For swimming pool owners and service professionals, understanding the role of Cyanuric Acid (CYA) goes beyond its basic function of protecting chlorine from sunlight degradation. This article explores the role of cyanurate alkalinity as a pH buffering system and how it influences the overall water balance.
Covered in this article:
- What is Cyanuric Acid (CYA)?
- Cyanuric Acid and Total Alkalinity
- Cyanurate alkalinity as a pH buffering system
- How to calculate cyanurate alkalinity
- Cyanurate alkalinity subtracted from TA
- Conclusion
What is Cyanuric Acid (CYA)?
We have many other articles about CYA in our blog and help center, so we will only do a quick summary here.
Cyanuric Acid (CYA) is used in swimming pools to bind with chlorine to protect it from sunlight UV degradation (called photochemical decomposition or photolysis).1,2 Chlorine bound to CYA is called stabilized chlorine, or more technically, chlorinated isocyanurates (or chloroisocyanurates). When bound to chlorine, CYA moderates chlorine's strength and speed. We discuss this more in another article and in several episodes of our Rule Your Pool Podcast.
Without CYA in the water, chlorine degrades quickly. Depending on factors like water depth and temperature, unprotected chlorine's half-life is only about 20-45 minutes (depending on the source cited), meaning about 75% of chlorine will be gone in under two hours.3,4 After three hours, all the chlorine may be gone. Good luck holding chlorine for an entire day in an outdoor pool...much less, a week.
Below is a helpful graphic from the late Bob Lowry. Chlorine either gets used up killing and oxidizing contaminants, or it gets broken down by UV light (sunlight). The reaction is:
2HOCl + UV → 2HCl + O2
2 Hypochlorous acid + UV → Hydrochloric acid + Oxygen
Source: Robert W. Lowry5. Note, the title of his graphic is "Liquid chlorine does NOT raise pH". We explain this concept and clarify it more in this article. Liquid chlorine only temporarily raises pH, because once it breaks down and does its job, the pH is brought back down by the release of hydrochloric acid (HCl).
Chlorinated isocyanurates are not nearly as susceptible to UV degradation as unstabilized free chlorine (particularly HOCl). Sources differ on how much longer stabilized chlorine can last. Some say eight times (8x) longer. We have not found studies that establish exactly how much longer stabilized chlorine lasts in sunlight because there are too many variables. We do know that most of free chlorine is bound to CYA, even at lower levels.
While we do not know exactly how much longer chlorine lasts, there are plenty of studies that show how CYA slows chlorine's killing speed.6 This is only one of many variables that determine how long free chlorine can remain in water.
We have also seen studies on how CYA impacts other aspects of pool chemistry.1 We could go on and on about how many ways CYA impacts pool chemistry, but this article focuses on CYA's impact on alkalinity, which in turn impacts the LSI.
Cyanuric Acid and Total Alkalinity
Alkalinity is a measure of the water's ability to resist changes in pH. Various acids and conjugate base pairs can both take and give away a Hydrogen ion (H+), which means these conjugate bases can neutralize acids. An acid and conjugate base pair that can do this is called a pH buffering system.
The dominant alkalinity species in swimming pools is the carbonic acid/bicarbonate buffering system, commonly called carbonate alkalinity. See the chart below:
Carbonic acid (dissolved carbon dioxide, CO2) is the acid, and bicarbonate ion is its conjugate base. When acid is added to water, most of the Hydrogen ions that acid introduces are then neutralized by the bicarbonate in the water. Adding Hydrogen to bicarbonate converts it into carbonic acid, which re-carbonates the water with dissolved CO2, resetting the physics explained by Henry's Law.
Bicarbonate is not the only buffering system in swimming pools. Stabilized outdoor swimming pools (with CYA) will also have cyanurate alkalinity. Below is the chart again, this time with Cyanurate alkalinity overlayed:
This chart shows the overlay of cyanurate alkalinity with carbonate alkalinity.
