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  1. #1

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    Potassium Iodide

    Just wondering if anyone has any information or experience re: the stability of potassium iodide solutions of from .25% to 2% stored in water.

    Sandy

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    They're stable.

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    Potassium iodide solutions (any concentration) must be kept in fridge, and with a piece of aluminium foil wrapped around the bottle - to prevent light from coming in. Once the solution turns yellowish due to oxidation to elementary iodine, it should be discarded. I estimate the shelf life of KJ solution as a year or two, definitely not more.

    Regards, Zhenya

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    You are confused between iodine (element) and iodide. Iodide (ion) is colorless in aquaous solution and is stable.

    Quiz. (5 points)
    What's the electron configuration of iodine? What about iodide? What's the difference and what can be said about their stability from that alone?

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    Let's try... iodine is in 7th main group of Mendeleev periodical table, that gives us s2p5... lowest oxidation level is -1, one electron more to form an octet. That configuration is indeed quite stable, but the atomic radius of iodine is seriously higher than that of, say, chlorine - and the electronegativity of iodine is lower, so the electron is more motile. Iodides are way wuch more prone to oxidation compared to other halides due to lower Gibbs energy of this reaction. If chlorides requires electrolysis or strong stuff like acidic permanganate to yield chlorine, iodides are much more fragile.

    Well, iodide is indeed colourless while fresh, but any other moiety with good oxidative properties (e. g. ozone, or peroxides from plasticware) turns its solutions yellow - for example, KJ + Cl2 = KCl + J2. We routinely use 6M solutions of KJ in our lab, and it becomes yellow with time - can't say what exactly oxidizes iodide to iodine.

    Cheers, and good luck - Zhenya

    Quote Originally Posted by Ryuji
    You are confused between iodine (element) and iodide. Iodide (ion) is colorless in aquaous solution and is stable.

    Quiz. (5 points)
    What's the electron configuration of iodine? What about iodide? What's the difference and what can be said about their stability from that alone?

  6. #6
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    Just FYI, Sodium Iodide is known to decompose spontaneously when in solution or as a solid, and Potassium Iodide is also known to decompose spontaneously but at a vastly slower rate than the sodium salt. It is so slow by comparison that it may be considered stable especially if kept in the dark or refrigerated.

    Sodium Iodide is not normally used in photographic preparations for this reason. The presence of free iodine can wreak havoc with solutions and emulsions.

    PE

  7. #7
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    Isn't a potassium iodide solution useful in testing papers for residual hypo somehow?

    Quote Originally Posted by eumenius
    Let's try... iodine is in 7th main group of Mendeleev periodical table, that gives us s2p5... lowest oxidation level is -1, one electron more to form an octet. That configuration is indeed quite stable, but the atomic radius of iodine is seriously higher than that of, say, chlorine - and the electronegativity of iodine is lower, so the electron is more motile....
    Talk like this about motile electrons and electronegativity gets me all hot and bothered.

    I once had a dream about "Francium flouride." Think about it.



    Joe

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    The potassium iodide test is used to (unreliably) test for the exhaustion of fixer. If exhausted, a yellow cloudy precipitate forms, but sometimes it gives a false positive or negative.

    PE

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    He-he, what a swell dream! It's almost like my dream about cesium astatide

    I was just solving hte quiz, it's five points! Can't really say what it means, but whatever - you ask the chemist about electrons, you get massive output

    Cheers,
    Zhenya

    Quote Originally Posted by smieglitz
    I once had a dream about "Francium flouride." Think about it.


    Joe

  10. #10
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    Correct me is I'm wrong (do I have to ask?) but I have seen potassium iodide on my druggist's shelf without any statement of limites shelf life.
    Gadget Gainer

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