The Cyanuric Acid to Free Chlorine Equilibrium

Background

Free available chlorine in water is not chemically stable under ultraviolet exposure. Sodium hypochlorite and calcium hypochlorite, the two chlorine sources that dominated pool sanitation through the 1950s, lose free chlorine rapidly in direct sunlight. Documented loss rates from that period show up to 75 percent of free chlorine destroyed within two hours of UV exposure, with near total loss possible within four hours in an unstabilized outdoor pool. This was not a minor inefficiency. It required pool operators to add chlorine multiple times daily to maintain any measurable residual, at real cost and with real gaps in coverage between doses.

Cyanuric acid entered the pool industry as the solution to this specific problem. Monsanto began producing and distributing cyanuric acid for pool use in 1956. By 1958, chlorinated isocyanurates, compounds that deliver chlorine and cyanuric acid together in a single product, were in commercial use in United States pools. A patent for chlorine stabilization using cyanuric acid was granted to Food Machinery Corporation in 1961, formalizing what by then was already an established commercial practice. The compound itself is older still. Cyanuric acid was first isolated by the chemist Friedrich Wöhler in 1829, more than a century before anyone applied it to recreational water.

The mechanism was, and remains, straightforward at the conceptual level. Cyanuric acid forms a weak, reversible bond with chlorine. Chlorine bound in this way is shielded from the ultraviolet-driven photolysis that destroys unprotected free chlorine, extending measured chlorine residual by a documented factor of up to eight at typical CYA concentrations. The chemistry that made this genuinely useful is the same chemistry that makes it require careful management: the bond that protects chlorine from the sun also holds it in a form that is not immediately available to sanitize.

The Equilibrium Model

The full quantitative relationship between free chlorine and cyanuric acid was established by O’Brien, working initially alone in 1972 and subsequently with Morris and Butler in a 1974 paper, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate,” published in Chemistry of Water Supply, Treatment, and Distribution. That work defined ten independent equilibrium reactions and two mass balance equations describing the behavior of cyanuric acid, its dissociation products, free chlorine, and the six possible chlorinated cyanurate species, at 25 degrees Celsius (77 degrees Fahrenheit). Wojtowicz extended and refined the associated equilibrium constants and their temperature dependence in subsequent work published in 2001 and 2004, and this expanded model remains the reference standard cited in current water chemistry literature, including recent peer reviewed work on chlorinated cyanurate behavior in both recreational and drinking water contexts.

The practical result of this equilibrium is that at any given moment, most of the chlorine in a stabilized pool is not present as free hypochlorous acid. It is bound in chlorinated cyanurate form, held in reversible association with cyanuric acid. In a pool at 30 ppm CYA and 3 ppm free chlorine, on the order of 97 percent of total chlorine is in this bound state. This fact has produced a persistent but inaccurate industry claim, commonly called chlorine lock, holding that chlorine bound to CYA becomes permanently unavailable. The equilibrium model does not support this. The association is dynamic and reversible by definition; as free hypochlorous acid is consumed through disinfection or oxidation, the equilibrium shifts and additional chlorine is released from the bound cyanurate pool to replace it. Chlorine lock, as a permanent condition, is not supported by the governing chemistry. What the equilibrium does establish, and what is functionally significant, is that the concentration of hypochlorous acid available at any instant is suppressed as a function of CYA concentration, and that suppression is what determines actual sanitizing capacity, independent of what a total or free chlorine test reports.

Where the 7.5 Percent Relationship Comes From

The commonly cited “7.5 percent rule,” stating that free chlorine should be maintained at a minimum of 7.5 percent of the CYA concentration, is not an arbitrary industry convention, though it is also not a value O’Brien himself calculated. The figure was developed in the 1970s by Ben Powell and later refined by Richard Falk, drawing on data from Wojtowicz, Stanley Pickens, Ellen Meyer, and others working from the O’Brien equilibrium model. It entered mainstream industry use later through Robert Lowry’s published work and training materials. The underlying logic is a practical linear approximation of the O’Brien equilibrium, developed because the full ten-reaction system is not something a pool operator can solve in the field. Across the CYA concentration range relevant to residential and commercial pools, the relationship between hypochlorous acid concentration and the ratio of free chlorine to CYA holds approximately constant within a usable margin. The 7.5 percent figure represents a conservative minimum within that range, chosen to maintain adequate hypochlorous acid concentration for routine bather protection under typical conditions. No party involved in its development has claimed the figure is exactly precise. Multiple variables affect the true minimum in any given pool, but the concept is chemically sound and functions as a reliable operating standard.

It is worth stating plainly what this approximation is not. It is not a claim that the underlying equilibrium chemistry changes character at any particular CYA concentration, including 50 ppm. The O’Brien and Wojtowicz models describe continuous equilibrium behavior; a pool correctly maintaining the proportional free chlorine target at 80 or 100 ppm CYA is governed by the identical equilibrium as a pool at 30 ppm. The approximation is a management tool derived from continuous chemistry, not a description of a threshold within that chemistry.

