Dechlorination Add a reducing agent to containers intended for the collection of water having residual chlorine or other halogen unless they contain broth for direct incubation of sample. Sodium thiosulfate (Na2S2O3) is a satisfactory dechlorination agent that neutralizes any residual halogen and prevents continuation of bactericidal action during sample transit. The examination then will indicate more accurately the true microbial content of the water at the time of sampling. For sampling chlorinated wastewater effluents add sufficient Na2S2O3 to a clean sample bottle to give a concentration of 100 mg/L in the sample. In a 120-mL bottle 0.1 mL of a 10% solution of Na2S2O3 will neutralize a sample containing up to 15 mg/L residual chlorine. For drinking water samples, the concentration of dechlorination agent may be reduced: 0.1 mL of a 3% solution of Na2S2O3 in a 120-mL bottle will neutralize up to 5 mg/L residual chlorine. See Table 9060:I for preparation of sodium thiosulfate solutions. Where possible, determine normal residual chlorine before sampling at a new site (e.g., pool water may contain a higher chlorine level than normal) to enable laboratory to prepare an adequate amount of dechlorination agent per sample bottle. Discard turbid (bacterial growth) 10% sodium thiosulfate stock solutions. Loosely cap bottle and sterilize by either dry or moist heat, as directed (Section 9040) and perform sterility checks as noted in Section 9020B.5d. Presterilized plastic bags or bottles containing Na2S2O3 are available commercially. Ref : Water board, GOI