By Marcelle Dibrell

Perhaps the most important and simultaneously most misunderstood topic in the pool industry is the relationship between chlorine and cyanuric acid. Cyanuric acid matters a lot, regardless of concentration.

It is not an exaggeration to say that it controls the oxidation and disinfection chemistry of chlorine. And that’s because when cyanuric acid is present in a chlorinated pool, most of the chlorine is bound to cyanuric acid.

At pH 7.5, with 3 ppm free chlorine and 50 ppm cyanuric acid, 97% of the chlorine is chemically bound to cyanuric acid. Half of the remaining 3% chlorine is hypochlorous acid, which is the active form of chlorine that does the disinfecting and prevents algae. The other half is the hypochlorite ion.

The disinfecting form of chlorine (hypochlorous acid, HOCL) is roughly proportional to the free chlorine cyanuric acid ratio: HOCl FC/CYA For example, when the free chlorine is 5% of the cyanuric acid, the hypochlorous acid is relatively constant. At a free chlorine concentration of 2 ppm and a cyanuric acid concentration of 40 ppm, the hypochlorous acid concentration is .02006. If the free chlorine and cyanuric acid are doubled – preserving the ratio of free chlorine to cyanuric acid, the hypochlorous acid is .02029 – just about the same.

So, keeping the ratio constant creates the same amount of hypochlorous acid, which means it will have the same level of disinfection and oxidation from the chlorine at either concentration (the same prevention of algae, the same oxidation of bather waste, the same disinfection of bacteria, etc).

In practice, what this means is that if the cyanuric acid is increasing, as will occur with continuous use of stabilized chlorine such as trichlor or dichlor, it is critical to increase the free chlorine proportionally to get the same efficacy. If you don’t do this, by keeping the free chlorine at 2 ppm but allowing the cyanuric acid to increase by adding trichlor, the hypochlorous acid will drop and problems such as algae growth will begin to occur.

That this occurs is what led to the notion of “chlorine lock,” or the idea that chlorine is irreversibly bound up in the cyanuric acid. If you simply raise the free chlorine proportionally with the increased cyanuric acid, the chlorine will have the same efficacy that it had at lower cyanuric acid levels. However, this may become impractical at truly high cyanuric acid levels.

For example, if 2 ppm free chlorine and 40 ppm cyanuric acid was effective at preventing algae, but continuous use of trichlor caused the cyanuric acid to grow to 200 ppm, it will now take a free chlorine concentration of 10 ppm to have the same effect. The accompanying table illustrates the concept.

This special issue of Service Industry News features additional articles about cyanuric acid, the all important chemical controlling chlorine in most outdoor pools.