Potassium Iodide

Discussion in 'B&W: Film, Paper, Chemistry' started by sanking, Sep 10, 2005.

  1. sanking

    sanking Member

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    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
     
  2. Ryuji

    Ryuji Member

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  3. eumenius

    eumenius Member

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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
     
  4. Ryuji

    Ryuji Member

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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?
     
  5. eumenius

    eumenius Member

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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

     
  6. Photo Engineer

    Photo Engineer Subscriber

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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. smieglitz

    smieglitz Member

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

    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.

    :smile:

    Joe
     
  8. Photo Engineer

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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
     
  9. eumenius

    eumenius Member

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

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

    Cheers,
    Zhenya

     
  10. gainer

    gainer Subscriber

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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.
     
  11. smieglitz

    smieglitz Member

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    Hmmm? Guess you had to be there.

    Joe
     
  12. eumenius

    eumenius Member

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    The potassium iodide itself keeps well in the dark and when dry - but I found some older preparations from our lab shelves (likely from 60s) to be quite yellow, and to possess the smell of iodine. Maybe something from the packaging or from outside spoiled them, that doesn't seem to be a spontaneous decomposition. Iodides of alkaline metals seem to be light-sensitive too, like silver iodide - but to a much much smaller degree.

     
  13. Justin Low

    Justin Low Member

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    I apologise for rehashing this old discussion, but I'd like to know, would making a 10% solution of KI be problematic in terms of solubility and longevity?

    I ask this because I have a 10g sachet of KI, but my scale is only precise to 1.0g. I was thinking that I could dissolve 10g of it in 100ml of water, and derive less concentrate solutions from that.

    If anyone's curious, I'm trying to mix up some FX-1, which calls for a minute amount of KI. I think the 10g will last me 'quite' a long time. :wink:

    Thanks in advance!

    Justin
     
  14. Photo Engineer

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    Justin, KI (Potassium Iodide) as stated here is not completely stable in solution and will decompose. Your 10% solution is easy to mix, but will probably go bad within a year unless kept refrigerated.

    Even then, a years keeping might be chancy but I'm not sure, never having tested it out. I refrigerate mine and keep it for about 6 months, then mix fresh.

    PE
     
  15. Justin Low

    Justin Low Member

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    PE, thanks for the information. I guess I should look for a more precise scale. :smile:
     
  16. Alan Johnson

    Alan Johnson Subscriber

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    For over 25 years starting 1962 the formula of FX-1 was published in BJP annual and referring to the 0.001% solution of potassium iodide the copies I have said "this keeps for 2 years at least".It is suggested to dissolve 1g in 1000ml water,dilute 100ml to 1000ml,then 100ml to 1000ml again to get a 0.001% solution.
     
  17. sanking

    sanking Member

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    A small amount? Yes, I would say that 5ml per liter of working solution of a 0.001% solution is indeed a small amount.

    Wonder why so little. I have experimented with potassium iodide in several formlas and it takes much, much more than that to have any anti-fogging qualities. But perhaps anti-fogging is not its purpose in FX-1?

    Sandy

     
  18. Alan Johnson

    Alan Johnson Subscriber

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    Originally (BJP Jan6 1961) G.W.Crawley wrote to the effect that the micro concentration of potassium iodide probably worked with some but not all films of that time by shortening the inductance period and "setting" fine surface detail early on.The effect of similar concentrations of oxidation products had been known for some time.The iodide ion may enfeeble the developing agency and predispose it to produce adjacency effects.Eberhard effects only form in a weak developer,metol<0.5g/L ,sulfite<6g/L.
    If this still is the case with modern films appears not to have been commented on.
     
  19. Alan Johnson

    Alan Johnson Subscriber

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    As a footnote to this,I wrote to Geoffrey Crawley to ask if the very small amount of potassium iodide in FX-1 produces edge effects with modern films. He replied (Amateur Photographer Apr 1 06) to the effect that" There is indeed no point in adding the iodide to FX-1 with modern films" FX-1 was designed for the era of "acutance "films.It remains a valid tool for processing slow and medium speed films but if some ,like T-max 100 ,give results which are a little flat,25mg potassium bromide can be added per working litre.
    FX-2 now seems to be the preferred high definition developer for modern films,he noted.