Determination of Total Chlorine Residual Description of the Method The chlorination of a public water supply produces several chlorinecontaining species, the combined concentration of which is called the total chlorine residual. Chlorine is present in a variety of chemical states, including the free residual chlorine, which consists of Cl2, HOCl and OCl–, and the combined chlorine residual, which consists of NH2Cl, NHCl2, and NCl3. The total chlorine residual is determined by using the oxidizing power of chlorine to convert I– to I3-. The amount of I3- formed is then determined by titrating with Na2S2O3 using starch as an indicator. Regardless of its form, the total chlorine residual is reported as if Cl2 is the only source of chlorine, and is reported as mg Cl/L. The description here is based on Method 4500-Cl B as published in Standard Methods for the Examination of Water and Wastewater, 20th Ed., American Public Health Association: Washington, D. C., 1998 Procedure Select a volume of sample that requires less than 20 mL of Na2S2O3 to reach the endpoint. Using glacial acetic acid, acidify the sample to a pH between 3 and 4, and add about 1 gram of KI. Titrate with Na2S2O3 until the yellow color of I3- begins to disappear. Add 1 mL of a starch indicator solution and continue titrating until the blue color of the starch–I3- complex disappear (see figure below). Use a blank titration to correct the volume of titrant needed to reach the end point for reagent impurities. Endpoint for the determination of the total chlorine residual. (a) Acidifying the sample and adding KI forms a brown solution of I3-. (b) Titrating with Na2S2O3 converts I3- to I– with the solution fading to a pale yellow color as we approach the end point. (c) Adding starch forms the deep purple starch–I3- complex. (d) As the titration continues, the end point is a sharp transition from a purple to a colorless solution. The change in color from (c) to (d) typically takes 1–2 drops of titrant. Questions 2. Why does the procedure rely on an indirect analysis instead of directly titrating the chlorine-containing species using KI as a titrant?
Determination of Total Chlorine Residual
Description of the Method
The chlorination of a public water supply produces several chlorinecontaining species, the combined concentration of which is called the total chlorine residual. Chlorine is present in a variety of chemical states, including the free residual chlorine, which consists of Cl2, HOCl and OCl–, and the combined chlorine residual, which consists of NH2Cl, NHCl2, and NCl3. The total chlorine residual is determined by using the oxidizing power of chlorine to convert I– to I3-. The amount of I3- formed is then determined by titrating with Na2S2O3 using starch as an indicator. Regardless of its form, the total chlorine residual is reported as if Cl2 is the only source of chlorine, and is reported as mg Cl/L.
The description here is based on Method 4500-Cl B as published in Standard Methods for the Examination of Water and Wastewater, 20th Ed., American Public Health Association: Washington, D. C., 1998
Procedure
Select a volume of sample that requires less than 20 mL of Na2S2O3 to reach the endpoint. Using glacial acetic acid, acidify the sample to a pH between 3 and 4, and add about 1 gram of KI. Titrate with Na2S2O3 until the yellow color of I3- begins to disappear. Add 1 mL of a starch indicator solution and continue titrating until the blue color of the starch–I3- complex disappear (see figure below). Use a blank titration to correct the volume of titrant needed to reach the end point for reagent impurities. Endpoint for the determination of the total chlorine residual. (a) Acidifying the sample and adding KI forms a brown solution of I3-. (b) Titrating with Na2S2O3 converts I3- to I– with the solution fading to a pale yellow color as we approach the end point. (c) Adding starch forms the deep purple starch–I3- complex. (d) As the titration continues, the end point is a sharp transition from a purple to a colorless solution. The change in color from (c) to (d) typically takes 1–2 drops of titrant.
Questions
2. Why does the procedure rely on an indirect analysis instead of directly titrating the chlorine-containing species using KI as a titrant?
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