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Cyanuric acid/chlorine: It’s all about ratio

By Marcelle Dibrell

Perhaps the most confusing, misunderstood and controversial topic within this industry is the chemical cyanuric acid. Cyanuric acid has been used in swimming pools since the 1950s to protect chlorine from degradation from the sun. Without cyanuric acid, chlorine is decomposed by ultraviolet light from the sun due to the photo-instability of both hypochlorous acid as well as the hypochlorite ion when they absorb this light. 

Before the ’50s, chlorine had to be added to residential swimming pools every day, and almost continuously to public pools due to a higher bather load. With the advent of cyanuric acid, chlorine typically needs replacement once or twice a week. That is because when cyanuric acid is added to water containing hypochlorous acid, a weak bond forms between chlorine and cyanuric acid, and this chloro-isocyanurate is stable against photo-degradation. Cyanuric acid is vital to maintaining chlorination in outdoor pools that are sanitized with chlorine, and many scientific experiments support this. 

For example, one study found that an initial chlorine concentration of 2.5 ppm in an outdoor pool dropped to almost zero in three hours in the absence of cyanuric acid (85°F, pH = 7), while for these same conditions, 70 percent of the chlorine remained after three hours with the addition of 2.5 ppm cyanuric acid. There are currently three ways to add cyanuric acid to the pool. It can be added in its pure form, which is a white powder that should be pre-dilluted prior to addition. It is also commonly added with chlorine directly to the water in the form of dichlor or tricolor. Unlike chlorine, cyanuric acid is  stable in water, and once added, does not usually go away except by splash-out, backwashing, or draining.  That means that if dichlor or trichlor is used to chlorinate the water, the cyanuric acid concentration can become quite high. 

The question is whether that matters:  experts agree that it does. The general consensus of the Center for Disease Control, World Health Organization, and “Ten State Standard” has lead the Association of Pool and Spa Professionals to recommend that cyanuric acid should be within 30 to 50 ppm with a maximum of 100 ppm.  Their service tech manual states that below 25 ppm, its (CYA) chlorine extending property is reduced, and levels above 50 ppm do not increase its effectiveness. This somewhat cautious language is likely due to an ongoing question concerning whether high levels of cyanuric acid are truly detrimental to chlorine’s efficiency such that it might affect bather health.

It has long been understood that cyanuric acid reduces the oxidation reduction potential of chlorine. That is because when cyanuric acid is present, most of the free available chlorine is bonded to the cyanuric acid, and only a fraction is present as hypochlorous acid. This fact reduces the speed of chlorination reactions, a fact that has been shown with respect to chlorine’s reactions with pathogens. Therefore, one of the reasons to limit the amount of cyanuric acid in a pool to within 30 to 50 ppm is because excessive cyanuric acid reduces the killing efficiency of chlorine to some extent. 

Numerous studies have shown that cyanuric acid increases the kill time for various bacteria, viruses, and oocysts in the presence of chlorine.  That is because when chlorine is attached to cyanuric acid, it is not as effective. The APSP addresses this point in its ANSI water standards manual. While it is understood that cyanuric acid reduces kill rates, the manual states: “We do not have any empirical evidence that a disease outbreak has been linked to any particular cyanuric acid level in a properly sanitized pool (i.e. when at least 1 ppm free available chlorine was present).”

However, many argue that it is important to consider prolonged kill rates because the amount of time that pathogens are able to survive in the water increases the chances that a person could become sick from them. The accompanying graph comes from a study conducted on the kill rate of Streptococcus Faecalis in the presence of various concentrations of chlorine and cyanuric acid. It clearly shows that increasing the cyanuric acid concentration also increases the amount of time it takes to kill the bacteria. 

For example, it can be seen that for the bacteria exposed to 1 ppm chlorine, the average time required for 99 percent  kill is about 25 minutes in 25 ppm cyanuric acid but goes up to about 55 minutes when the cyanuric acid is raised to 50 ppm. It should also be noted that cyanuric acid continued to slow the reaction time after 50 ppm, indicating that while levels above 50 ppm may not increase cyanuric acid’s protection ability, they do decrease chlorine’s sanitation ability. It is interesting to note that as the chlorine concentration is raised, increasing the cyanuric acid seems to have less of an effect on the kill time. 

This effect points to an important issue: When thinking about chlorine’s killing capacity, one should consider the ratio of chlorine to cyanuric acid, rather than simply the concentrations of each. Industry standards currently recommend maintaining chlorine concentrations between 2 and 4 ppm and cyanuric acid levels between 30 to 50 ppm. However, while these numbers are an effective starting place, this recommendation fails to recognize the relationship of hypochlorous acid to the ratio of free chlorine and cyanuric acid. 

Hypochlorous acid is proportional to the ratio of free chlorine to cyanuric acid.


It is essential to recognize this relationship because it is the concentration of hypochlorous acid that determines the rate of all of chlorine’s reactions. Some people mistake the free chlorine concentration as the total amount of chlorine that is working to sanitize a pool. This is not so. 

Even in the absence of cyanuric acid, a free chlorine measurement is actually about half of the chlorine that is working to sanitize the pool since at normal pool pH levels. The active form of chlorine is in equilibrium with the inactive form. Hypochlorous acid is in equilibrium with the hypochlorite ion and at pH 7.5, they are split about 50/50. When cyanuric acid is added to the mix, the active portion of chlorine is reduced further still. The reason for this is fairly complicated, having to do with a lot of equilibrium reactions of chlorine with cyanuric acid. 

When cyanuric acid is present in the water, most of the chlorine forms a bond with cyanuric acid, leaving very little active chlorine in the form of hypochlorous acid.  But it is the hypochlorous acid that is the needed form of chlorine to sanitize. It is important to remember that the concentration of hypochlorous acid is proportional to the ratio of the measured free chlorine and cyanuric acid. There is 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, a service professional and creator of Pool Solutions, as well as Richard Falk, a frequent contributor to Trouble Free Pools, have done extensive research on the mitigating effects of cyanuric acid to the hypochlorous acid concentration. Their research has indicated that the absolute minimum concentration of hypochlorous acid should be at least .03 ppm to control algae growth.

This corresponds to a free chlorine that is about 7.5 percent  of the cyanuric acid concentration. Ideally, however, the hypochlorous acid concentration that they have found is best at preventing algae is a little higher, at .05 ppm.  This corresponds to a free chlorine concentration that is about 11.5 percent of the cyanuric acid.

The accompanying table shows what the hypochlorous acid concentration may be as a function of changing the free chlorine and cyanuric acid levels. When the hypochlorous acid concentration is above .01, the pool may be safe against most bacteria, but values closer to .05 ppm have been found to provide better protection against algae. The concentration of hypochlorous acid is what determines the rate of chlorine’s reactions. 

For chlorine to be effective against algae, it must kill the algae faster than it can reproduce. Experience has shown this concentration to be about .05 ppm hypochlorous acid.  It is immaterial whether that value is achieved by maintaining 2 ppm free chlorine with 15 ppm cyanuric acid, or 5 ppm chlorine with 40 ppm cyanuric acid. 

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