Chlorine has played a vital role in water treatment for centuries. More recently in swimming pools, the main treatment for clean, clear, and safe water has been chlorine-based sanitizers.

The chemistry is fairly simple. When chlorine is added to water, it immediately disassociates into hypochlorous acid HOCl and also an ionized form known as hypochlorite ion OCl-. The HOCl is the primary killing agent of the chlorine while OCl- is very weak. In basic chemistry, the amount of HOCl that can be produced in pool water is based on water balance. The pH controls how much of either HOCl or OClis produced. At lower pH, more of the killing agent HOCl is produced, while at higher pH, more of the weak OCl- is produced.

HOCl is an excellent disinfectant, but it has one weakness: It is sensitive to ultraviolet light, which breaks it down into ineffective forms.

In the mid 1950s, Monsanto began to apply for patents for the use of cyanuric acid as a stabilizer to prevent the rapid degradation of HOCl in water. The introduction of cyanuric acid in the pool industry marked a significant breakthrough, offering a solution that enhanced the stability and longevity of chlorine in outdoor pool environments, ultimately revolutionizing water treatment practices and contributing to the modern standards of pool maintenance.

That’s because when unstabilized chlorine is added to pool water, 9095% will be lost within 2 hours from direct exposure to sunlight. Within 4 hours, all chlorine can be gone in a pool without any cyanuric acid. With 30 ppm of cyanuric acid in a pool, the chlorine will last up to 8 times longer.

Cyanuric acid is often used in swimming pool maintenance to help protect chlorine from degradation caused by sunlight (UV radiation). It forms a complex with chlorine, creating a more stable compound that slows down the breakdown of free chlorine in the presence of sunlight.

Cyanuric acid serves as a shield for chlorine against the degrading effects of sunlight. It forms a stable compound with chlorine, slowing down its breakdown in the presence of UV radiation. Available in granular form at 100% strength, 3.3 pounds of cyanuric acid per 10,000 gallons results in about a 40-ppm concentration.

To find out how much cyanuric acid is added to the pool, use the simple formula:

(Cyanuric Acid Content) (÷) (Pool Volume x 8.34) x 1,000,000 = Cyanuric Acid Addition (ppm)

*Note: A gallon of water weighs 8.34 pounds.

Substituting the values from the above example:

(3.3 pounds)(÷) (10,000 x 8.34) x 1,000,000 = 39.5 ppm

The Pool and Hot Tub Alliance recommends maintaining cyanuric acid levels in swimming pools between 30-50 ppm.

Cyanuric acid can be introduced through chlorine products classified as iso-cyanurates, including trichlor tablets, tri-chlor granular, and sodium dichlor granular chlorine (dichlor). Understanding and managing cyanuric acid levels are integral to optimizing the longevity and effectiveness of chlorine in pool water.

That’s because both studies and experience have shown that at elevated levels, cyanuric acid can get in the way of disinfection and make it more difficult to prevent and control algae.

In recent years, there has been quite a bit of discussion about cyanuric acid and the role it plays in controlling chlorine’s ability to kill unwanted intruders. During the ‘90s, there was a big dispute about chlorine lock — the notion that you can put so much cyanuric acid in a pool to “lock up the chlorine,” and make it ineffective. The reality is a little more nuanced.

One of the biggest revelations of recent years is the understanding that the hypochlorous acid concentration is actually determined by the cyanuric acid concentration, so we need to know what that corresponds to in terms of measured chlorine and cyanuric acid concentrations.

We also need to know how much hypochlorous acid we actually need, realizing that measured chlorine is NOT the same as measured hypochlorous acid.

So how much hypochlorous acid do we need?

The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have determined that a minimum of 650 mV (ORP) is required for instantaneous inactivation of most pathogens. That corresponds to about 0.01 ppm hypochlorous acid. Meanwhile, some studies have shown that 0.05 ppm hypochlorous acid kills most algae. So basically, we know we want a hypochlorous acid concentration somewhere between 0.01 and 0.05 ppm.

Next, we need to figure out how that range corresponds to our measured chlorine and cyanuric acid. Unfortunately, the exact math to get there is tricky and involves solving a bunch of equilibrium equations.

Fortunately, someone else has done the work for us.

In the early 2000s, water chemistry expert Richard Falk solved those equations to determine the amount of cyanuric acid and chlorine that taken together result in a given hypochlorous acid concentration. See accompanying graphic.

The graph shows free chlorine increasing at the top from left to right and cyanuric acid increasing on the left from top to bottom. Colored numbers on the inside are calculated hypochlorous acid concentrations. Those marked red are hypochlorous acid concentrations lower the WHO standard for pathogen inactivation, while those marked in blue are higher than 0.05 ppm hypochlorous acid needed to kill common algae. Green numbers indicate chlorine and cyanuric acid combinations that should be effective for most pools.

The graph also shows that a desired hypochlorous acid range of between 0.01 and 0.05 ppm can result from a variety of chlorine and cyanuric acid combinations, from 0.1 to 10 ppm chlorine, and from 0 to 100 ppm cyanuric acid.

All that is left is the constraints of industry guidelines and regulations.

The 2023 Model Aquat ic Health Code (MAHC) states that a cyanuric-acid-to-free-chlorine ratio that exceeds 45:1 is grounds for closure. That means that chlorine should be greater than 2.2 percent of the cyanuric acid concentration. It stipulates a maximum cyanuric acid concentration of 90 ppm and chlorine levels between 2 and 10 ppm. It’s a very broad allowance, and pool operators adhering to the standard may find that some combinations within this range may actually succumb to algae.

According to Falk and others, the sweet spot for protection against algae has a higher chlorine-to-cyanuricacid ratio.

Their experience is that maintaining the chlorine at 7.5 percent of the cyanuric acid is generally adequate to protect typical pools from green algae, and that a ratio of about 15 percent is good for yellow/mustard algae. Lowering your phosphates (algae food) may allow operators to follow the MAHC’s low estimate of 2 percent.