Potassium Dichromate christals

[edwoo11]

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 !

[brucemuir]

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.

[Gerald C Koch]

The solubility of potassium dichromate is not constant but depends on temperature, a 6.5% solution is stable at 10^oC (50^oF). So if the room temperature gets too cold then some of your 13% solution will crystallize out.

[AgX]

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.)

[johnielvis]

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

[gmikol]

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

[edwoo11]

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...

[Gerald C Koch]

In photography the substitution of a sodium salt for the potassium one and vv should be done cautiously. There may be an unintended effect.

[Gerald C Koch]

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!

[edwoo11]

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...