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weiming1998
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Quote: Originally posted by elementcollector1  | Well, it reduced just fine, sulfuric acid and isopropyl really does the trick.
I'm still wondering about the sodium carbonate. I poured some boiling water over the bicarbonate, and it fizzled and bubbled as if decomposing (the
gas was likely CO2). Is this pure sodium carbonate? What say you, blogfast25?
(I don't know, it still looks like Gatorade to me. Then again, there's always that 'named colors' dilemma with our eyes...) |
Using bicarbonate to directly precipitate Cr(OH)3 will work. Any potential Cr (III) carbonates/bicarbonates formed can be decomposed to the hydroxide
easily by heating the solution after the bicarbonate had been poured into the acid and the solution stops fizzing.
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elementcollector1
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Well, I have the bicarbonate/carbonate solution dripping through a sep-funnel into a good-sized flask overnight, we'll see what has happened come
morning.
Incidentally, the reason I couldn't add the carbonate solution all at once was because the bubbling was so strong that I thought it might overbubble
from the flask I have it in. I'm guessing the huge formation of bubbles is due to a bunch of CO2 from the reaction between the sulfuric acid (from the
acidification + reduction step) and the sodium carbonate / bicarbonate (both basic). There's only a small amount of the Cr (III) sulfate solution in
the flask, so I'll probably have to repeat this a couple times.
Incidentally, can anyone give me a picture of what fresh Cr(OH)3 looks like? Everyone describes it as 'grayish-green," or some variation, but I've
never actually seen a picture. I always assumed it was just a grayer shade of the deep green of Cr2O3.
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elementcollector1
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Well, I have no Cr(OH)3, and my solution (after quite a bit of dilution because of all the sodium carbonate/bicarbonate solution used) is now
blue-green. I'm assuming this is chromite, so can I still prepare the hydroxide from this?
(I tried neutralizing the acid beforehand with NaOH, and all it did was immediately turn back to chromate.)
So, boil it down, and try again with more concentrated solutions?
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blogfast25
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Cr(III) oxide (or hydroxide) is amphoteric: it can be dissolved in alkali (yielding chromites) or acids (yielding Cr(III) salts). Careful
neutralisation of chromite will precipitate Cr(OH)3 but add too much acid and it will dissolve again...
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elementcollector1
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I'm happy to report that I have a precipitate of light green Cr(III) sulfate from the reduced Cr(III) solution boiled down (there's still supernatant,
though). I can try test tube-precipitations of this saturated solution with ammonia (speculated to work well, due to the poor amphoterism of Cr(OH)3
in ammonia which can be removed completely by boiling), sodium bicarbonate, and sodium carbonate.
This is pure chromium sulfate, correct? The acetone produced from reduction should have boiled off, as should the acid. My only concern is Na+ ions,
which shouldn't be much of a problem considering I'm after an insoluble end-product.
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elementcollector1
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Unfortunately, that precipitate was probably mostly sodium sulfate. Going to try concentrating it, removing as much sodium sulfate as possible,
boiling to dryness to get rid of any remaining acid, and react with a few weak bases I have on hand to precipitate Cr(OH)3. Failing that, does anyone
know a good method to convert sodium chromate or chromium sulfate to ammonium dichromate? I want to try the 'chemical volcano' anyways 
Elements Collected:52/87
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