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REDOX titration of bleach help!

  1. Nov 7, 2007 #1
    I am performing a REDOX titration of a diluted bleach sample in my analytical chemistry class. I am using sodium thiosulfate as the titrant to determine the amount of sodium hypochlorite ion in solution. I am using a starch indicator, which is supposed to turn from blue to colorless at the endpoint. The solution itself turns colorless at a point, but excess blue particles, of what i am assuming are starch, still float around. Does anybody know if the endpoint of the titration is when the solution itself becomes colorless, or if by adding excess titrant the endpoint is when the particles in the colorless solution become colorless???
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  3. Nov 7, 2007 #2


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    Your description would be more thorough if you stated the use of excess KI to allow for the formation of triiodide. You also titrate in the presence of acid? to help the titration reaction? The blue color occurs by the interaction or reaction between triiodide and starch. When enough thiosulfate has been added to reduce the iodine (thereby eliminating the triiodide), the blue color disappears.

    The starch used as an indicator makes seeing the triiodide easier.
  4. Nov 7, 2007 #3
    iodide from excess KI is a reducing agent, it is oxidised by the hypochlorite ion to iodine. Iodine is an oxidsing agent. The iodine liberated will then oxidise the thiosulphate.

    Iodine is normally brown when in solution. actually it reacts with iodide from the excess KI to form tri iodide ion, and in this form, the iodine becomes soluble.

    the tri iodide gives a blue black colour with starch. when the blue black colour disappears, this means that all the tri iodide has reacte with the thiosulphate.

    the particles you see are iodine which has not dissolved. the iodine has a tendency to reappear during such titrations. you should shake the solution well, or maybe you should try adding another drop of thiosulphate, and then shake again.

    the end point is when the solution becomes colourless.
  5. Nov 7, 2007 #4


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    The amount of starch in the solution should be small enough that it doesn't form chunks. How much starch did you add?
  6. Nov 7, 2007 #5
    I made the starch indicator with 2 grams of soluble starch in 500 mL of heated water, I used about 1-2 mL for the titration. The analyte consisted of a 10 mL aliquot of dilute bleach (10 mL pure bleach in 250 mL water), 1 mL 3 M H2SO4, and 2 grams of excess KI. The titration required about 67 mL of .01 M Sodium Thiosulfate to reach the "endpoint" where the solution turns from dark greenish-blue to clear, with small bits of dark purple-blue particles floating around. When about 10 mL more titrant is added, the blue particles become white.
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