Boffis
International Hazard
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Chrome alum from chromate oxidation residues
Over the years I have used a fair bit of one chromate/bichromate or another and after removal of the organics and the reduction of the remaining Cr6+
with potassium metabisulphite, hoped to recover the chromium 3+ as potassium chromium sulphate. I have even tried adding further potassium sulphate to
assist the process when chromic acid or sodium dichromate were used but to no avail. The solution after evaporation to a thick deep bluish green
liquid only ever deposits crystals of sodium or potassium sulphate depending on the nature of the starting material.
Looking into the chemistry of Cr3+ it appears that it forms a rather stable sulphato-complex that does not readily swap its sulphate ions for water
molecules. It tends to form when the solution is boiled with excess sulphuric acid. A measure of the stability of this complex is the fact that you
can mix it with excess sodium hydroxide solution until strongly alkaline without getting a precipitate of chromium oxide hydrate.
Does anyone know how to persuade the chrome complex to revert to a hydrated sulphate? Simply boiling the solution down doesn't help, you simply get a
more viscous bluish black gloop.
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Amos
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The blue-green sulfato complex forms at elevated temperatures; try to avoid raising the temperature above 40C as much as possible. According to
woelen, the exchange of those sulfate ligands for H2O does slowly occur but takes WEEKS at room temp.
Using bisulfite as a reducing agent instead of an alcohol should also come as a last resort; perhaps because of the excess sulfate present afterward.
If you've used excess alcohol and obtained a bluish to bluish-violet complex, this should produce crystals of chrome alum upon gentle evaporation.
If you still can't get this to work, add COLD potassium or sodium carbonate solution to the sulfato complex and wait (do not let it heat substantially
as the excess sulfuric acid is neutralized). After a duration the basic sulfate (that's what I think it is, anyway) will precipitate as an indigo-grey
solid. This can be dissolved in fresh aqueous sulfuric acid, yielding a highly dichroic solution, and will give crystals of chrome alum upon the
addition of excess potassium sulfate and patient evaporation at room temp.
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Boffis
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Thankyou Amos. I have just checked the beaker full of green liquid and found to my delight that it is starting to deposit small purple isometric
crystals. I have placed the liquid after filtering off the crystals, in a schott bottle and will leave it for a month or so and see what happens.
Its a bit difficult to avoid heating above 40-50 C since this material is waste from reactions that operated at circa 100 C. But patience appears to
be rewarded  .
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