Cyanuric Acid: A Discussion on Chlorine Stabilizer


Welcome, students. I’ve prepared a lecture today on the chemical compound Cyanuric Acid (CNOH)3.

You may know CYA as Pool Stabilizer or Conditioner. This curious chemical seeks out and attaches itself to the chlorine molecule. In doing so, it acts as a sun shield, absorbing sunlight, and reducing the sun’s degradation to your chlorine.

Friedrich Wöhler


The chemical was discovered in 1829, by Friedrich Wöhler a German chemist (much like myself, sans the sideburns!). It was not until the 1950’s however, that a class of disinfectants known as chlorinated iso-cyanurates were developed. Soon after, savvy chlorine manufacturers …ahem, began adding it to “stabilize” chlorine tablets.

Cyanuric acid that is sold as pool stabilizer or conditioner is a white, flaky powder, similar in appearance to calcium chloride. It is a weak acid, and is not corrosive or considered hazardous.

How is Cyanuric Acid Used to Swimming Pools?

pool-stabilizer-labelStabilizer should be added to refilled pools to raise the level to the range of 30-50 ppm. After this foundation, the small amount of CYA pressed into your stabilized tablets should help replace the CYA lost during backwash, splashout or winterization.

The TriChlor tablets and the DiChlor shock that we sell are examples of chlorine products that are combined crystallized with the salts of cyanuric acid. These are known as Stabilized Chlorine, and are meant to help maintain the residual of cyanuric acid in your pool.

Cyanuric Acid Reduces Chlorine Degradation

cyanuric-acid-protectsI like to think of the CYA molecule holding a sun parasol, shielding the chlorine. While it absorbs up to 50% of the sun, you can expect to see a reduction of up to 50% of your chlorine usage. After adding CYA to the pool, you’ll immediately notice the increase in your chlorine’s lifespan, and your wallet size.

A pool without stabilizer can lose 90% of it’s free chlorine with just a few hours of bright sun. When stabilized, your chlorine level will last longer, and you’ll use about half of the tablets to maintain your chlorine residual.

Too Much Cyanuric Acid

Many users of chlorinated isocyanurates report problems with a build-up of CYA over time, to levels beyond 50ppm. To lower the levels of CYA, there is no magic potion (yet, but we’re working on it!). Lowering CYA levels is accomplished by dilution; that is, draining and refilling a portion of the pool water.

If you are using stabilized chlorine (Trichlor or Dichlor), and you have problems with too much CYA, without the ability to drain and refill – you can switch chlorine types. Cal Hypo tablets and shock can be used, which do not contain cyanurates. Trichlor and Dichlor are a nearly 50% cyanurate compound.

Another solution is to switch to a salt water chlorinator. You’ll still want to add CYA to the pool, to help your salt cell from working too hard, but you won’t be adding additional CYA from stabilized tablets or shock.

To lower the CYA level by dilution, here’s the math. If your cyanuric acid level was at 100 ppm (way too high), and you want to lower it to 50 ppm, you will need to replace half the pool water (assuming your fill water has 0 ppm of CYA).

Not Enough Cyanuric Acid

Outdoor pools without a residual of cyanuric acid in the water have trouble maintaining a chlorine residual during a sunny day. Even small levels of CYA in the water will have a pronounced protective effect.

Indoor pools can also benefit from a very low residual (5-10 ppm) of CYA, as it helps to prevent off-gassing of chlorine and reduces chloramine formation.

If you need to raise your cyanuric acid level, you simply pour the granular white powder directly into the pool, at a rate of about 1 lb. per 5,000 gallons, to raise it about 10ppm.

Keeping the Proper Level of Cyanuric Acid

Testing the water for a Cyanuric Acid level is the first step. The test for cyanuric acid is interesting; it’s called a turbidity test. The presence of CYA will turn the water sample cloudy, or turbid. Aquachek-pro-II test strips for Cyanuric Acid, Hardness and TDS

If you don’t have a turbidity test for CYA, you can use these test strips to check your Cyanuric Acid, Hardness and Total Dissolved Solids (TDS) levels. Since you only need to check these levels monthly, it makes sense to have separate test strips for these tests.

Maintaining the level around 30-50 ppm is recommended. There is little advantage in exceeding the level, and in fact, the higher the level, the higher the kill time for your chlorine becomes.

Other effects of high cyanuric acid levels include problems with cloudy water, and a difficulty in maintaining a free chlorine level in the pool, with a corresponding increase of chloramines.

So students, here ends another lesson in swimming pool chemistry. In summary; it’s prudent and necessary to use some level of cyanuric acid. Test monthly to be sure that it doesn’t rise much above 50 ppm over time, and if it does – lower by dilution, until we come up with a better solution.

Back to the lab!

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Cyanuric Acid: A Discussion on Chlorine Stabilizer — 2 Comments

  1. With a cya level of 50, would it ever be necessary to add more stabilizer to my pool, since cya levels will not decrease without dilution?

    • Yes, it can for pools that receive a lot of rainfall, or for pools that are winterized using safety covers, for which the pool water level is often lowered 12-18″ when closing. In both cases, when the pool is diluted with enough rain/snow, and the following season may need a cya booster