What is Cyanuric Acid?
Cyanuric Acid (CYA) is a very weak organic acid with the following chemical structure: [C(O)NH]3
It is a “triazine”, consisting of a ring of 3 Cyanic Acid [CONH] molecules. The hydrogen atoms shift positions, causing the molecule to exist in 2 different structures (aka “tautomers”) that readily interconvert:
This means that these 2 different molecules are constantly present in the pool in an endless cycle.
A Note About Equilibrium
In chemical reactions, things are not always static nor one way. Often some reactions are moving forward while others move backwards – like in the above example. When we look at the amount of compounds in a solution, we are typically saying “at any one time”. We can influence how much there is of any compound at a given moment by changing conditions in the water.
What does CYA do?
Cyanuric acid is used to “stabilize” chlorine in the presence of UV light (aka sunlight).
When UV rays hit free chlorine in the pool water, the chlorine becomes oxidized. This speeds up it’s breakdown (which we refer to as “chlorine burn”). Direct sunlight and water temperature are the two main forces that drive chlorine burn.
Cyanuric acid combines with chlorine to slow it’s oxidation and thus the rate it burns due to UV light.
Cyanuric acid is also a component of many stabilized chlorine compounds used in pool sanitation:
“Trichlor” (Trichlor-s-triazinetrione/trichloroisocyanuric acid)
“Trichlor” is most commonly used as slow dissolving chlorine tablets. It has the most available chlorine by weight and has the following chemical structure and formula: [C(O)NCl]3
As you can see, the “trichlor” comes from the 3 chlorine molecules attached on the cyanuric acid ring.
“Dichlor” (Dichloro-s-triazinetrione/dichloroisocyanuric acid)
“Dichlor” is a quick dissolving chlorine, typically used for “shock” treatments. It has the following chemical structure and formula: [C(O)NCl]2[C(O)NH]
The “dichlor” part of the name comes from the two chlorine atoms on the edge of the cyanuric acid ring. Instead of a third chlorine, there is a hydrogen molecule instead.
While chemically “monochlor” does exist, it is not a type of chlorine compound added to pools. Rather, it is one of the results of adding trichlor or dichlor to your water.
Concerns over levels of Cyanuric Acid
Cyanuric acid is considered helpful in slowing down chlorine burn. But it is also thought that too high a level of CYA can render chlorine ineffective at killing algae and bacteria. This is often referred to as “chlorine lock”.
Since cyanuric acid breaks down much more slowly than chlorine, the level of pool stabilizer tends to become very high. The fear is that the high ratio cyanuric acid to chlorine is how the chlorine becomes “trapped” or “locked”.
However, this isn’t supported by the science of how cyanuric acid and chlorine work in solution.
How CYA releases chlorine
Chlorinated CYA still releases chlorine molecules to keep the water sanitized. Otherwise, CYA containing compounds like dichlor and trichlor would be useless for sanitation. To understand how Cyanuric acid releases chlorine, we will look at the reaction of Trichlor tabs in water.
The release of chlorine by the addition of “Trichlor” can be looked at as a multistage process. Each reaction exists in an equilibrium, influenced primarily by the pH of the water.
The overall reaction is as follows
Trichlor ←→ Dichlor + Chlorine ←→ Monochlor + Chlorine ←→ Cyanuric Acid + Chlorine
At every stage of the reaction we see the release of chlorine.
Addition of any of the listed compounds will cause a shift in equilibrium. For example, the addition of Cyanuric Acid will cause the reaction to shift more to the left. However, it takes more and more chemical energy to shift the reaction through subsequent equilibrium points.
A big factor in determining how much chlorine is released is the pH.
How pH impacts CYA levels and chlorine
pH has a significant impact on the formation of hypochlorite from chlorinated cyanuric acid (aka stabilized chlorine). The higher the pH the more available hydroxides as a reactant, shifting the equilibrium to the right. Inversely, the lower the pH, the more the reaction shifts to the left forming more chlorinated cyanurates.
[C(O)NCl]3 + OH- ←→ [C(O)NCl]2[C(O)NH] + OCl-
[C(O)NCl]2[C(O)NH] + OH- ←→ [C(O)NH]2[C(O)NCl] + OCl-
[C(O)NH]2[C(O)NCl] + OH- ←→ [C(O)NH]3 + OCl-
Simply put, this means that a higher pH helps CYA release more chlorine in the form of Hypochlorite.
Deep Dive: pK values of Chlorinated Cyanurates
Various chlorinated cyanurates have different dissociation constants, influencing the concentration of reactants. Looking at pK values which are derived from dissociation constants make it relatively easy to compare how these compounds dissociate.
