Imagine finding out that something you have been taught in pool industry trade shows, classes and literature for half a century is incorrect. Yes, that is difficult to believe.
We have all seen it: the species distribution charts that shows hypochlorous acid (HOCl, the fast killing form of free chlorine) at about a 50:50 percent ratio with hypochlorite ion (OCl—, the slower form at a pH of 7.5. At a pH of 7.8 the HOCl is around 33 percent (see Chart 1). When the pH is 8.0, the HOCl content is about 22%, and at a pH of 8.2, it is 16%.
Chart 1 – HOCl/OCl— at 0 ppm CyA
The fact of the matter is, however, that this scenario is ONLY VALID IN UNSTABILIZED WATER.
Think about that… how many outdoor, chlorinated pools are maintained without cyanuric acid? Only a small minority.
As documented in a recent scientific paper written by Dr. Stan Pickens, a chemist and former Chairman of the APSP’s Recreational Water Quality Committee, we learn that the effect of CyA and pH on chlorine efficacy (algae and bacteria killing power, derived almost completely from its HOCl form) has not been correctly understood and taught by our industry.
So, what are the correct HOCl/OCl— equilibria for STABILIZED pools? See Chart 2 below. In stabilized pools, suddenly the major determining factor on chlorine killing power is no longer the pH, but rather cyanuric acid (CyA). Here, the pH actually plays only a very minor role in the chlorine activity.
To demonstrate, let’s look at a few examples. Using a chlorine level of 2 ppm in pool water with a pH of 7.5, we expect an HOCl level of 1 ppm, or 50% of the 2 ppm total (Chart 1). But with 25 ppm of cyanuric acid (CyA) added, the HOCl content is only 2.1%! That means only 0.042 ppm of HOCl! The rest of the chlorine is present as OCl— (2.4%) and stabilized chlorine, CyA·Cl, (95.5%).
Chart 2 – HOCl/OCl—/Cl·CyA at 25 ppm CyA and 2 ppm of total chlorine
In that same scenario, but with 50 ppm of CyA, the HOCl concentration is down to 1%, and at 100 ppm of CyA, the HOCl is just 0.5%. (Note: at higher chlorine levels, the percentage of HOCl increases somewhat. For example, at 4 ppm of chlorine, the percentage of HOCl is 2.4% instead of 2.1% with 2 ppm of chlorine at pH of 7.5 and 25 ppm of CyA. But the vast majority of the chlorine is present as stabilized chlorine, or chlorine attached to the cyanuric acid molecule.)
Yes, those HOCl percentages seem very low. But millions of pools have been and are being maintained with CyA levels from 25 ppm to 100 ppm without unsafe pool water health issues. Therefore, since 2%, 1%, and even 0.5% HOCl has kept millions of pools safe from harmful bacteria and algae, then how in the world can 22% HOCl (a pH of 8.0 with zero CyA) be considered too low? Or, how do we know that 22% isn’t too high when no CYA is present? That science and aspect has yet to be thoroughly investigated or researched.
Let’s realize that even at a pH of 8.4, where the HOCl content in unstabilized water is about 10%, there is still 10 times more HOCl than when pool water contains 50 ppm of CyA (1% HOCl) and at a pH of 7.5.
What many industry people may not understand is that when pool water does not contain any CyA, a 1 to 4 ppm of chlorine (as the EPA requires) generally has far more killing power (chlorine efficacy) than is needed to eliminate harmful bacteria and algae in typical pool water conditions. And that is still true even when the pH is as high as 8.4.
Others may not understand that the distribution of HOCl, OCl—, and CyA·Cl is very dynamic. That means that when HOCl is “used” in oxidation processes, OCl— and CyA·Cl are converted to HOCl extremely fast to replace what was used up.
So, if cyanuric acid plays such a major role in chlorine speciation in pool water, what are the relative benefits of using it?
- First, of course, cyanuric acid stabilizes chlorine, or protects it from UV degradation. In direct sunlight, chlorine is lost relatively rapidly from pool water, but CyA protects the chlorine.
- CyA buffers pH at a higher pH range than bicarbonate. In other words, when acid is added to a non-stabilized pool the pH drops further than it would if the same amount of acid were to be added to a stabilized pool.
- CyA lowers the LSI – which, depending on the situation, may be a benefit or a disadvantage.
- The presence of CyA may slow the rate of formation of disinfection byproducts in swimming pool water.
- The use of CyA allows a higher level of chlorine without significantly higher HOCl, which may affect chemical toxicity or sensitivity, bleaching, or other related side effects.
And what are the disadvantages resulting from CyA?
