Cyanuric acid is added to swimming pools to reduce photochemical decomposition of free available chlorine (av. Cl) by ultraviolet (UV) rays in sunlight. This is accomplished by formation of chloroisocyanyurates that do not appreciably absorb UV light. Recent data show that the av. Cl loss rate (at 80-85ºF) caused by sunlight is only 1-2% per day at 138 ppm CA. This is consistent with the fact that over 99% of the av. Cl in CA stabilized pools is in the form of chloroisocyanurates. Virtually all of the decomposition (about 14% per day) observed in a bather-free pool in the summer time in the Phoenix area is due to thermal decomposition of chloroisocyanurates, which increases by a factor of 2 for each 5º rise in temperature. Bather load can appreciably increase the av. Cl loss rate.
Cyanuric acid, in combination with its ionized form (i.e., cyanurate ion), also functions as a buffer to help stabilize pH.