Why carbonate alkalinity matters to CYA
The use of cyanuric acid comes with consequences and one of the most overlooked consequence is that it affects total alkalinity. It is not a problem, unless it is not accounted for, and some pool care technicians fail to recognize its effect on this crucial water balance parameter.
Low total alkalinity is damaging to pool surfaces, corrosive to metals, and makes it difficult to keep the pH stable.
When cyanuric acid is added to the pool, it increases the total alkalinity, but not the carbonate alkalinity, which is the alkalinity that matters in water balance.
When measuring total alkalinity, one is really interested in carbonate alkalinity, because it is high or low carbonate levels that cause corrosion or scaling respectively.
When cyanuric acid is added to the pool water, it increases the total alkalinity due to the formation of the cyanurate ion, but not the carbonate alkalinity, which is the part of the alkalinity that is relevant.
Total alkalinity is actually a measure of carbonate, bicarbonate, hydroxyl and cyanurate ions.
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 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 this occurs, the measured total alkalinity may fall within normal ranges, but the true carbonate alkalinity might be exceedingly low, causing an extremely corrosive environment. This circumstance would cause etching to plaster surfaces as well as corroded metals.
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 corrected =
Alkalinity total – 1/3 Cyanuric Acid Thus, if the test for total alkalinity shows 90 ppm, while the test for cyanuric acid shows 100, the carbonate alkalinity is roughly 57, which is a little low.
90 – (.33 x 100) = 57 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. Therefore, it is best to use corrections factors that account for this pH dependency. The cyanuric acid correction factors may be found in the accompanying table.