When cyanuric acid is added to pool water, it forms cyanurate ions. These ions contribute to the Total Alkalinity of the water. Cyanuric acid (H3C3N3O3, or H3Cy) is the acid, and cyanurate ion (H2C3N3O3-, or H2Cy-) is its conjugate base.
As the pH increases, around 11.2, another Hydrogen breaks away from the Cyanurate ion, creating Hydrogen Cyanurate (HCy2-).7 This dissociation is well above the normal pH range for a swimming pool, so may never occur in most pools. The important pH value for cyanurate alkalinity is its pKa value of 6.88 pH.8
Cyanurate Alkalinity as a pH Buffering System
Below is a quote from John Wojtowicz's paper entitled "Swimming Pool Water Buffer Chemistry." If you want to see how deep the rabbit hole goes, we encourage you to read it. Before quoting it, let's give some context.
When we publish numbers and specific values (like pKa values), they are impacted by ionic strengths and other relevant parameters like TA, CYA, temperature, etc.. So pay attention to the values mentioned: 7.5 pH, 1000 ppm TDS and 80ºF. If you move those values enough, it can alter what he's saying here:
"On a molar basis, the cyanuric acid/cyanurate [buffering] system provides more effective swimming pool water buffering at pH 7.5, 80ºF, and 1000 ppm TDS because its pH of maximum buffering is closer to pool pH.
However, on a ppm basis, the buffer intensity of the carbonic acid/bicarbonate and cyanuric acid/cyanurate systems are roughly comparable over the recommended pH range (7.2 - 7.8) and greater than that of borate at a pH < 7.8.
At pH 7.8, the buffering of the three systems are roughly comparable on a ppm basis."
- John Wojtowicz 9
Like the carbonic acid/bicarbonate buffering system, the cyanuric acid/cyanurate buffering system buffers better against a reduction in pH. This is because its pKa value (6.88 pH) is lower than swimming pool pH levels. If the pKa were above swimming pool pH levels, like borate, it would buffer better against a rise in pH.
Because its pKa is closer to operational pool pH levels than that of carbonate alkalinity, cyanurate alkalinity is considered a stronger buffer against pH reduction by molar weight...but by ppm, there is not nearly as much cyanurate in water as there is bicarbonate. If (for some reason) cyanurate and bicarbonates were in the water in equimolar amounts, technically CYA would be a stronger buffer. But they are not equimolar. There is much more bicarbonate, and therefore carbonate alkalinity is the dominant buffering system in the water.
But how do we know how much cyanurate alkalinity we have in our water?
How to calculate Cyanurate Alkalinity
We need to know two water chemistry parameters to calculate how much cyanurate alkalinity is present in water. They are:
- Cyanuric acid (ppm)
- pH
The rule of thumb is multiplying the cyanuric acid level by one-third, and that's the cyanurate alkalinity level.1,9 This must be subtracted from Total Alkalinity (TA) to "correct" the alkalinity into carbonate alkalinity for an LSI calculation.
Rule of thumb (estimate): 1/3 of CYA ppm
0.33 x (CYA ppm) = cyanurate alkalinity ppm
The exact math is very specific, but thankfully, there are simplified charts that help us do this by hand.10
Simplified equation: CYA correction factor (see chart) multiplied by CYA ppm
(Cyanurate correction factor @ current pH) x (CYA ppm) = cyanurate alkalinity ppm
Example at 7.8 pH, 90 ppm CYA:
(0.35) x (90 ppm) = 31.5 ppm cyanurate alkalinity
Cyanurate alkalinity subtracted from TA
You might notice that we can calculate our cyanurate alkalinity ppm without knowing the total alkalinity (TA). But if we have CYA in our water and want to calculate our carbonate alkalinity, we must subtract our cyanurate ppm from our TA. Assuming no borates are used in the water, it looks like this:
Total Alkalinity (TA) - Cyanurate Alkalinity = Carbonate Alkalinity (aka corrected alkalinity)
The carbonate alkalinity level is required for calculating the LSI and pH ceiling values.11
Conclusion
The carbonic acid/bicarbonate buffering system is not the only pH buffering system in swimming pools. Outdoor pools that use cyanuric acid (CYA) have additional buffering capacity against a reduction in pH, called cyanurate alkalinity.
