Chlorine Stabilization

Ideal range: 60 – 80 ppm

chlorine stabilizationIn 1958, cyanuric acid (a week acid often abbreviated CYA) was discovered to inhibit chlorine depletion by sunlight. In other words, cyanuric acid stabilized the free chlorine residual from sunlight destruction. When CYA is dissolved in water at 25 ppm, FC (free chlorine) lasts three to five times longer than pool water not containing CYA. Above 50 ppm, only marginal stabilization benefit is observed. Note: Cyanuric acid stabilizes hypochlorous acid Regardless of the form of chlorine used!

Cyanuric acid is an odorless, white granular substance. It is also known by such names as stabilizer, isocyanuric acid, conditioner and CYA. When dissolved in pool water, FC residuals last three to five times longer in the presence of sunlight.


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Although cyanuric acid possesses all these benefits, it also has a significant downside. In the process of stabilizing FC, cyanuric acid causes the chlorine to slow down or become less effective both as an oxidizer and a sanitizer. This must be compensated for by using a higher concentration of chlorine in the pool.



In 1958, two new products  for pool water sanitation were introduced. Called chlorinated isocyanurates, or simply “isos,” they are sanitizer and stabilizer all in one. The key to understanding the stabilized chlorines “trichlor” and “dichlor” is that their chlorine is attached to a CYA molecule. Unstabilized chlorines – chlorine gas and sodium, calcium, and lithium hypochlorites – do not have a CYA component. Outdoor pools sanitized with unstabilized chlorine must have CYA added seperately.

Although this one-step process has made isos very popular, operators need to be aware that CYA can build up unintentionally to an objectionable level. This is because after the chlorine part of an iso compound has been depleted, the cyanuric acid component remains behind, dissolved in the water. CYA can accumulate faster than splash-out, carry-out, and backwashing removes it. For this reason, better test kits allow you to monitor CYA.

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