ROLE OF CYANURIC ACID (CYA) IN CHLORINATED OUTDOOR POOLS

People usually refrain from discussing politics and religion in an effort to avoid controversy. In the pool industry, the question of cyanuric acid's effect on total alkalinity can be as contentious.

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 stablized 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 stablization.

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 recommended pH and cyanuric acid levels, the cyanuric acid/cyanurate system significantly contributes to the buffering of pool water.

In addition to stabilizing available chlorine and buffering pool water, 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.

To calculate the carbonate alkalinity use the following formula:

AlkC = AlkTA - (Cy x CyF)
where AlkC = carbonate alkalinity; AlkTA = measured total alkalinity;
Cy = measured cyanuric acid; and
CyF = cyanuric acid factor.

For waters within recommended pH and cyanuric acid levels, using a CyF equal to 0.33 is appropriate. For waters outside the recommended pH range or waters containing high cyanuric acid, it becomes more important to use a CyF from the table below:

pH

6.5

7.0

7.2

7.4

7.8

8.0

CyF

.11

.22

.26

.30

.35

.36


For example: if pH = 7.4; AlkTA = 100 ppm; and Cy = 50 ppm:
AlkC = AlkTA - (Cy x CyF)
AlkC = 100 - (50 x 0.30)
AlkC = 85 ppm

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