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 !

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.

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.

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.)

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

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

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...

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

Solubility of Potassium Dichromate is 12.3% @ 20C,

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

Gerald C Koch said:

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

Click to expand...

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...

edwoo11 said:

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...

Click to expand...

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

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.

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.

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.

I have a 20% solution of AD in a brown glass stoppered bottle and I find even when not having any liquid near the top while working that there are crystals forming about the stopper and entrance to the bottle.
I guess this must also be due to dropping temperatures at night but there aren't any crystals at the bottom of the bottle. AD is faster than PD for exposure times.

As other's noted, it's because of the temperature change. As the temperature falls, the solution becomes super saturated and is unstable. So some potassium dichromate drops out of the water and forms crystals, which settle on the bottom. Heat energy is required to keep the potassium dichromate suspended in the water. I have a few bottles of potassium dichromate in my darkroom of varying strengths. The weaker solutions don't form crystals, while the stronger solutions do. What I do with the stronger solutions when they form crystals is heat them up in the microwave and remix them. Just a few seconds will do. Don't just add more water, as that will dilute your solution and throw your development times off. It's important to keep your potassium dichromate to water ratio consistent.

I usually mix up a liter of solution at a time and store it in a plastic bottle. Then I pour out a small amount (50mL or so) into a small glass bottle with an eye dropper. I use that small glass bottle to work with. It's small size and material make it easy to microwave (usually only takes 3-5 seconds to warm it up enough to remix the crystals), and I can put the lid on it and shake it instead of stirring it. So getting rid of the condensed crystals is no problem and takes less than a minute. Then, when that bottle is empty, I pour out my liter solution into a large, glass, microwavable container, and warm it up and remix it. Then I refill the small glass bottle and the liter bottle with that solution which still has the proper ratio. That way I only have to measure everything out once or twice a year, and don't have to warm up and mix such a large bottle every time I use it to keep it super saturated.

You can come up with your own solution, but my advice is that whatever you decide to do, make sure it involves keeping your potassium dichromate to water ratios consistent.