In this Sulfo-Cy5 NHS ester section, three finished water samples and one tap water sample were collected to examine the feasibility and applicability of the NaAsO2 selective quenching method. The total residual chlorine concentrations of these four samples were ranged from 0.54 to 1.51 mg/L as Cl2 (shown in Table S1). The proportion of [NHCl2] + [Organic chloramines] in the total residual chlorine was 46.3, 60.7, 39.4 and 32.4% (wt%) in Fig. 4a–d, respectively, which was non-negligible in all cases. After dosing the calculated amount of NaAsO2 using Eq. (8) to quantify ineffective chlorine, two groups of data were observed. In Fig. 4a and b, proportions of ineffective chlorine reached as high as 27.8% and 21.0%, respectively, while in Fig. 4c and d, the values were only 3.7% and 3.3%, respectively. The significant difference in the proportion of ineffective chlorine could be related to the characteristics of water samples, especially the DON concentration. It has been proven that the yield of organic chloramines increased with the increase of DON/DOC mass ratio , which indicated that more DON might react with chlorine to produce more organic chloramines. Because ineffective chlorine is involved in the part of organic chloramines, the relationship between ineffective chlorine and DON should also be the same as that between organic chloramines and DON, which can also be observed in Table S1. The samples with high proportions of ineffective chlorine (DWTP1 and TW in Fig. 4a and b) had high DON values (0.6 and 0.5 mg-N/L, respectively, in Table S1), while the samples with low DON values (<0.1 and 0.3 mg-N/L in Table S1) had low proportions of ineffective chlorine (DWTP2 and DWTP3, respectively in Fig. 4c and d). Considering humic acid solution at similar DON concentration of 0.1 to 0.2 mg-N/L (corresponding to DOC = 3 and 5 mg-C/L, respectively, in Fig. 3), the same conclusion could also be drawn that water samples with low DON values resulted in low percentages of ineffective chlorine in total chlorine (1.0% and 2.3%, respectively). Therefore, it indicated that the proportions of ineffective chlorine obtained in this study should be reasonable, and this selective quenching method by NaAsO2 should be feasible and applicable.