Lambda-Eyde
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Chromium trioxide from barium chromate
I have 500 g of barium chromate lying around, which I need to find a use for.
I thought that I could add sulfuric acid, giving me BaSO<sub>4</sub> as a precipitate and CrO<sub>3</sub> in solution (since
both barium compounds are insoluble, there would obviously be some kind of equillibrium here). I made a small test: The result was a slightly orange
solution with some BaCrO<sub>4</sub> still at the bottom of the beaker.
Slightly disappointed and not sure what the heck happened, I consulted Google. Turns out that barium dichromate is soluble at low pH (I
didn't even know it existed - I need to improve my researching skills) - thus tearing down my little thought experiment.
So, my question to you, is there any way to make CrO<sub>3</sub> from barium chromate? I thought the presence of
Ba<sup>2+</sup> would be an advantage, making it easy to precipitate out. Seems like that's not so easy to do without also crashing
CrO<sub>4</sub><sup>2-</sup> out of the solution.
Would bubbling CO<sub>2</sub>/adding dry ice to the acid solution do any good? I'm thinking it would precipitate barium carbonate while
leaving the chromate alone. Ie. precipitating barium in acidic solution, thus keeping BaCrO<sub>4</sub> from doing the same (which only
does so at a higher pH).
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sternman318
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Did you try diluting or neutralizing your test solution? BaSO4 is apparently soluble in concentrated sulfuric acid. If so, diluting it or neutralizing
it might precipitate it .
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Lambda-Eyde
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I first added concentrated sulfuric acid, then diluted it. It's now a bright orange solution with a pale yellow precipitate (looks just like barium
chromate). I don't see how neutralizing it would selectively precipitate the barium as sulfate instead of chromate.
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Xenomorph
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-
[Edited on 5-7-2011 by Xenomorph]
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Magpie
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Quote: Originally posted by Lambda-Eyde  |
Would bubbling CO<sub>2</sub>/adding dry ice to the acid solution do any good? I'm thinking it would precipitate barium carbonate while
leaving the chromate alone. Ie. precipitating barium in acidic solution, thus keeping BaCrO<sub>4</sub> from doing the same (which only
does so at a higher pH). |
I don't think any carbonates are going to remain in an acidic solution, ie:
CO3-- + 2H+ ----> CO2 + H2O
Here's something you might try from the qualitative analysis scheme for Gp IV metals, although it seems the same as what you just tried:
"Dissolve the ppt (BaCrO4) in 2 drops of 12M HCl. Add 1 drop of 4M H2SO4. A white ppt (BaSO4) confirms the presence of barium"
The single most important condition for a successful synthesis is good mixing - Nicodem
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sternman318
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http://www.public.asu.edu/~jpbirk/qual/qualanal/barium.html
According to this :
BaSO4 is extremely insoluble in water, alkalies, or acids, but is slightly soluble in hot, concentrated sulfuric acid.
and
Barium chromate is soluble in mineral acids, but only slightly soluble in acetic acid. In strong acids, an orange solution of barium dichromate is
formed
With this information, could you add a NaSO4 solution to your BaCrO4, then dissolve it with HCl? BaSO4 should precipitate out.
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woelen
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Another thing which may be interesting to try is to use a strong solution of a carbonate and leave the BaCrO4 in that solution for a long time. BaCO3
also is highly insoluble and I can imagine that there is exchange of carbonate and chromate ions. But do not expect complete exchange, at best you'll
get a mix of carbonate and chromate, but for many experiments involving chromates and dichromates such a solution may be suitable.
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Lambda-Eyde
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| Quote: Originally posted by Magpie | I don't think any carbonates are going to remain in an acidic solution, ie:
CO3-- + 2H+ ----> CO2 + H2O
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Wow, I can't believe I didn't think about that. So incredibly obvious!
| Quote: Originally posted by woelen | | Another thing which may be interesting to try is to use a strong solution of a carbonate and leave the BaCrO4 in that solution for a long time. BaCO3
also is highly insoluble and I can imagine that there is exchange of carbonate and chromate ions. But do not expect complete exchange, at best you'll
get a mix of carbonate and chromate, but for many experiments involving chromates and dichromates such a solution may be suitable.
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I will try that. However, my goal is to isolate pure CrO<sub>3</sub>, not to make other (di)chromates. For that purpose I have potassium
dichromate.
Looks like this is going to be harder (if not impossible) than I expected. I'll have a look at the solubility tables and see if I can come up with
something.
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Magpie
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| Quote: Originally posted by Lambda-Eyde |
However, my goal is to isolate pure CrO<sub>3</sub>, not to make other (di)chromates. For that purpose I have potassium dichromate.
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This may be of use:
http://www.sciencemadness.org/talk/viewthread.php?tid=6116#p...
The single most important condition for a successful synthesis is good mixing - Nicodem
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