Quantifying the Public Health Stakes

The consequence of operating below the effective free chlorine to CYA ratio is not cosmetic. A 2015 study examining Cryptosporidium parvum inactivation under varying CYA concentrations at fixed free chlorine levels provides direct quantitative evidence of the scale of this effect. At 20 mg/L free chlorine, the CT value (the product of disinfectant concentration and contact time) required for 3-log10 inactivation of Cryptosporidium was 17,800 mg times minutes per liter at 8 mg/L CYA, rising to 31,500 mg times minutes per liter at 16 mg/L CYA, a near doubling in required contact time from a comparatively modest increase in CYA. At 48 mg/L CYA, only 1-log10 inactivation, a tenfold weaker standard than the 3-log10 target, was achievable within the study’s tested conditions, with a CT value of 76,500 mg times minutes per liter.

This data does not describe a hypothetical risk. It describes a documented, measured reduction in disinfection capacity against a chlorine-resistant pathogen already responsible for recreational water illness outbreaks, occurring at CYA concentrations well within what much of the industry has historically treated as an acceptable operating range.

The Practical Ceiling: An Operational Finding, Not a Chemistry Limit

Maintaining proper sanitization above 50 ppm CYA is a documented operational problem, not a chemistry problem. The equilibrium relationship described above is continuous, and the 7.5 percent proportional relationship holds mathematically at CYA concentrations well above 50 ppm. Nothing about the underlying chemistry changes character at that threshold.

The difficulty is a separate, non-chemical constraint: as CYA rises, the corresponding minimum effective free chlorine target rises with it, and sustaining that higher target consistently requires equipment and operational discipline that the majority of residential and commercial pool systems do not reliably have. At 80 ppm CYA, the calculated minimum effective free chlorine is 6 ppm. At 100 ppm, it is 7.5 ppm. Salt chlorine generators, standard chemical feeders, and manual dosing routines, the equipment actually in use across the overwhelming majority of pools, are not consistently capable of holding those higher targets day over day against normal bather load and demand fluctuation. Observed field outcomes at these higher CYA levels are not chemistry failures. They are the predictable result of an operationally demanding proportional target going unmet, producing exactly the invisible under-sanitization the equilibrium model predicts when the ratio is not maintained. A practical operating ceiling near 50 ppm reflects this reality rather than a limitation in the underlying chemical model.

Summary

The relationship between cyanuric acid and free chlorine is governed by a well established, quantitatively defined chemical equilibrium, first modeled by O’Brien in 1972 and 1974 and refined by Wojtowicz in subsequent work. The widely cited 7.5 percent minimum ratio is a practical approximation of that equilibrium, not an independent rule. The public health consequence of ignoring that ratio is measurable and significant, as demonstrated by direct pathogen inactivation data. Practical operating ceilings on CYA, including the 50 ppm threshold commonly recommended, are properly understood as findings about equipment and management capacity under real field conditions, not as statements about where the underlying chemistry stops functioning as described.

References

O’Brien, J.E. (1972). Hydrolytic and Ionization Equilibria of Chlorinated Isocyanurate in Water. Doctoral dissertation, Division of Engineering and Applied Physics, Harvard University, Cambridge, Massachusetts.

O’Brien, J.E., Morris, J.C., Butler, J.N. (1974). Equilibria in Aqueous Solutions of Chlorinated Isocyanurate. In Rubin, A.J. (Ed.), Chemistry of Water Supply, Treatment, and Distribution, 1973 Symposium. Ann Arbor Science Publishers, pp. 333 to 358.

Anderson, J.R. (1965). A Study of the Influence of Cyanuric Acid on the Bactericidal Effectiveness of Chlorine. American Journal of Public Health and the Nation’s Health, 55(10), 1629 to 1637.

Wojtowicz, J.A. (2001). Relative Bactericidal Effectiveness of Hypochlorous Acid and Chloroisocyanurates. Journal of the Swimming Pool and Spa Industry, 2(1), 34 to 41.

Wojtowicz, J.A. (2001). Oxidation of Cyanuric Acid with Hypochlorite. Journal of the Swimming Pool and Spa Industry, 4(2), 23 to 28.

Wojtowicz, J.A. (2004). Effect of Cyanuric Acid on Swimming Pool Maintenance. Journal of the Swimming Pool and Spa Industry, 5(1), 15 to 19.

Murphy, J.L., Arrowood, M.J., Lu, X., Hlavsa, M.C., Beach, M.J., Hill, V.R. (2015). Effect of Cyanuric Acid on the Inactivation of Cryptosporidium parvum Under Hyperchlorination Conditions. Environmental Science and Technology, 49(12), 7348 to 7355.

Wahman, D.G. (2018). Chlorinated Cyanurates: Review of Water Chemistry and Associated Drinking Water Implications. Journal AWWA, 110(9), E1 to E15.

Wahman, D.G., Alexander, M.T. (2019). A Drinking Water Relevant Water Chemistry Model for the Free Chlorine and Cyanuric Acid System from 5 to 35 Degrees Celsius. Environmental Engineering Science, 36(3), 283 to 294.

This piece is part of the Pools Scientific Compendium, an ongoing effort to build dedicated, cited chemistry research for recreational water, the kind of research this industry has never systematically produced for itself.