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Cyanuric acid dictates chlorine levels

By Marcelle Dibrell

If there is one topic within the pool and spa service community that remains poorly understood among many service professionals it is cyanuric acid and its role in mitigating chlorine’s reaction rate.

The problem is not that the information isn’t out there. After all, cyanuric acid was introduced to the swimming pool industry in the 1950s, and its complex chemistry with chlorine has been understood since at least the early 1970s.

Despite this, there has been a lot of misinformation printed on the subject. The controversy over the existence or non-existence of “chlorine-lock” didn’t help. Industry guidelines about proper chlorine and cyanuric acid levels fall short in their simplicity, although they do provide a useful starting place for beginners.

The industry’s historical seeming lack of concern in either learning or disseminating the facts about cyanuric acid’s role in swimming pools is interesting. 

Some say that cyanuric acid concentrations kept in check along with the corresponding proper chlorine levels renders the use of specialty chemicals unnecessary. 

However, as for learning the facts, the chemistry is complicated — not easy for a non-chemist (and even some chemists) to grasp.

Over the last several years, there have been attempts to improve the industry’s understanding of just how important a parameter cyanuric acid is. Cyanuric acid, also called stabilizer or conditioner, plays a vital role in chlorine’s efficacy. Without cyanuric acid, chlorine breaks down quickly in sunlight, leaving the water without a sanitizer residual, prone to algae outbreaks, and bathers unprotected against infection. Too much cyanuric acid and the water may suffer some of the same problems.

Too much cyanuric acid causes the industry-recommended chlorine concentrations to lose their power to prevent algae from taking hold and slows other reactions. In other words, it retards chlorine’s efficacy. Isn’t this what people meant when they said “chlorine-lock?”   

Of course the word “lock” isn’t the best description since lock sounds permanent and chlorine can escape its bond with cyanuric acid. Nonetheless, decades of quibbling over this little word has led to colossal confusion.

Most industry professionals know that cyanuric acid must be added to pools. Most know the industry guidelines: 30-50 ppm; recommended not to exceed 100 ppm.

What many don’t know is that the guidelines make sense only for corresponding concentrations of chlorine, and the current industry guidelines may not always be effective against algae.

The way that cyanuric acid impacts chlorine’s efficiency can and has been explained by chemists. They use a series of equilibrium reactions to solve for the hypochlorous acid concentration as a function of both free chlorine and cyanuric acid.

This is done because it is understood that hypochlorous acid is the species that does chlorine’s heavy lifting in the pool. 

What results is an equation that says that hypochlorous acid is proportional to the ratio between free chlorine and cyanuric acid.

So now what?

If it were known what concentration of hypochlorous acid was necessary to defeat pathogens and algae, it would then be possible to determine the required free chlorine levels for a given cyanuric acid level.

What is that level?

There has been some disagreement as to what the minimum concentration of hypochlorous acid ought to be in order to provide a safe bather environment. The World Health Organization (WHO) has set a minimum ORP level at 650 mV, corresponding to a minimum hypochlorous acid level of about .01 ppm. 

This is a level that has been shown to deactivate many pathogens. 

However, the consensus is that this concentration may not be effective against algae. 

Ben Powell was among the earliest pioneers who set the stage for the current understanding of the chlorine-cyanuric acid relationship.

Powell, a service professional (and the creator of both PoolSolutions, an information site, and The PoolForum) had an idea that swimming pool chlorine levels should be adjusted according to stabilizer levels. This was based on years of his own experience servicing commercial pools, lots of input from people who used his forum, a technical paper by John Wojtowicz, and some math.

The problem is: given various stabilizer levels, how much chlorine should be present to prevent algae?  And there is also a related problem: given various stabilizer levels as well as an algae bloom, how much chlorine should be present to shock the algae away? 

He set about the problem empirically, with the notion that as the stabilizer concentration goes up, so should the chlorine. The result, likely first printed in the late ‘90s, is the famous “Ben’s Best Guess Guide to Swimming Pool Chlorination.”

See accompanying graphic. 

Ben’s Best Guess is practical in that it proscribes chlorine as a function of stabilizer, and it’s an easy, simple-to-use graphic that if followed, will keep algae away. 

Much has been learned and expanded upon Ben’s Best Guess in the years that followed. Richard Falk, known as Chem Geek at the online forum Trouble Free Pool, was perhaps the biggest contributor to working out the relationship between chlorine’s efficacy as a function of cyanuric acid. Using published equilibrium constants and chemical equations, Falk was able to refine Powell’s work and the result is more exact.

Falk’s contribution was to derive a simple formula for a minimum free chlorine to cyanuric acid ratio to prevent algae. 

He did this by recognizing that hypochlorous acid (the killing form of chlorine) is proportional to the ratio of free chlorine to cyanuric acid:


Beginning with Powell’s best guesses on free chlorine values that are effective for a given cyanuric acid concentration, Falk determined that one should have a minimum free chlorine to cyanuric acid ratio of 7.5 percent to prevent algae in traditionally chlorinated pools. 

Falk’s ratio has made doing the math to prevent algae incredibly easy.

FC = 7.5% x CYA

For example, if the measured cyanuric acid in a swimming pool is 30, then a pool operator should maintain a minimum free chlorine level of 2.25 ppm. 

2.25 ppm FC = 7.5% x 30 ppm CYA

If the cyanuric acid is at 70 ppm, the free chlorine should be maintained at a minimum of 5.25 ppm. 

5.25 ppm FC = 7.5% X 70 ppm CYA

In this case using an industry recommended guideline of 3 ppm chlorine may not control an algae outbreak and a pool operator might infer that “chlorine lock” has occurred. This is not the case; the pool simply needed more chlorine.

Falks derived his ratio in part by recognizing that even if the free chlorine is the same, the concentration of hypochlorous acid (effective chlorine) changes when cyanuric acid is introduced at different levels.

Falk found that when Powell’s proscribed minimum and maximum free chlorine and cyanuric acid levels are converted to hypochlorous acid, the implied target value corresponds to about 0.05 ppm hypochlorous acid (minimum, 0.03 ppm; maximum, 0.07 ppm).

His 7.5 percent rule corresponds to the minimum 0.03 ppm hypochlorous acid level. This is for prevention of green and black algae. Yellow/mustard algae would require a higher 15 percent rule. These rules are for preventing algae growth independent of algae nutrient (phosphate, nitrate) levels. If algae nutrient levels are lower, then lower FC/CYA ratios would prevent algae growth.

Falk’s ratio shows that industry guidelines fall short of where they should be to control and prevent algae, particularly when swimming pools have high cyanuric acid levels. It demonstrates why high cyanuric acid pools tend to develop problems — they probably don’t have enough chlorine in them.

Falk’s work is well known to the millions of online visitors of Trouble Free Pool, a swimming pool help and information exchange forum. Due to the high traffic at that site his ratio is increasingly being used by pool operators to maintain an algae free pool. 

Falk’s ratio, derived in 2006, has also informed the recommendations of the well known chemical consultant and author Robert Lowry, who in 2012 included the ratio in his article “Cyanuric Acid: It Controls Your Pool."

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