The Short, Short Version
Cyanuric acid stabilizes free chlorine by preventing it from evaporating from UV sunlight, which increases the lifespan of free chlorine but decreases its ability to sanitize pool water.
How Does Sunlight Lower Free Chlorine?
When you add chlorine to your pool, a series of chemical reactions take place that result in the sanitization of your pool water. The first thing that happens is the chlorine reacts with the water to form hypochlorous acid and hypochlorite ions.
These chemical compounds bind with bacteria and other microorganisms in a process called Oxidation Reduction Potential (ORP). In simple terms, chlorine makes it impossible for bacteria to reproduce in your pool water.
The sun is an incredibly powerful source of energy, like me on the dance floor. Its energy is transmitted as heat through light, including ultraviolet light (UV light). When the energy from UV light interacts with hypochlorous acid and hypochlorite ions, it causes them to break down and evaporate in a process called photolysis. In fact, direct sunlight can evaporate half your pool’s free chlorine in less than 20 minutes.
How Does Cyanuric Acid Work?
Have you seen Spider-Man Homecoming? Remember the part when the ship splits in half and Spider-Man and Iron Man use their superhero strength to stabilize the two halves of the ship from falling over? That is exactly what cyanuric acid does to hypochlorous acid and hypochlorite ions that begin to undergo photolysis.
Cyanuric acid is often called chlorine stabilizer because it quite literally stabilizes the bonds between the hydrogen, oxygen, and chlorine atoms of hypochlorous acid and hypochlorite ions that would break down due to the overwhelming energy of UV light from the sun. For a more in-depth read, check out The Relationship Between Pool Chlorine and Cyanuric Acid.
What are the Benefits of Cyanuric Acid (Pool Chlorine Stabilizer)?
The most apparent and obvious benefit of using cyanuric acid to stabilize your pool chlorine is cost savings. Without stabilizer, you will go through chlorine faster than puppies go through chew toys. Up to 90% of your free chlorine may be lost within 2 hours of exposure to direct sunlight. How expensive would it be to replace your chlorine every few hours?
The introduction of cyanuric acid chlorine stabilizer can increase the lifespan of your pool’s free chlorine by 3 to 5 times, depending on the amount of direct sunlight your pool receives. To really stretch the value of adding stabilizer, we recommend adding chlorine after sunset. This can increase your free chlorine’s lifespan by an additional 2 to 3 times compared to adding chlorine in the morning.
What are the Detriments of Using Cyanuric Acid (Pool Chlorine Stabilizer)?
At this point, you may be thinking cyanuric acid is a godsend. It is to chlorine what that drug was to Bradley Cooper in the movie Limitless. However, just like there were downsides to taking the Limitless drug, there are also downsides to using pool chlorine stabilizer.
The most important downside is that cyanuric acid reduces the chlorine’s Oxidation Reduction Potential (ORP). Meaning, it reduces free chlorine’s ability to sanitize bacteria and other harmful microorganisms. So, if you use chlorine stabilizer, you will also need to use more chlorine to get the same sanitizing effect as unstable chlorine.
A reliable heuristic is to maintain your free chlorine levels at 7.5% of your cyanuric acid levels. If you maintain cyanuric acid at 50 ppm, you should maintain free chlorine at 3.75 ppm.
The diminishing effect cyanuric acid has on free chlorine’s sanitizing ability is generally not a problem with swimming pools, so long as free chlorine levels are maintained. However, it is a problem with hot tubs. Cyanuric acid slows free chlorine’s sanitizing ability to the point that it can’t sanitize pseudomonas aeruginosa bacteria (hot tub itch) fast enough.
For that reason, it is ill-advised to add stabilizer to your hot tub. Instead, use test strips to regularly test your hot tub’s chlorine levels and add spa-specific chlorine as necessary.