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  1. #1

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    Potassium Dichromate christals

    I have done a solution of 13% potassium dichromate, and for unknown reason after 2 weeks some christal developed in the bottle, then i have to dissolve them again making the solution warm.
    Quite annoying, I am not even at saturation point and it's stored in the dark.
    do you know what is happening and how to prevent it ?
    could it be the bottle material? mine is a plastic one, for chemicals (made of PET i suppose)
    thank you !

  2. #2
    brucemuir's Avatar
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    Edwoo, you can determine what grade bottle you have by examining the bottom (most likely place most of the time) for the triangle symbol with a number inside it.
    http://en.wikipedia.org/wiki/Resin_identification_code


    As far as your crystal problem I'll let one of the chemists help you out in that dept.

  3. #3

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    The solubility of potassium dichromate is not constant but depends on temperature, a 6.5% solution is stable at 10oC (50oF). So if the room temperature gets too cold then some of your 13% solution will crystallize out.
    Last edited by Gerald C Koch; 01-05-2013 at 02:31 PM. Click to view previous post history.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  4. #4
    AgX
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    I can't think of the crystallisation caused by the bottle.

    (I once looked up some lists on the stability of PET as a fellow member reported a leaking PET-bottle and I learned that it is somewhat resistant against weak alcali but non-resistant against strong alcali!
    These lists are often not very practible, but what I learned make me think that PET should not be used in the darkroom.)

  5. #5

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    Solubility of Potassium Dichromate is 12.3% @ 20C, from here.

    So you are, in fact, right at or above the saturation point. It has nothing to do with the bottle you're storing it in, IMO. This is un-advisable, since you can never be sure what your concentration is, unless every use you make sure to warm it to dissolve all crystals.

    If you need a stable solution of that concentration, I'd recommend ammonium dichromate, if your process is compatible with it. (e.g. carbon is OK, but I've never done gum, so I don't know.) It's solubility limit is 35.6% @ 20C.

    --Greg

  6. #6

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    Thanks for the replys !
    My bottle is a class 2- iso 3164, high density polyethylene (HDPE), then I don't really think that could be the problem, i bought that bottles in a Chemical supply store, looking good for any chemicals, even strong acids i believe.
    For the temp, that's interesting because the last 2 weeks it was store at 16° Celsius, but it hapened also at 19° as well, but probably slowly.
    Thanks Gerald, that must be a temperature issue, for now i will keep dissolving crystals when they appear and prepare only small quantities of solution.

    other thing : the quantity of crystals is quite large, way more of what could be up the saturation threshold (if that is clear ^^), it looks like the Potassium dichromate in the solution would all go back to crystals, that is strange, I never read that anywhere.


    to gmikol : i was looking for ammonium dichromate also, but it's very difficult to get, and makes expo time much higher. though I 'd like to try...

  7. #7

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    In photography the substitution of a sodium salt for the potassium one and vv should be done cautiously. There may be an unintended effect.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  8. #8

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    Solubility of Potassium Dichromate is 12.3% @ 20C,
    Should the temperature drop from 20C to 10C then roughly half the potassium dichromate will come out of solution!
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  9. #9

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    Quote Originally Posted by Gerald C Koch View Post
    Should the temperature drop from 20C to 10C then roughly half the potassium dichromate will come out of solution!
    That's right, I will experience with higher storage temp to see the difference, but now it's winter, atelier is colder than 20°C...

  10. #10

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    Quote Originally Posted by edwoo11 View Post
    i was looking for ammonium dichromate also, but it's very difficult to get, and makes expo time much higher. though I 'd like to try...
    What process is this? I've never heard about ammonium dichromate being slower than potassium (though I have heard about sodium dichromate being less active than either of the other 2).

    You need to make sure you account for the difference in molecular weight of the 2 compounds, but I don't have the numbers at-hand to give you.

    --Greg

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