What is Cyanuric Acid (CYA)?
Cyanuric acid, often called stabilizer or conditioner, is an organic chemical compound added to pool water. CYA does not naturally occur in water and will only be in water if added.
What does Cyanuric Acid do in Pool Water?
Cyanuric acid interacts with chlorine in two distinct and very important ways:
- Cyanuric acid protects chlorine from degradation by UV light
- Cyanuric acid buffers the harshness of chlorine.
How Does Cyanuric Acid Work in Pool Water?
Cyanuric acid, when dissolved in chlorinated pool water, forms various compounds with chlorine called chlorinated cyanurates. These are chemical compounds very similar to the original cyanuric acid where one of the hydrogen atoms has been replaced with chlorine.[1]
In this way, the cyanurate compound buffers the chlorine in your water. When the chlorine atom attaches itself to the cyanuric acid molecule, it is effectively “held in reserve” and will not participate in disinfection or chemical oxidation reactions. This makes the pool water much less harsh in terms of exposure of the swimmer to active chlorine compounds (hypochlorite and hypochlorous acid).
It is important to stress this point - the cyanuric acid concentration determines the amount of active chlorine (hypochlorous acid + hypochlorite ion) available in pool water. This is why the CYA level is so critical to know and why TFP teaches that there is no such thing as an absolute Free Chlorine level which is safe; the safe level of Free Chlorine is determined by how much CYA is in the water.
This is shown in the Chlorine/CYA Chart. Any Free Chlorine level up to the SLAM FC value for a CYA level is safe.
The entire pool industry gets this concept totally wrong when they state that a 1-3ppm Free Chlorine is all you need. THAT. IS. WRONG! Your Free Chlorine level is determined by your CYA level, FULL STOP! The higher your CYA level, the more Free Chlorine you need in order to maintain a proper level of active chlorine.
How Does Cyanuric Acid Protect Chlorine From the Sun's UV Rays?
Cyanuric acid and the related chlorinated cyanurates protect chlorine from degradation, or loss, due to UV light.
Chlorine degrades when exposed to UV light
When chlorine in water is exposed to the UV light from the sun, the hypochlorite ion (OCl-) and hypochlorous acid (HOCl) both react with UV light and get degraded into oxygen gas and chloride ion (Cl-). Both types of active chlorine have different rates of extinction by UV light, but, on average, chlorine in water with no CYA present will have a half-life (ie, the time it takes for the initial concentration to decrease by half) of approximately 35mins. So if you put chlorine in your water and wait 2 hours, there will be less than 5% of the original dose left simply from loss to UV light.
Cyanuric acid protects chlorine from UV degradation
When cyanuric acid is added to water and chlorine is able to bind to it, the half life of the chlorine increases to a maximum of 8.4 hours depending on how much CYA is present. The greater your CYA level, the less chlorine is lost.
There are two reasons for this:
- The presence of cyanuric acid causes chlorine to become bound to it and held in reserve. Because of this, there is much less active chlorine (hypochlorous acid and hypochlorite) present in the water. Since there is less active chlorine, there is less chlorine to react with UV and, ultimately, lose.
- Chlorine bound to CYA is much more stable when exposed to UV light and will not readily degrade. Because of this, CYA is very good at protecting chlorine form UV loss. The CYA molecule, with no chlorine attached to it, also absorbs some UV and radiates it back at a different wavelength of light and so it, all by itself, will reduce the amount of UV hitting the water volume (as UV light passes into the water, it is just as likely to interact with a molecule of CYA as it is with a molecule of chlorine). This inherent UV shielding effect is seen at higher levels of CYA.
What are the Correct CYA Levels for a Pool?
The recommended levels of CYA depend on the way the pool is chlorinated.
A CYA level of 30-50 is recommended when chlorinating with liquid chlorine. Pools in hot areas with intense sunlight can benefit from higher CYA levels to offset the degradation described below.
Salt Water chlorine Generation (SWG) pools seem to require a higher level of CYA, about 70-80 ppm, to operate efficiently. The theory is that the CYA is slow to "store" the chlorine as it is being generated so without enough CYA there is a build-up of chlorine that degrades the performance of the salt cell.[2]
Does CYA affect TA?
See Is TA is affected by CYA levels?
How to Increase CYA Level in Pool Water
Solid stabilizer or Liquid Stabilzer?
You increase CYA by adding cyanuric acid, often sold as stabilizer or conditioner. CYA is available as a solid and as a liquid. The liquid costs a lot more (usually about 3X the price of granular solid CYA), and generally is not worth the extra expense.
How Should Solid Stabilizer be Added to Pool Water?
Solid stabilizer is best added by placing it in a sock or nylon and hanging that sock in front of fast moving return(s). Shaking and squeezing the sock while in the pool water will help the stabilizer dissolve faster.
Alternatively, the sock can be placed into the skimmer basket BUT it should only be left in there if the pump is continuously running and not turned on and off by a timer. CYA is acidic when it dissolves into water and so it is not a good idea to leave the sock in the skimmer without the pump running.
Pouring granular CYA into the pool directly, or into the skimmer as recommended on stabilizer bottles, is not recommended as it takes a long time to dissolve and the granules can sit against the pool surface creating locally low pH levels.
The pump should be run for 24 hours after adding solid stabilizer and you should avoid backwashing/cleaning the filter for a few days if stabilizer was added to the skimmer.
When to Test for CYA after Adding Stabilizer?
Solid stabilizer can take up to 24 hours to dissolve into the pool water no matter the method of addition and so it is important to wait for it to dissolve and for the pump to homogenize (mix) the pool water well before testing. It has been found that if you wait approximately 24-48 hours after the last bit of CYA has dissolved to test that those results will show up on a test.
Be aware that the current test for CYA (melamine turbidity test) can only detect CYA difference of 10ppm at best. So if you are adding small amounts of CYA to your pool, the test might not show it.
How to Lower CYA Levels in Pool Water
In nearly all cases the best way to lower CYA is to replace water. Pool water can be replaced by draining or by a water exchange process that we describe in Draining.
If replacement water is extremely expensive or your local water utility has restrictions in place then you might want to look into a reverse osmosis water treatment.
How is CYA Lost and Degrade in Pool Water?
In general, once you add CYA to pool water it will be in the water unless you drain the water that contains the CYA. Some CYA is lost as water splashes out from pools.
Water leaking from pools will take CYA with it. Unexplained dropping of CYA levels, especially if you have an auto-fill automatically adding fresh water to the pool, can indicate a water leak.
Evaporation of water from the surface of the pool will not cause a loss of CYA. The water will evaporate and the CYA will stay in the remaining water.
High pool water temperatures will cause the chlorine to oxidize Cyanuric Acid. This tends to show in water temperatures of 90+ degrees. Every 10F increase in temperature results in roughly doubling the rate of degradation.[3]
Chlorine breakdown in sunlight causes CYA degradation by hydroxyl radicals. This can cause a loss from 2 ppm per month to 10 ppm per month depending on the amount of sunlight the pool is exposed to.
In an area with 90+ pool water temperatures and extreme sunlight exposure 10+ ppm of CYA a month can be lost through degradation.
There is bacteria that feeds on Cyanuric Acid and will create ammonia in pool water. This can only occur if the Free Chlorine falls to 0 allowing the bacteria to live. This generally happens in pools closed for the winter and discovered at pool opening.
