Abstract
Breakpoint chlorination occurs in swimming pools and generates •OH and nitrosating agents. While •OH facilitates micropollutant removal, nitrosating agents promote nitrosamine formation. Cyanuric acid, a common chlorine stabilizer, reacts reversibly with free chlorine to form chlorinated cyanurates, effectively "locking" free chlorine and altering breakpoint chemistry. Accurate estimation of free chlorine is essential for evaluating these impacts and depends on the hydrolytic dissociation constants of chlorinated cyanurates, yet reported values vary widely. This study re-evaluates these constants and examines how cyanuric acid influences breakpoint reactions, focusing on the trade-off between •OH-driven micropollutant degradation and nitrosamine formation. Using phenolic probes, we show that electrochemically determined hydrolytic dissociation constants more accurately predict free chlorine concentrations under pool-relevant conditions than spectrophotometric values. Kinetic experiments reveal that chlorinated cyanurates participate in breakpoint reactions, chlorinating NH(2)Cl and NHCl(2) with rate constants approximately half of those of HOCl. Cyanuric acid also catalyzes NHCl(2) formation from NH(2)Cl. Under simulated pool conditions, cyanuric acid enhanced micropollutant removal, suppressed NCl(3) formation, but promoted nitrosamine formation. A refined kinetic model captured these trends and provided mechanistic insights. Cyanuric acid, while mitigating scavenging of •OH and nitrosating agents by oxidants, prolongs NCl(3)-NHCl(2) interactions, thereby increasing •OH and nitrosating agent yields while lowering residual NCl(3).