By Marcelle Dibrell
If it weren’t for cyanuric acid, you probably wouldn’t have a job.
Residential pool service that relies on once-weekly visits would not be possible without the introduction of cyanuric acid.
That’s because in the absence of cyanuric acid, the sun’s ultraviolet rays break down chlorine at an alarming rate.Added to water, chlorine exists in two forms: hypochlorous acid and hypochlorite ions. And while hypochlorous acid is relatively stable to UV decomposition, the hypochlorite ion (whose absorption maximum occurs from 290 nm out to about 350 nm) is readily decomposed by sunlight.
Because of this, without cyanuric acid, after just one hour of exposure of chlorine to sunlight, 75 percent is lost to decomposition reactions. Add a little cyanuric acid, and that loss is reduced by a lot.
In fact, the presence of 30 ppm cyanuric acid saves about 80 percent of free chlorine after an hour’s exposure to the sun.
And because of that, it is possible to add chlorine to a pool just once a week and return to find it still clean, clear, and algae free.
But there’s actually a lot more going on behind the scenes for pools that use cyanuric acid.
Here are six things we know that cyanuric acid does in a swimming pool:
• Cyanuric acid protects chlorine from breaking down in sunlight.
• Cyanuric acid controls the amount of hypochlorous acid in the water.
• Cyanuric acid slows chlorine’s oxidation/disinfection reactions.
• Cyanuric acid lowers ORP.
• Cyanuric acid changes the composition and concentrations of disinfection by-products.
• Cyanuric acid affects the Langelier Saturation Index (LSI).
A lot of these items are interrelated. For example, cyanuric acid protects chlorine from breaking down in sunlight by forming weak bonds with chlorine. This new complex doesn’t absorb wavelengths in the range that reach the earth’s surface.
Without cyanuric acid, chlorine exists in the water as hypochlorous acid and the hypochlorite ion. Hypochlorous acid absorbs at 235 nm. Hypochlorite absorbs at 292nm.
Meanwhile, cyanuric acid absorbs at about 215 nm, at a wavelength shorter than reaches the earth.
For this reason, it is much more stable to disentigration.
But because it forms these weak bonds with chlorine, it lowers the amount of hypochlorous acid that is in the water at any given time.
And because there is less hypochlorous acid in the water at any given time, it slows chlorine’s reactions, which are determined by its concentration. This is because reactions generally slow down when concentrations go down. Also determined by concentration is the oxidation reduction potential. Because there is less hypochlorous acid in the water at any given time, the ORP goes down.
And also because there is less hypochlorous acid in the water, fewer disinfection by-products can form.
Cyanuric acid and its complicated relationship with chlorine are only just beginning to be understood and embraced by the pool and spa industry. Some of what has been learned about the ways it impacts chlorination chemistry is so new, it has yet to make its way into the codes and guidelines adopted by municipalities.
In this issue of Service Industry News, we’ll take a closer look at how cyanuric acid controls your pools.
Cyanuric Acid And Kill Rates
The effect of cyanuric acid on chlorinated pool water is to lower the concentration of hypochlorous acid. However, this effect is the result of an equilibrium reaction between cyanuric acid and free chlorine. That means that as unbound hypochlorous acid gets used up performing its various oxidation/ sanitation reactions, cyanuric acid releases more to replace it.
However, because there is less hypochlorous acid present at any given time, it does perform a little more slowly as shown in the accompanying graphic.
In the graphic, one can see CT on the vertical axis. A CT value is simply the concentration (C) multiplied by the time (T) needed to kill or inactivate an organism. These values are generally reported for 3-log reductions of organisms.
A 1- log reduction would inactivate 90%, a 2-log reduction would inactivate 99%, and a 3-log reduction would inactivate 99.9% of the organisms.
CT values are usually given in the units of ppm·minutes. For this graphic, chlorine’s CT value of 1 ppm minute means that 1 ppm of chlorine was able to kill 99.9% of the organisms in 1 minute.
A CT value of 100 ppm minutes would means either that:
• 100 ppm was able to kill 99.9% of the organisms in 1 minute
• 1 ppm was able to kill 99.9% of the organisms in 100 minutes.
For all of the organisms shown in the graphic, it is evident chlorine’s CT values go up as cyanuric acid is increased. That means either: the same amount of chlorine takes longer to achieve killing, or more chlorine is needed to kill the organisms in the same amount of time.
What holds for the organisms studied also holds for algae, although there are fewer studies to support this because algae does not represent a public health risk. However, the chemical manufacturer Lonza performed a three-monthlong experiment at a test facility in Florida in which they added algae and synthetic bather load to swimming pools that contained varying concentrations of cyanuric acid. Each week, two days after the contaminant additions, they shocked the pools with 10 ppm calcium hypochlorite.
