blogfast25
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Chromium alum crystallisation problem
As the byproduct of a few reductions of K2Cr2O7 with alcohol I posses a few grams of chromium alum, KCr(SO4)2.12 H2O, worth crystallising IMHO. Acc.
Wiki the RT water solubility is rather high: 244 g/100 g.
Contained in about 300 ml of fairly weak solution, I boiled this down to about 20 ml or so and a few well formed crystals appeared, in supernatant
liquid, upon cooling. That was then left alone for some days.
Then I decided to reduce it some more: the crystals disappeared on heating. I boiled off some more water and the solution became quite syrupy but no
crystals formed on cooling. Today I added some more clean water, then reduced it again but no crystals are forth coming at all. Any suggestions?
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unionised
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I think that solubility should be 22.4 or 24 rather than 224.
I have had a similar problem with chrome alum. I think the Cr(+++) ions form some sort of hydrate or other complex which is green and doesn't come out
of solution as the alum.
If you saturate the solution with ordinary alum you might get mixed crystals which are also pretty colours.
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blogfast25
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My concentrated alum solution is also green, not purple. The dituter solution was purple cold and green when hot...
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woelen
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This is a common problem with chromium(III) sulfate in aqueous solution. The purple solution contains free aquated Cr(3+) ions and free aquated
SO4(2-) ions. When the solution is boiled, then a ligand-exchange reaction occurs and the Cr(3+) ion exchanges water-ligands for sulfate ligands and a
sulfato-complex of chromium(III) is formed. This sulfato-complex is extremely soluble and also rather hygroscopic. It is impossible to crystallize
this complex. On standing in the cold, the sulfate-ligands are released again as free sulfate ions, very very slowly. It may take weeks or even a few
months before all chromium(III) again is present as free aquated Cr(3+).
For this reason, making chrome alum from dichromate by reduction with e.g. ethanol always must be done such that the solution does not heat up well
beyond 40 C. Higher temperatures favor the formation of the sulfato-complex and once that has happened you have to be very very patient.
I made a webpage about this interesting phenomenon:
http://woelen.homescience.net/science/chem/exps/crIII/index....
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blogfast25
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That's a very satisfying explanation, Woelen. Nice web page too, as always... Thank you. 
[Edited on 4-9-2010 by blogfast25]
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Rogeryermaw
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would a seed crystal not help? i have read that a fairly pure supersaturated solution will sometimes need help getting started crystallizing.
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ScienceSquirrel
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I have carried out this preparation using potassium dichromate purchased as a dyer's mordant, drain cleaner grade sulpuric acid and purple methylated
spirit as the alcohol source.
It went very well and gave a good yield of very clean purple crystals.
It is important not to let the solution get too hot or it will not crystallise
An outline preparation is here
http://www.sas.org/E-Bulletin/2003-02-07/chem/column.html
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blogfast25
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Quote: Originally posted by Rogeryermaw  | | would a seed crystal not help? i have read that a fairly pure supersaturated solution will sometimes need help getting started crystallizing.
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No, the chemical structure has been altered, it's not the same substance anymore. It's a mixture of substances, evaporate more water and eventually
you'll end up with something glassy: an amorphous supercooled liquid! A seed crystal cannot reverse the changes made to the substance.
I've now collected and diluted the liquid into a glass bottle and will observe it over time for changes: will it turn purple again eventually?
@ScienceSquirrel: Thanks for the link...
[Edited on 6-9-2010 by blogfast25]
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woelen
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Yes, it will turn purple, but this will take a long time. After a few days, the color already will have changed quite a lot in the right direction,
but before it really has turned purple like a pure chrome alum solution will take much longer.
[Edited on 6-9-10 by woelen]
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blogfast25
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Reading up on the phenomenon, Cr3+ seems to do something similar with ammonia complexes, where the ammonia can take up different ligand positions.
Interesting...
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tetrahedron
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| Quote: Originally posted by woelen | | It may take weeks or even a few months before all chromium(III) again is present as free aquated Cr(3+). |
thanks woelen for the detailed explanation..so there IS hope to get from green to purple without going to the trouble of redox =) and in crystal
growing, what's a few weeks?
on a sidenote, over 10 years ago i was able to buy (ammonium) dichromate from a local industrial lab supplier, no questions asked..today the same
substance has five warning pictograms (ghs), sixteen R phrases, and the sale is restricted to professional users (due to teratogenicity)
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