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Cyanuric acid alters chlorine/pH link

By Marcelle Dibrell

Cyanuric acid might be the one of the most commonly used and least commonly understood chemical added to swimming pools. Most know that cyanuric acid is used in outdoor swimming pools to prevent chlorine from rapidly dissipating due to UV degradation. Some are aware that cyanuric acid affects chlorine’s disinfection rates. But not many understand why. 

Service professionals should be familiar with the relationship between active chlorine and pH. It is well understood that as the pH goes down, chlorine becomes a better oxidant and disinfectant. (See accompanying table.)  If it were safe for surfaces and equipment, it would be nice to maintain a pH of 6, where about 96 percent of the chlorine is in its killing form. Unfortunately, such a low pH would be corrosive to equipment. Therefore, pools are kept at a minimum pH of 7.2, where the table shows that about 63 percent of the chlorine is in its active form.

What is not well understood is that the relationship between active chlorine and pH is substantially altered in the presence of cyanuric acid. For example, just 30 ppm of cyanuric acid reduces the amount of active chlorine to about 1.6 percent at a pH of 7.2.  Many outdoor pools are maintained with cyanuric acid, either added directly, or through the use of dichlor or trichlor tabs. But some users neglect to account for cyanuric acid’s dramatic effect on chlorine’s efficacy. 

Disinfection, oxidation, and algae inhibition rates are significantly reduced in the presence of cyanuric acid. Luckily, it doesn’t require much hypochlorous acid to kill many pathogens. Furthermore, chlorine bound to cyanuric acid is readily converted into active chlorine as the hypochlorous acid is used up. 

Cyanuric acid moderates chlorine’s strength, and on the positive side, it helps reduce bleaching to swim suits, or oxidizing hair. There is also evidence that it alters the amount and type of certain volatile disinfection by-products. These are all arguments for why it is appropriate to use a little cyanuric acid, perhaps about 15 ppm, in indoor pools. However, neglecting to account for cyanuric acid’s impact on the chlorine kill rate makes setting the current absolute limits on chlorine concentrations in swimming pools irrational. 

Specifically, industry recommendations currently set a maximum chlorine concentration of 4 ppm in swimming pools. In the presence of 100 ppm of cyanuric acid, which is the maximum allowed cyanuric acid according to many industry guidelines, 4-ppm chlorine would be ineffective against algae if the conditions were right to support its growth. 

When cyanuric acid is added to a pool, it affects the amount of active chlorine far more significantly than the pH. But without cyanuric acid, pH has an enormous effect. Without cyanuric acid, as the pH moves from 7.8 to 8.5, active hypochlorous acid moves from 30 percent to 8 percent. However, in the presence of 30-ppm cyanuric acid and 3-ppm chlorine, active hypochlorous acid only moves from 1.3 percent to 1 percent. 

To say it another way, with 3-ppm free chlorine and no cyanuric acid, changing the pH from 7.8 to 8.5 changes the actual hypochlorous acid concentration from about 0.96 to 0.26 ppm HOCl.  With 3-ppm free chlorine and 30-ppm cyanuric acid, changing the pH from 7.8 to 8.5 changes the active hypochlorous acid concentration from about 0.038 to 0.031.  Once cyanuric acid is present, pH has nowhere near as strong an effect on hypochlorous acid as it does without. Much more than pH, when cyanuric acid is present in a swimming pool, it controls the chemistry.

An accompanying article explores these ideas in greater detail. The take home message is that at the usual levels of chlorine and cyanuric acid, disinfection, oxidation, and algae inhibition rates are proportional to the free chlorine, cyanuric acid ratio.  It is this message that must inform industry recommendations concerning the maximum allowable absolute limits on chlorine concentrations.

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