Interference in Melamine–based Determination of Cyanuric Acid Concentration

Isocyanuric acid (commonly referred to in the swimming pool/spa industry as cyanuric acid, stabilizer, conditioner, CYA, etc., and referred to simply as cyanuric acid in this paper) is a member of the symmetrical triazine family, and it is used in outdoor swimming pools and spas to protect chlorine against loss from ultraviolet radiation from the sun. Depending on factors such as season and climate, UV–instigated loss of chlorine can be substantial and costly, and consequently the swimming pool industry makes use of cyanuric acid to retard this loss. The effect of cyanuric acid on the sanitizing capability of chlorine has been investigated and documented. The greatest benefit is, of course, the reduction of chlorine loss due to UV. UV degradation takes as much as four times longer in water containing 35 mg/L cyanuric acid and 1 to 3 mg/L chlorine. However, as the concentration of cyanuric acid increases in water with a constant concentration of chlorine, the ORP decreases and the kill–time increases. Anderson notes that “the addition of 25 mg/L of cyanuric acid at a pH of 7.0 results in a greater decrease in the bactericidal properties of chlorine than an increase in pH of 7.0 to 9.0 in the absence of cyanuric acid” (Anderson 1964). Although pH levels of 9.0 are not common in swimming pools, standard pool pH levels of 7.4 to 8.0 and cyanuric acid levels of 25 to 200 mg/L are common, and are great enough in range for the decrease in biocidal activity to be in evidence. Cyanuric acid is generally added to swimming pools/spas via one of two methods: the addition of specific amounts of cyanuric acid powder or granules to the water to achieve a specific desired concentration, or the use of chlorinated isocyanurates, which method controls the chlorine residual but continuously increases the cyanuric acid concentration regardless of the necessity for additional cyanuric acid in the water. Both methods form the desired compounds in the water. “The effect of cyanuric acid is the same at a given concentration whether it is derived from the acid itself or whether it enters the pool with one of the convenient forms of chlorinated isocyanurate disinfectants” (Monsanto 1975 VI–1).