Role Of Cyanuric Acid (CYA) In Chlorinated Outdoor Pools

What is the role of cyanuric acid in pool water? Does it affect the total alkalinity reading? Should the test results be adjusted? If so, by how much?

Here's our take on cyanuric acid, total alkalinity, and their relationship.

As a quick refresher, total alkalinity is the measure of the ability of water to resist changes in pH, or its "buffering capacity."

Cyanuric acid, also called stabilizer, is commonly used in outdoor pools to reduce photodecomposition of available chlorine. When added to pool water, a fraction of the cyanuric acid (H₃Cy) ionizes to form cyanurate (H₂Cy^-). The fraction that ionizes is pH-dependent.

When cyanuric acid is added to pools using unstabilized chlorine, the ionized cyanurate combines with available chlorine to form stable chloroisocyanurates.

In pools using stabilized chlorine, stable chloroisocyanurates are formed without the separate addition of cyanuric acid since stabilized chlorine sanitizers contain both chlorine and cyanuric acid as part of the molecule. However, an initial dosage of 20 ppm cyanuric acid is recommended to provide enough cyanurate for immediate stabilization.

Cyanuric acid also affects the buffering of pool water. A buffer system is composed of a weak acid and its salt. In pool water containing cyanuric acid, the predominate buffer systems are carbonic acid/bicarbonate and cyanuric acid/cyanurate. The buffer intensity of these buffer systems is both pH- and concentration-dependent. At ideal pH (7.4 to 7.6) and cyanuric acid levels (30 to 50 ppm), the cyanuric acid/cyanurate system will not significantly contribute to the buffering of pool water. However, as the cyanuric acid levels increase, they will have an effect on the total alkalinity test result.

The cyanuric acid/cyanurate system contributes to total alkalinity since total alkalinity is the sum of all titratable alkaline substances and cyanurate is a titratable alkaline substance. Therefore, the total alkalinity titration measures both carbonate and cyanurate alkalinities. This affects water balance calculations because the alkalinity term in the Saturation Index equation is strictly carbonate alkalinity.