- CyA increases the CT of chlorine. (CT is the contact/time required to affect an organism.)
- CyA lowers the LSI (a benefit or drawback, depending)
When CyA is used, the amount of free chlorine should be maintained higher than in unstabilized pools. Chlorine levels are often above 4 ppm in residential pools, and many service techs and pro-active pool owners super-chlorinate their pools on a regular basis.
CyA will continue to be widely used in the industry, so it is important to understand what impact it has on the chlorine and other chemistry in the pool. We always feel that we should not sell pool professionals short: lets teach the right chemistry and let them learn it and use it as they deem best for their conditions and situations.
Let us rehearse, then, some of the myths associated with these concepts.
Myth #1 is that the species distribution chart we have all been taught is applicable to stabilized water… it isn’t.
Myth #2 is that pH levels of 8 or 8.2 cause unsafe conditions because they make the chlorine ineffective. They don’t. Dr. Picken’s paper shows that when CyA is added to pools, whether the pH is at 7.8 or 8.4, there is very little change to the amount of HOCl. As the pH rises, the CyA, interestingly, buffers the HOCl concentration, and the percentage of HOCl is nearly the same. Going from 7.8 to 8.2, the HOCl level is only decreased by 10%. A higher pH is okay to maintain. Cyanuric acid also stores a large percentage of the chlorine in a stabilized form, thus minimizing the amount of HOCl even when the chlorine level is very high, which makes the water safer both in sanitizing power and for bather comfort.
Myth #3 is the concept that the presence of CyA makes water more aggressive to plaster. Although it is true that CyA decreases the percentage of the total alkalinity that is bicarbonate, thus lowering the LSI, this effect can and should be compensated for. But CyA also buffers pH drop when acid is added to a pool, thus buffering pH and protecting plaster.
Myth #4: some industry material claims that pH levels above 7.8 also results in cloudy water. That claim is technically untrue… it is high LSI, resulting in precipitation of calcium carbonate that these people are referring to. pH is indeed the primary player in LSI, but not the only factor, and even high pH can be compensated for with lower alkalinity, lower calcium, or higher TDS.
Myth #5: Higher pH causes eye irritation. A 1973 study by Rylander, et al. on eye irritation from pool water reported that a pH of up to 9.0 had no significant influence on the amount of eye irritation! The same study shows that reducing the pH from 8.0 to 7.0 resulted in a higher frequency of eye irritation. This clearly shows that a pH of above 7.8 does not cause eye irritation as has been taught. Incidentally, in some parts of the country tap water has a pH of as high as 9.0, and the EPA allows a pH of up to 8.5 for drinking water.
Of course, there are some conditions where a higher pH may not be best, such as pool water that contains a very high calcium level, when bleach or Cal hypo is being added, or where the tap water and/or pool water contains copper (copper is less soluble at higher pH), and especially when the LSI is above +0.5.
But when the calcium level is low, or when using Trichlor, bromine, PHMB, or ozone; maintaining a higher pH can be a more effective program. As mentioned, the effect of a higher pH raising the LSI can be offset somewhat by lowering the alkalinity level if needed, which in turn can also help in stabilizing the pH, since high alkalinity levels tends to promote higher pH above 8.2. (However, trichlor use generally means higher alkalinity is needed). Often, maintaining a pH around 7.8 to 8.0 can make the balancing of swimming pool water much easier.
It is now understood that the commonly accepted or allowable chlorine levels (1 to 4 ppm) in stabilized pools is too low. Interestingly, at a pH of 7.5 and 25 ppm of CyA, it requires about 11 ppm of chlorine to achieve the same amount of HOCl as having 1 ppm of free chlorine with zero CyA at a pH of 7.5. And even higher levels of chlorine are needed to achieve the same HOCl content when the CyA levels are above 25 ppm. But remember, most authorities agree that 1 ppm of chlorine with zero CyA has more killing power than is needed to kill most bacteria.
The above-mentioned science may be why the CDC, MAHC, and other state agencies are currently entertaining the possibility, and in some cases even suggesting and/or permitting higher amounts of free chlorine (currently up to 10 ppm) for swimming pools when CyA is present. The MAHC is continually reviewing this science, and in its annex, is openly calling upon the industry to contribute additional data that could result in making changes to chlorine and pH standards.
The EPA’s chlorine limit of 1 to 4 ppm does not seem to be based on pool water that contains CyA. Higher amounts of chlorine should be allowed in stabilized pools, which in many cases would better help to ensure safe, healthy, and enjoyable pool water.
Click on this link to access Dr. Pickens paper on this topic. Dr. Pickens provides consulting services regarding pool water chemistry.