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    mirna sherry's Avatar
    mirna sherry Posts: 3, Reputation: 2
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    #1

    May 8, 2009, 02:22 PM
    Why after titration of I- with S2O3(2-) using starh as an indicator a blue reappears
    When I titrated iodide with S2O3(2-) I used starch as an indicator, at equivalence the color changed to colorless. Then I left the solution aside after a while the blue color reappeared.
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
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    #2

    May 8, 2009, 02:33 PM

    Iodide was probably oxidized by the oxygen in air to iodine. The only other explanation is that there is a trace of iodine that was bound in some precipitate or something like that -- when that iodine was released, it reacted with the starch. I'm not sure if I can think of any other explanation.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #3

    May 8, 2009, 10:41 PM

    Or perhaps did some Iodine was on the inner wall of your flask, then it slowly came down, adding to the titrate more iodine, thus the blue colour reappeared.
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