In order to verify the existence of ineffective chlorine, humic tsa trichostatin solutions with different DOC concentrations were prepared to react with free chlorine. The DOC, TDN, NO3−, NO2− and NH4+ values of humic acid stock solution were determined as 22.7 mg-C/L, 1.1 mg-N/L, 0 mg-N/L, 0.03 mg-N/L and 0.05 mg-N/L, respectively, and the DON was calculated as 1.0 mg-N/L with DON/DOC mass ratio of 0.04. Based on the previous study, organic chloramines were formed fast during chlorination at the first hour . Because ineffective chlorine is one part of organic chloramines, the chlorination time in this study was set as 30 min at initial free chlorine dose of 5 mg/L as Cl2 and pH 7.0 controlled by 5 mM phosphate buffer. As Fig. 3 shows, the total residual chlorine decreased as DOC increased from to 15 mg-C/L, but the ineffective chlorine quantified after the proposed NaAsO2 quenching method increased from <0.02 to 0.08 mg/L as Cl2. The results indicated that ineffective chlorine existed at an unnegligible proportion in total chlorine, which increased with increasing DOC concentration during chlorination. From Fig. 3, with the increase of DOC, the free chlorine concentration decreased obviously while [NH2Cl] and [NHCl2] + [Organic chloramines] increased slightly (NHCl2 and organic chloramines were put together because they were indivisible). At DOC concentrations higher than 10 mg-C/L (corresponding to DON ? 0.4 mg-N/L), almost all free chlorine was consumed, but the concentration of ineffective chlorine kept increasing, which indicated that ineffective chlorine could be produced fast before free chorine was totally consumed, and spindle apparatus could exist stably during the whole chlorination process. Otherwise, ineffective chlorine concentration should decline due to the shortage of free chlorine and the competitive reactions between chlorine and other organic compounds.