Breakpoint chlorination is a key concept in pool chemistry. In May 2017, we published a two-part article about pool sanitizers. In part one we compared different chlorine types, and their pros and cons. This article dives deeper into the science of chlorination. We explore the difference between breakpoint chlorination and hyperchlorination (shocking).

What is breakpoint chlorination?

Breakpoint chlorination is the point where chlorine levels exceed the oxidant demand, and the water begins to build a residual of free available chlorine (FAC). Theoretically, exceeding the “breakpoint” prevents increased levels of disinfectant byproducts (like chloramines).

breakpoint chlorination, chlorination graph, pool shock, combined chlorine, free available chlorine

What happens when chlorine is added to a pool?

Let’s look at the graph above. When you first add chlorine to water, it immediately begins to oxidize metals like iron and manganese. This initial reaction wipes out a certain portion of chlorine, which is why nothing shows up on the graph until point (A). As you continue to add chlorine to water, it reacts on contact with other contaminants—not just germs, but non-living organics too—which creates byproducts. The ammonia and nitrogen-based contaminants (NH3) become variations of chloramines when met with chlorine. This will be explained more in depth in a moment…but know that chloramines actually carry some disinfection potential, therefore adding to the total chlorine level…initially.

The chemical reaction that creates Monochloramine (NH2Cl) looks like this:

2NH3 + 2HOCl 2NH2Cl + 2H2O

Ammonia + Hypochlorous Acid yields Monochloramine + Water

Further chlorination of monochloramine creates Dichloramine (NHCl2):

2NH2Cl + 2HOCl 2NHCl2 + 2H2O

Monochloramine + Hypochlorous Acid yields Dichloramine + Water

And of course, even further chlorination yields the most noxious of chloramines that off-gasses from pools, Nitrogen Trichloride, aka Trichloramine (NCl3):

NHCl2 + 3HOCl NCl3 + 3H2O

Dichloramine + Hypochlorous Acid yields Trichloramine + Water

All of these reactions depend on pH and temperature.

Chloramines are weak disinfectants

As noted before, chloramines are disinfectants–which is why they are referred to as disinfectant byproducts (DBPs). In fact, many water treatment plants add chloramines to their water as a secondary disinfectant. Albeit weak and slow, chloramines first contribute to the total chlorine levels because they help with disinfection. This, however, reaches a threshold where chlorine turns on chloramines, indicated at point (B). In other words, chlorine oxidizes all contaminants, which includes chloramines after point (B) on the graph. That’s why the total chlorine level drops with the addition of more free chlorine (the X axis on the graph).

The downward trend on the graph shows chlorine starting to “win the fight” against contaminants until it oxidizes all but the combined chlorine residual. This level of chlorine residual is shown on the graph at point (C). If chlorine cannot overcome the oxidant demand, your water’s chlorine demand rises, and the ORP drops. This would look like a more prolonged downward trend toward breakpoint, because breakpoint would be at a much higher dose of chlorine. When the chlorine can meet the oxidant demand, the water has reached breakpoint chlorination.

After breakpoint is achieved, a free chlorine residual builds

Only after the oxidant demand has been addressed can disinfection occur. Therefore, only after breakpoint chlorination has been exceeded can a residual of free chlorine build. Up until that point, chlorine has its hands full trying to oxidize its way to breakpoint.

Shameless product plug: Part of our mantra of more meaningful pool care is increasing chlorine efficiency and reducing chlorine demand. On the graph above, that would mean reaching breakpoint earlier, with less chlorine in the water. This can be achieved by using our CV-600 or CV-700 enzymes to help address the bather load (oxidant demand). The less non-living organic contamination in the water, the faster chlorine can reach breakpoint and build a residual of free available chlorine.

Another Orenda product to help improve chlorine efficiency is PR-10,000 phosphate remover. Removing phosphates reduces the dissociation of Hydrogen from hypochlorous acid (HOCl), meaning you have more of the strong chlorine. The video at the end of this article explains more about weakening chlorine.

Free available chlorine (FAC) is needed as a residual sanitizer in the water. Combined chlorine (CC) is the used-up chlorine that combined with ammonia and other oxidants prior to reaching breakpoint. Combined chlorine includes chloramines, as described earlier. Many pool operators refer to combined chlorine as chloramines. It is the most direct measurement of disinfectant byproducts we can test for. Total available chlorine (TAC) = FAC + CC. We measure all types of chlorine in parts-per-million.

Just remember, test kits cannot tell the difference between hypochlorous acid (HOCl), and its dissociated, weak form, hypochlorite ion (OCl-). So even though you may read a good amount of free available chlorine (FAC), if your pH is high or you have high phosphates, you may still have weak chlorine in your water. If so, your ORP reading will reflect it.

You can calculate any of the three with addition and subtraction.  Most test kits measure free and total chlorine, so you simply subtract:

Total Chlorine – Free Chlorine = Combined Chlorine

To eliminate the ammonia-based chloramine byproducts, it takes a 12:1 ratio of chlorine to chloramines to reach breakpoint. Does that sound efficient to you? We don’t think so.

What about shocking the pool?

Shocking the pool is hyperchlorination. When pool operators shock a pool, they add way more chlorine than normal in an attempt to wipe out the oxidant demand and get past breakpoint. Recommendations range from 10x the combined chlorine levels to 30 ppm. Pool operators usually shock pools for one of two reasons:

  1. there was a fecal incident, and shocking is required by health codes for disinfection, or
  2. the pool cannot reach breakpoint chlorination, and needs help oxidizing.

If your swimming pool struggles to reach—and exceed—breakpoint chlorination, the chlorine you have is not enough to do the job. The oxidant demand is greater than the chlorine available to handle it. The oxidant demand in these cases can be chloramines, non-living organics, or any combination of both.

Popular non-chlorine “shock” products are oxidizers…not sanitizers. That’s because shocking a pool to get past breakpoint is to oxidize contaminants…not to sanitize. Non-chlorine shocks (like monopersulfate) oxidize…not sanitize. Therefore, do not use them in the event of a fecal incident.

Yes, we have an opinion about hyperchlorination

If you’re shocking your pool frequently to reach breakpoint chlorination, ask yourself how you got there. Clearly the normal chlorine levels in your pool are not enough to meet the demand that keeps you going back through the same procedure. We have seen pools running 10 ppm chlorine, yet still cannot hit breakpoint. Cyanuric Acid stabilizer can dramatically affect a shock too. Remember the rule of 7.5% when it comes to CYA. That is, if you have 100 ppm CYA, you have effectively zero chlorine until you exceed 7.5 ppm. Then you go through the graph above.  As you can tell, many things weaken chlorine.

We are in favor of a minimalist approach. Why throw more chlorine at the problem? Chlorine is not designed to be a primary oxidizer! It is designed to be a sanitizer and disinfectant. If you are routinely hyperchlorinating your pool, we hope you will reconsider your practices. Applying the right chemistry for the right situations can minimize costs, maximize efficiency and improve the overall swimmer experience. If we do it right, breakpoint chlorination will be easy to reach, and you can have a safe residual of free available chlorine to keep the water safe.

Thanks for taking the time to read this long, in depth article. Want to learn more about it? Just ask us.