General applications of redox titrimetry environment public health and industrial analyses - chlorination of public water supplies. Chlorination of public water supplies Efficiency of chlorination depends on the form of the chlorinating species. free chlorine residual, due to Cl2, HOCl, and OCl- combined chlorine residual, formed by NH2Cl, NHCl2, and NCl3. Total chlorine residual: the oxidizing power of chlorine to convert I- to I3- I3- formed is then determined by a redox titration using S2O32- as a titrant and starch as an indicator. Chlorination of public water supplies Procedure sample volume: less than 20 mL of S2O32- to reach the end point. pH 3 to 4 (glacial acetic acid) add about 1 g of KI titrate with Na2S2O3 until the yellow color due to I3- begins to disappear. add 1 mL of a starch indicator solution, continue titration until the blue color of the starch-I3- complex disappears. Indirect analysis method The redox half-reaction when I- is used as a titrant is 3I- = I3- + 2e- Reaction product, I3-, is itself colored, the color of the solution containing the analyte changes with each addition of titrant. So it is difficult to find a suitable visual indicator for the titration's end point.
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