The accomapnying chart shows clear evidence of increased algae growth with elevated cyanuric acid levels.
At this point, readers may be convinced that an excessive amount of cyanuric acid might not be a good thing. But what is that amount?
It depends on how much chlorine you want to use… Industry expert Richard Falk derived a simple formula for the minimum free chlorine needed to prevent algae based on the amount of cyanuric acid. This formula is based two things:
• The amount of chlorine that pool operators have found to be effective in the presence of cyanuric acid.
• The concentration of hypochlorous acid this calculates out to be.
That minimum hypochlorous acid concentration corresponds to a free chlorine level that is 7.5 percent of the cyanuric acid level.
And while that may seem complicated, the math to prevent algae is easy.
FC = 7.5% X CYA
So, for example, if the measured cyanuric acid in a swimming pool is 40 ppm, then a pool operator should maintain a minimum free chlorine level of 3 ppm to prevent algae.
And at a measured cyanuric acid level of 80 ppm, you would need 6 ppm chlorine for algae prevention.
See where this is going? Keep in mind that the EPA mandates chlorine levels not exceed 10 ppm. And most municipalities set a cyanuric acid limit at 100 ppm.
But beyond the actual regulations, in the battle against algae, maintaining excessive levels of cyanuric acid is simply an unneccesary waste of chlorine.
Cyanuric Acid and Total Alkalinity
When cyanuric acid is added to the pool, it increases the total alkalinity, but not the alkalinity that matters. If this is not recognized, it can cause problems. That’s because it is the carbonate alkalinity that provides protection for plaster. If cyanuric acid is not subtracted from the total alkalinity test result, the service tech may believe that the alkalinity is fine, when in fact it may be dangerously low.
Low total alkalinity is damaging to pool surfaces, is corrosive to metals, and it makes it difficult to keep the pH stable.
Because the cyanurate ions don’t play any real role in corrosion or scaling, though, it should be subtracted out of the measured total alkalinity to get a number that is useful for understanding the water’s balance.
This is especially important when the cyanuric acid concentration is exceptionally high. When this occurs, the measured total alkalinity could lie within normal ranges, but the true carbonate alkalinity might be low, causing a corrosive environment. That would cause etching to plaster surfaces, corroded metal, and pH instability.
To calculate carbonate alkalinity at normal pH levels, subtract out one third of the cyanuric acid concentration. In other words, total alkalinity is corrected for cyanurate alkalinity by the following equation to yield carbonate alkalinity: Alkalinity corrected = Alkalinity total – 1/3 Cyanuric Acid Thus, if the test for total alkalinity shows 100 ppm, while the test for cyanuric acid shows 100 ppm, the carbonate alkalinity is roughly 47 ppm, which is too low.
80 – (.33 x 100) = 47 Note that the one third correction factor is pH dependant. That is because the cyanuric acid and cyanurate ion concentrations are also pH dependant. Therefore, it is best to use correction factors that account for this pH dependency.
The cyanuric acid correction factors may be found in the accompanying table.
Cyanuric Acid and ORP
ORP ( oxidat ion reduction potential) is a measurement that shows how effectively a single molecule is able to oxidize another molecule. It is generally used to determine how effective chemical disinfectants (usually chlorine) are at oxidizing others. When chlorine is added to a swimming pool that’s filled with contaminants, the chlorine should get rid of the contaminants. Chlorine is an oxidizing agent, which means that it takes electrons from the things that it encounters, both living and not. This serves to kill bacteria and algae, inactivate viruses, and break down bather-introduced waste. Measurements from an ORP sensor can help operators to identify if the chlorine or similar disinfectant is working as intended.
As has been mentioned in accompanying articles, cyanuric acid slows chlorine’s ability to react with contaminants in swimming pool water. This effect is reflected in the fact that it lowers the ORP measurement of the water, similar to how ORP measurements are lowered by increasing the pH. Hypochlorous acid is the predominant oxidizing agent in chlorine.
Because the cyanuric acid loosely bonds with chlorine, the primary oxidizer doesn’t get measured by ORP, and the ORP goes down. The World Health Organization recommends a minimum of of 650 mV for microbio-safe water. This number will correspond to different levels of free chlorine, depending on how much cyanuric acid is in the water.
See accompanying graphic. Note that even at 4 ppm free chlorine, the measured ORP sinks to 640 mV in the presence of 70 ppm cyanuric acid.
Its also important to note that ORP sensors’ measurements may vary from one manufacturer to another.
For this reason, for pools not subject to regulatory oversight, we recommend setting reasonable chlorine concentrations, as dictated by a chlorine-to-cyanuric-acid ratio, and determining the ORP that corresponds to these levels.