By molar weight, cyanurate is a stronger buffer against acid than bicarbonate because its pKa value is closer to normal pool chemistry. But on a ppm basis, because there is so much more bicarbonate in the water, carbonate alkalinity is the dominant pH buffering system in swimming pools.
While the Orenda Calculator™ does all of these calculations and corrections for you automatically, it helps if pool owners and operators understand that the LSI and pH ceiling depend on the carbonate alkalinity level, which can be calculated by subtracting cyanurate alkalinity from total alkalinity.
The more CYA you have in your water, the lower your carbonate alkalinity (compared to TA), and therefore the lower the LSI and pH ceiling. Surprisingly, however, this additional CYA provides more buffering against acids. Taking all of this into consideration, we teach the importance of keeping CYA levels to a minimum in our Fourth Pillar.
1 Wojtowicz, John. (2004). Effect of Cyanuric Acid on Swimming Pool Maintenance. Journal of the Swimming Pool and Spa Industry. Vol. 5 (1), pp. 15-19.
2 US Environmental Protection Agency. (1992). Chlorinated Isocyanurates. R.E.D. Facts, US EPA Office of Prevention, Pesticides and Toxic Substances. EPA-738-F-92-010.
3 Lowry, Robert W. (2016). IPSSA Basic Training Manual (2016 Revised Edition), pg. 108.
4 Technically, the half-life of Hypochlorous acid (HOCl) is about one hour (at a pH of 5.0), while the half-life of Hypochlorite ion (OCl-) is only about 12 minutes (at a pH of 8.0). When no CYA is present, the pH strongly influences the half-life of chlorine. With CYA, however, the percentages of both HOCl and OCl- are much lower than in a non-stabilized pool. These numbers come from:
Nowell, Lisa N., Hoigné, Jürg. (1992). Photolysis of aqueous chlorine at sunlight and ultraviolet wavelengths––I. Degredation rates. Water Research. Vol. 26 (5), pp. 5993-598.
5 Lowry, Robert W. (2018). Pool Chemistry for Residential Pools. Pool Chemistry Training Institute (PCTI). Ch. 5, pg. 46.
6 Pulsar (Retrieved 2024). The Effect of Cyanuric Acid on Disinfection. Pulsar Technical Bulletin.
7 There appears to be no agreed-upon naming convention for HCy2-. There's Cyanuric acid, then cyanurate ion, then...what? We read every source we could find, and none of them had a name for it, because the focus was more on cyanurate ion. So until we are otherwise corrected, we decided to call it Hydrogen Cyanurate. This is following the naming convention of similar substances. At the very least, it gives us something to call it. Just like the "pH ceiling", we may have just coined the name. Who knows. If YOU know the proper name for HCy2-, please let us know, and we'll update this immediately.
8 National Center for Biotechnology Information (2024). PubChem Compound Summary for CID 7956, Cyanuric acid.
9 Wojtowicz, John. (2001). Swimming Pool Water Buffer Chemistry. Journal of the Swimming Pool and Spa Industry. Vol. 3 (2), pp. 34-41.
10 The Orenda Calculator™ does not use simplified numbers for this calculation. It uses precise numbers that adjust in real-time as you change dials on the calculator. But for the purposes of this article, following along with the chart, or simply using the 1/3 rule-of-thumb is sufficient to get the concept across. The point is we need to be aware that approximately 1/3 of our CYA ppm will be in the form of cyanurate alkalinity, which must be subtracted from TA.
11 The Orenda Calculator™ asks you to input the total alkalinity because that is what test kits measure. It then displays the carbonate alkalinity level in real-time as you adjust other factors like CYA and pH. We decided to display carbonate alkalinity because we had so many customer questions about which alkalinity was supposed to be entered in the calculator. So yes, we do factor these variables into the calculator automatically. Just input your TA, CYA, and pH.