Potassium Dichromate christals

Discussion in 'Alternative Processes' started by edwoo11, Jan 5, 2013.

  1. edwoo11

    edwoo11 Member

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    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. brucemuir

    brucemuir Member

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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. Gerald C Koch

    Gerald C Koch Member

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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.
     
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  4. AgX

    AgX Member

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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. johnielvis

    johnielvis Member

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    yes--this does happen...just shake it up till it dissolves or add more water.

    a better solution (pun) is to use sodium dichromate--WAY more soluable---mucho perferable to the potassium salt.....too strong a solution of that and same poblem of crystallization, but it's much more easily soluable
     
  6. gmikol

    gmikol Member

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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
     
  7. edwoo11

    edwoo11 Member

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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...
     
  8. Gerald C Koch

    Gerald C Koch Member

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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.
     
  9. Gerald C Koch

    Gerald C Koch Member

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    Should the temperature drop from 20C to 10C then roughly half the potassium dichromate will come out of solution!
     
  10. edwoo11

    edwoo11 Member

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    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...
     
  11. gmikol

    gmikol Member

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    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
     
  12. Loris Medici

    Loris Medici Member

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    I agree with Greg here, ammonium dichromate isn't slower than potassium dichromate - given the solutions have the same dichromate ion concentration. When mixing by weight/volume, 1g A.D. is equal to 1.167g of P.D. - that is, A.D. is more sensitive per given weight. (Because you'll end up with more dichromate ions per weight.)

    Regards,
    Loris.
     
  13. edwoo11

    edwoo11 Member

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    Good to know thanks, I think I read that somewhere, may be this person was mixing less amonium dichromate to get longer exposure times...
    nice for me if is the same, if i can have some later... is it cheaper ? because you need twice as much.

    I am doing gum prints.
     
  14. Loris Medici

    Loris Medici Member

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    Hi again,

    You don't actually need twice the amount; you aren't supposed to always use saturated dichromate solutions. If using PD, try to keep the solution strength below or around 10%, with AD the upper (safe) limit would be something around 25%. I never used something stronger than 15% for gum myself - and that was for a short period of testing paper negatives! My usual dichromate concentration is 5%, and the exposure times are around 3-6 minutes with UVBL lightsource - nothing slow here...

    Regards,
    Loris.