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).
Chlorine vs. Nitrogen
Let's look at the graph above. When you first add chlorine to water, it immediately begins to oxidize metals like iron and manganese, which reduce chlorine. This initial reaction wipes out a certain portion of chlorine, which is why nothing shows up on the graph until point (A). As more chlorine is added to water, it reacts on contact with other contaminants—not just germs, but non-living organics and nitrogen compounds too—which create byproducts. Organics are carbon-based, and get oxidized by chlorine, further reducing it. But nitrogen? Nitrogen is not oxidized so easily.
Ammonia (NH3) and nitrogen-based contaminants like urea get oxidized, and become variations of chloramines when combined with chlorine. This will be explained more in depth in a moment...but know that chloramines actually carry some disinfection potential, and therefore are measured with total chlorine...initially.
So what happens to ammonia (NH3) when met with hypochlorous acid (HOCl)? Well, chlorine starts replacing hydrogens. Let's start with ammonia.
The chemical reaction that creates Monochloramine (NH2Cl) looks like this:
2NH3 + 2HOCl → 2NH2Cl + 2H2O
Ammonia + Hypochlorous Acid yields Monochloramine + Water
Notice that one of the three (3) Hydrogens in the ammonia was replaced by a Chloride (Cl).
Further chlorination of monochloramine creates Dichloramine (NHCl2):
2NH2Cl + 2HOCl → 2NHCl2 + 2H2O
Monochloramine + Hypochlorous Acid yields Dichloramine + Water
Here again, one more Hydrogen has been replaced by a Chloride (Cl).
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
Finally all Hydrogens have been replaced by chlorides to create Nitrogen Trichloride.
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.
FAC residual after breakpoint chlorination
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.
Free available chlorine (FAC) is needed as a residual sanitizer in the water. Combined chlorine (CC) is the chlorine that combined with ammonia and other nitrogen compounds (including chloramines and other DBPs). Combined chlorine is the most accurate measurement of disinfectant byproducts we can test for. Total available chlorine (TAC) = FAC + CC. We measure all types of chlorine in parts-per-million (ppm).
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 will reflect that.
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 combined chlorine, it takes a surge of chlorine, called hyperchlorination (or shocking) to overcome the load. The conventional wisdom in the pool business is a shock of 10x your combined chlorine level in additional free chlorine. But according to renown chemist Richard Falk, the 10x figure is not accurate. Here is a direct quote from Richard on the PoolGenius Network forum:
"The molar ratio of chlorine to ammonia is 1.5:1 or 3:2, but since ammonia is measured in ppm N units while chlorine is measured in ppm Cl2 units, with the factor of 5.06 difference this is a ppm ratio of 7.6 to 1. Because forming dichloramine requires 2 moles of chlorine for 1 mole of ammonia and because of side reactions that can occur, the actual chlorine to ammonia ppm ratio is around 8-10x which is where the 10x rule came from. However, this is wrong since CC is in ppm Cl2 units (so no factor of 5.06) and monochloramine already has 1 of the 1.5 chlorine attached to it already. To oxidize monochloramine, it takes from 0.5 to 1.0 times the CC level. Even if the CC were urea, it takes 2-3 times the CC level, not 10x. Of course, the higher the FC level the faster reactions occur, but there is no magic 10x amount." - Richard Falk
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.
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. So think about how the demand itself got there. High combined chlorine is generally because of ammonia being introduced to the pool. Find out what chemicals are being used in and around the pool. Think of pool deck cleaners (many of them are ammonia-based), and algaecides (many of them are also ammonia-based).
We are in favor of a minimalist approach. Why throw more chlorine at the problem, without making an effort to discover the root cause of 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. Identify the sources of ammonia, and prevent them from getting in the water. We also recommend supplementing chlorine with enzymes to remove non-living organics and oils.
Thanks for taking the time to read this long, in depth article. Want to learn more about it? Just ask us.