combined chlorine is 0.8 ppm, then 8ppm of chlorine is needed to achieve breakpoint.
Read more on pool chemicals.