In any discussion of cyanuric acid, it is essential to mention that cyanuric acid contributes to total alkalinity. This is important to realize for those responsible service professionals who wish to maintain their pools within the recommended Langelier Saturation Index (LSI) range.
The use of cyanuric acid comes with consequences, and among the most overlooked consequence is that it affects total alkalinity. It is not a problem, unless it is not accounted for, but some pool care technicians fail to recognize its effect on this crucial water balance parameter.
Low total alkalinity is damaging to pool surfaces, is corrosive to metals, and it makes it difficult to keep the pH stable.
When cyanuric acid is added to the pool, it increases the total alkalinity, but not the alkalinity that matters. And if this is not recognized, it can cause problems.
When measuring total alkalinity, we are really interested in carbonate alkalinity, because it is high or low carbonate levels that cause corrosion or scaling respectively.
Water needs a certain amount of minerals to be balanced, and if it doesn’t have them, it will draw them from various pool surfaces in a process called corrosion. If, on the other hand, the water is oversaturated with minerals, the minerals will precipitate out in a process called scaling.
When cyanuric acid is added to the pool water, it increases the total alkalinity due to the formation of the cyanurate ion.
That’s because total alkalinity is actually a measure of carbonate, bicarbonate, hydroxyl, and cyanurate ions.
But because the cyanurate ions don’t play any real role in corrosion or scaling, it should be subtracted out of the measured total alkalinity in order to get a number that is useful for understanding the water’s balance.
This is especially important when the cyanuric acid concentration becomes particularly high. When that occurs, the measured total alkalinity may fall within normal ranges, but the true carbonate alkalinity might be exceedingly low, causing a corrosive environment. This circumstance would cause etching to plaster surfaces as well as corroded metals and pH instability.
At normal pool pH levels, to calculate carbonate alkalinity, 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 total – 1/3 Cyanuric
Acid = Alkalinity corrected
Thus, if the test for total alkalinity shows 90 ppm, while the test for cyanuric acid shows 100 ppm, the carbonate alkalinity is roughly 57, which is a little low.
90 ppm – (1/3 x 100 ppm) = 57 ppm
It is important to note that the one third correction factor is pH dependent. That is because the cyanuric acid and cyanurate ion concentrations are pH dependent, where at higher pH levels, the effect of the concentration of the cyanurate ion is more significant. 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. How much CYA am I adding when…
One common question is: How much cyanuric acid is introduced to pools from the addition of a tab of trichlor, or from dosing with dichlor.
The following practical examples illustrates how dosage calculations help pool operators make informed decisions about chemical additions for optimal water balance.
Trichlor tabs typically contain 54 percent cyanuric acid. The most common tabs are either 1-inch or 3-inch slow-release tabs.
A 3-inch tab weighs about 8 ounces, or 0.5 pounds, because there are 16 ounces in a pound. Of that, 0.27 pounds is cyanuric acid (54% x 0.5 pounds = 0.27 pounds).
So how much cyanuric acid is added to 10,000 gallons every time we add a 3-inch tab? It’s about 3 ppm:
(0.27 pounds) (÷) (10,000 x 8.34) x 1,000,000 = 3.24 ppm
And because there is little loss of cyanuric acid from a pool, it is easy to see how quickly a once-weekly addition of a tab will result in an over-accumulation of cyanuric acid.
Do the same math for dichlor.
Common usage instructions for shock use of dichlor recommend adding 1 pound of dichlor to 12,000 gallons of water.
Dichlor contains about 57 percent cyanuric acid, which means that 0.57 pounds of that is cyanuric acid (57% x 1 pound = 0.57) so:
(0.57 pounds) (÷) (12,000 x 8.34) x 1,000,000 = 5.6 ppm
If you started the season with 40 ppm cyanuric acid, a weekly shock addition of dichlor would bring the cyanuric acid to nearly 130 ppm by the end of the summer.
That is well beyond currently accepted industry guidelines.