Where pK = pH, we can assume that 50% of the reaction exists in equilibrium. So half will be the chlorinated cyanurate (stabilized chlorine), while the remaining is hypochlorites.
For example, Trichlor has a very low pK value, indicating that it is a strong acid. This means that it fully dissociates in water and does not exist in solution.
Below are reference pK values for each of the above equilibrium reactions:
- [C(O)NCl]3 + OH– ←→ [C(O)NCl]2[C(O)NH] + OCl– (pK = <0.3)
- [C(O)NCl]2[C(O)NH] + OH– ←→ [C(O)NH]2[C(O)NCl] + OCl– (pK = 3.1)
- [C(O)NH]2[C(O)NCl] + OH– ←→ [C(O)NH]3 + OCl– (pK = 4.1)
Dichlor and Monochlor are considered relatively weak acids with some degree existing in solution. Based on pK values, we can determine that these weak acids exist in 50% concentration where pK = pH. This means Dichlor releases 50% hypochlorite ions at a pH of 3.1, while Monochlor releases 50% at a pH of 4.1. As pH rises, so does the amount of available hypochlorite.
CYA reduction by Hypochlorites
Hypochlorites are able to oxidize cyanuric acid, resulting in their decomposition and their reactants gassing out of solution.
The overall reaction is as follows
2[C(O)NH]3 + 9 OCl- ←→ 3N2 + 6CO2 + 9Cl- + 3H2O
Both carbon dioxide and nitrogen gas are released from the water resulting in a very linear reaction. In theory, the addition of the reactants would result in the formation of Cyanuric Acid. However there is not enough atmospheric pressure to lead to the reaction in any meaningful amount.
However, this does outline how cyanuric acid decomposes overtime. This is slow due to the required high ratio of hypochlorites to cyanuric acid. For example, 5 ppm of hypochlorites will only reduce Cyanuric Acid 1 ppm. This would be shocking with cyanuric acid free chlorine to achieve 10 ppm total chlorine at around a pH of 7.5.
CYA and Alkalinity Levels
Some argue that cyanuric acid should be taken into consideration when calculating alkalinity. This is done by subtracting the amount of cyanuric acid from total alkalinity readings to get a “true” value.
But this is wrong when we consider what alkalinity is and how it functions.
Alkalinity as a pH buffer is a measurement of weak acids that “trap” hydrogen ions. The trapping of this hydrogen keeps the pH stable.
In swimming pools Alkalinity measures carbonates, which form the weak acid carbonic acid. Carbonic acid then breaks down into CO2 that is gassed from the water. This reduces the Alkalinity over time as acids become trapped and are broken down.
Cyanuric acid is also a weak acid and acts as a buffer to some degree. But it doesn’t behave exactly like carbonic acid. Cyanuric acid is much more stable, and doesn’t have the same buffering capacity. It also has different solubility rules than carbonates.
Rather than trying to compare apples and oranges, you’re better off simply viewing each one separately.
Conclusions about Cyanuric Acid Levels, pH and Chlorine
Based on the above science, we can make a few conclusions about Cyanuric acid levels in pools.
Chlorine Lock is a Myth
Although cyanuric acid does indeed have an effect on the amount of chlorine available, chlorine lock is a myth. This is because each level of stabilized chlorine exists in equilibrium. Therefore, there is always some chlorine made available, and is much more dependent on the pH level.
High pH makes more chlorine available
Most pools are maintained in the pH range of 7.2 to 7.6 according to the Langelier Saturation Index. This range is favored due to the increased presence of hypochlorous acid which is well documented:
HClO ⇌ ClO− + H+ (pKa = 7.53)
At a pH of 7.53, 50% exists at Hypochlorous Acid. Just as in any other equilibrium reaction, the reaction will shift as amounts of reactants change. So consuming hypochlorous acid will result in hypochlorites reacting to form more hypochlorous acid to maintain equilibrium.
However, the lower the pH the more chlorinated cyanurates exist in equilibrium as well. So while more hypochlorous acid will exist at a lower pH, more chlorine will be “trapped” at any single time.
Cyanuric acid’s impact is reduced as more hypochlorites are available to form hypochlorous acid as equilibrium dictates. As hypochlorous acid is consumed, more hypochlorites will convert. In addition, this can marginally accelerate the decomposition of chlorinated cyanurates through oxidation of hypochlorites.
Cyanuric acid and it’s chemistry in pools is indeed complex. But when you take everything into account you see that at normal pH ranges of pools (especially higher pH values), CYA levels become less of a big deal. Just make sure to maintain proper chlorine and pH and your pool will stay clean.