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    The titration
    Cr2O7 2- +6I- --> 2Cr- +3I2+ 7H2O
    And
    I2+ 2S2O3 2- --> 2I- + S4O6 2-
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    (Original post by Hazel99)
    The titration
    Cr2O7 2- +6I- --> 2Cr- +3I2+ 7H2O
    And
    I2+ 2S2O3 2- --> 2I- + S4O6 2-

    Rather, your question should be "when"....so and so.

    Starch in the presence of iodine would be blue-black and when all iodine is not around anymore, then discolouration to colourless occurs.

    Or vice versa, depending on what you start with and what you expect to end up with.
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    The equivalent questions is, "why do you add methyl orange (et.al.) to an acid/alkali titration?"

    To see an obvious colour change at molar equivalence (or nearby).
 
 
 
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