The objectives of this study

The objectives of this study are (1) to compare different reduction reagents used as quenchers for total chlorine, (2) to optimize the quenching condition for ineffective chlorine determination using 'Sulfo-NHS-LC-Biotin' NaAsO2, (3) to examine the formation of ineffective chlorine during chlorination of Suwanne River humic Sulfo-NHS-LC-Biotin and in finished waters from DWTPs, and (4) to investigate the sterilization efficacy of ineffective chlorine by comparing with other residual chlorine species.
2. Materials and methods
2.1. Chemicals
All reagents used were at least of analytical grade. Sodium hypochlorite (NaOCl, 4–4.99%), ammonium chloride (NH4Cl, ?99.5%), DPD sulfate (?99.0%), potassium iodide (KI, ?99.0%), potassium phosphate monobasic (KH2PO4, ?99.0%) and sodium phosphate dibasic (Na2HPO4, ?99.0%) were all purchased from Sigma–Aldrich (USA). Suwannee River humic acid (standard II) was purchased from International Humic Substances Society (IHSS, USA). Other chemicals including sodium sulfite (Na2SO3), sodium hydrogen sulfite (NaHSO3), sodium thiosulfate (Na2S2O3), sodium arsenite (NaAsO2) and ascorbic acid were obtained from Sinopharm Chemical Reagent Co., Ltd. (China) without further purification. All solutions were prepared using ultrapure water produced from a Milli-Q water purification system (Millipore, USA) with total chlorine content under the method detection limit (MDL).