Does cyanuric acid raise or lower pH?

The short answer is yes. Cyanuric acid will lower the pH of pool water.

Cyanuric acid is a real acid and the pH of 0.1% cyanuric acid solution is 4.5. It doesn’t seem to be very acidic while the pH of 0.1% sodium bisulfate solution is 2.2 and the pH of 0.1% hydrochloric acid is 1.6. But please note that the pH of swimming pools is between 7.2 and 7.8 and the first pKa of cyanuric acid is 6.88. This means that most cyanuric acid molecules in the swimming pool can release a hydrogen ion and the ability of cyanuric acid to lower pH is very close to that of sodium bisulfate which is usually used as a pH reducer.

For example:

There is an outdoor swimming pool. The initial pH of pool water is 7.50, the total alkalinity is 120 ppm while the cyanuric acid level is 10 ppm. Everything is in working order except for the zero cyanuric acid level. Let us add 20 ppm of dry cyanuric acid. Cyanuric acid slowly dissolves, usually taking 2 to 3 days. When cyanuric acid is completely dissolved the pH of pool water will be 7.12 which is lower than the recommended lower limit of pH (7.20). 12 ppm of sodium carbonate or 5 ppm of sodium hydroxide is needed to add to adjust the pH problem.

Monosodium cyanurate liquid or slurry is available in some pool stores. 1 ppm monosodium cyanurate will increase cyanuric acid level by 0.85 ppm. Monosodium cyanurate is fast soluble in water, so it is more convenient to use and can quickly increase cyanuric acid levels in swimming pool. Contrary to cyanuric acid, monosodium cyanurate liquid is alkaline (the pH of 35% slurry is between 8.0 to 8.5) and slightly increases the pH of pool water. In the above-mentioned pool, the pH of pool water would increase to 7.68 after adding 23.5 ppm of pure monosodium cyanurate.

Do not forget that cyanuric acid and monosodium cyanurate in pool water also act as buffers. That is, the higher the cyanuric acid level, the less likely the pH will drift. So please remember to retest the total alkalinity when the pH of pool water is needed to adjust.

Also note that cyanuric acid is a stronger buffer than sodium carbonate, so pH adjustment requires adding more acid or alkali than without cyanuric acid. 

For a swimming pool in which the initial pH is 7.2 and the desired pH is 7.5, the total alkalinity is 120 ppm while the cyanuric acid level is 0, 7 ppm of sodium carbonate is needed to meet the desired pH. Keep the initial pH, the desired pH and the total alkalinity is 120 ppm unchanged but change the cyanuric acid level to 50 ppm, 10 ppm of sodium carbonate is needed now.

When pH needs to be lowered, cyanuric acid has less impact. For a swimming pool in which the initial pH is 7.8 and the desired pH is 7.5, the total alkalinity is 120 ppm and the cyanuric acid level is 0, 6.8 ppm of sodium bisulfate is needed to meet the desired pH. Keep the initial pH, the desired pH and the total alkalinity is 120 ppm unchanged but change the cyanuric acid level to 50 ppm, 7.2 ppm of sodium bisulfate is needed — only a 6% increase of the dosage of sodium bisulfate.

Cyanuric acid also has an advantage that it will not form scale with calcium or other metals.