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Why can't potassium hexacyanoferate(III) be used to test for Fe3+?

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    Just quoting in Puddles the Monkey so she can move the thread if needed
    (Original post by Puddles the Monkey)

    (Original post by Shiv Loves Maths)
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    Potassium hexacyanoferate(III) already contains iron in the +3 oxidation state. There is no change when Fe3+ interacts with the hexacyanoferrate ion.

    However, when Fe2+ interacts there is a possibility of charge transfer between the two iron ions in different oxidation states which absorbs energy in the visible region of the spectrum and an intense blue colour is produced.

    The same thing happens if hexacyanoferrate(II) reacts with Fe3+ ions.

    [Fe(CN)6]3- + Fe2+ --> [FeFe(CN)6]-

    [Fe(CN)6]4- + Fe3+ --> [FeFe(CN)6]-

    The potassium salt of the compounds used to be called Turnbull's Blue and the other Prussian Blue, but they are chemically identical.
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