shivam - 5-11-2015 at 10:18
Hey guys!
I have tried (and failed) precipitating Chromium(III) Carbonate many times from an aqueous solution of Potassium dichromate reduced by sodium
metabisulfite
Now, I know Chromium makes complexes with metabisulfite ions and it is hard to precipitate
I could reduce K2Cr2O7 by Isopropyl alcohol but it is quite costly (at least where I live)
So, any suggestions on precipitating Cr2(CO3)3 from such a solution reduced by sodium metabisulfite?
Amos - 5-11-2015 at 12:39
First it should be noted that chromium(III) carbonate isn't a compound that can be isolated; if you add sufficient sodium carbonate, you'll get a
precipitate of chromium(III) hydroxide, a blue-grey solid, accompanied by the evolution of carbon dioxide.
I'd also like to make sure everything is as it should be with regards to your procedure.
- Is your chromium(III) solution fully reduced? It should be slightly translucent green or blue-green solution; the latter being more favorable. If
the solution is not yet one of these colors, you may need to acidify your chromium solution and if that isn't enough, add more metabisulfite. If your
solution is decidedly green, it may have been exposed to too great a temperature at some point; Once you've passed about 30 degrees C, your
chromium(III) ions start getting snatched up into complexes that are very stubborn and hard to work with.
- Have you absolutely added enough carbonate to the solution to neutralize all of the acid that may still be present, as well as precipitate the
hydroxide? Keep in mind that sodium carbonate forms hydrates which contain a very significant mass percentage of water.
I've done a lot of work with chromium, but I only have one foggy recollection of precipitating the hydroxide from a solution I neutralized with sodium
metabisulfite. I'm not sure if I trust that recollection. If you can't ever get it to work, I'd advise acidifying some dichromate with sulfuric acid
and reducing with any available alcohol; it doesn't have to be isopropanol. Do take great care if you use methanol, though, as this will produce no
small quantity of formaldehyde. If you need to be warned of the risks of formaldehyde, then I advise you not use methanol.
Amos - 5-11-2015 at 12:42
Oh, almost forgot: If you want to reduce your dichromate with an alcohol, use sulfuric acid in at the very least a 3:2 molar ratio to your dichromate,
and use a large excess of alcohol. The acid is needed for a smooth and quick reduction of the hexavalent chromium.
nezza - 6-11-2015 at 11:38
Cr(III) is a real bummer. There are numerous aquo complexes with sulphate , chloride etc incorporated in them and reduction conditions need to be
carefully controlled to get a Cr(III) solution which will precipitate all of the Cr(III). If not some or all of the Cr(III) can remain in solution as
complexes.
j_sum1 - 6-11-2015 at 13:42
I recommend studying a pourbaix diagram for your system. Sign in to https://www.materialsproject.org using your favourite social media account and you willl have access to an excellent and useful tool for this kind
of thing.
I am endebted to deltaH for this link.
[edit] Stuck "L" key on my keyboard. Distastefully ambiguous statement made. 
[Edited on 6-11-2015 by j_sum1]