sven1982
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Solubility of chromium sesquioxide (Cr2O3)
Hi people,
I'm searching for a way to dissolve Cr2O3 (99.9% pure) in an aqueous fluid (preferably an acid, but I can cope with alkalines too  )
I tried quite a lot, every almost acid in the book (nitric-, sulfuric-, hydrochloric-, hydrofluoric-, hydrobromic-, fosforic- and perchloricacid in
each in every combination (with or without hydrogen peroxid), but that compound simply doesn't dissolve... Even in a ratio as low as 0.1g/L,
while the Merck index states: "slightly soluble in acid & alkali". Best results are achieved with a mixture of hydrofluoric, nitric and
hydrochloric acid, but still the estmited dissolution is very low (lower than 0.1%)
I also tried an alkaline fusion with Lithium tetraborate (with or without lithium fluoride), even with less succes I fear...
Has anyone here an idea how I could totally dissolve an amount of the chromium sesqiuoxide?
Perhaps with an other alkaline fusion? Or with an acid I oversaw?
Thanks in advance...
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neutrino
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Cr<sub>2</sub>O<sub>3</sub> is notoriously slow to dissolve in acids. I'd suggest making it into a (di)chromate and
reducing to Cr (III). The synthesis of dichromates has been discussed around here, e.g. here.
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sven1982
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I'm going to try some of these things tomorrow...
Starting with the NaOH/H2O2 method and ending with the Na2CO3/KNO3/Cr2O3 2:2:1...
I'll let you know in what I succeed...
Especially the fusion methods appeal to me, since I need total dissolution dissolution of the sample... Looks like I'm going to dust off my Pt-Ir
crucibles and test them again 
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Borek
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I can be completely wrong as it is a way too late here for proper thinking, but wouldn't Cr2+ traces help in dissolving Cr(III) compounds?
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S.C. Wack
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It is alleged in several books that it can be mostly dissolved by fusion with persulfate or pyrosulfate (e.g. bisulfate at the proper temp for
conversion to pyrosulfate), giving the sulfate, after a while. I was not impressed by my tests of this, maybe the time was short and the temp too low.
The old analysis books seem to prefer fusion with Na2O2, IIRC. And if not that, nitrate/carbonate or chlorate/carbonate.
I had difficulty reproducing my results in the other thread with a much purer MnO2, especially with the H2SO4. it would have helped if I had measured
and written things down. I've been meaning to try different ratios so I could figure this out, guess it hasn't been much of a priority.
EDIT: I think I know what I did wrong in the second round of testing with MnO2 - I've just done a third round. And this time it's the HCl
experiment that isn't going well! The other two tries with HCl went much better. But at least I got the H2SO4 version to work this time.
I must have used too concentrated an acid for the conditions last time, didn't use wet glassware, or I don't know what. This time, MnO2
made from MnSO4/KMnO4 (that pottery-grade crap from the first test was pyrolusite at best) and the same 90% H2SO4 and Cr2O3 was used. The MnO2 started
to dissolve right away with O2 evolution, but the Cr2O3 did nothing until a couple of mls of water was added. It then dissolved rather quickly with
more O evolution, giving a brown solution. It looks just like the MnCl3 solution, just without the layer of unreacted Cr2O3 on the bottom. Why the
sulfuric version is brown/clear this time and green/clear before I don't know. Maybe more MnO2 was used this time. Once again, this was just a
quick gram-scale test without measuring.
EDIT 2: The HCl experiment did not work at all this time and has been abandoned.
A large amount of water was added to the sulfuric experiment and the Cr2O3 precipitated. It was separated, washed, and mixed with an equal volume of
MnO2. A little conc. H2SO4 was added, just enough to dissolve the MnO2. Then a little water was added, just enough to dissolve the Cr2O3. this
happened more or less instantly, along with the appearance of the black solid as usual. The mixture was brown, and became purplish with the addition
of more H2SO4. It was filtered giving a clear purple soln.
The black precipitate became a fine brown Mn oxide on treatment with water. With bicarbonate instead, gas was slowly evolved and a mixture of heavy
black solid and a green fine solid precipitated, I am for now assuming that this is MnO and a higher oxide. The green solid dissolved instantly in
H2SO4, giving the expected color.
H2O2 was added to the purple soln, turning it green. Bicarbonate was added, giving a cloudy yellow mixture that cleared up with a little H2SO4 to
decompose some excess bicarbonate. There was no precipitate here, nor was there any hint of Cr2O3 in the black precipitate, assumed at present to be a
higher Mn oxide. But what is what for sure is yet to be determined.
[Edited on 26-7-2005 by S.C. Wack]
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neutrino
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MnO2?
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sven1982
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Tried it all out 
NaOH/H2O2 => no reaction at all (well, a bit of bubbling, but no dissolution at all)
Na2O2 => Something happened, well, the Cr2O3 was gone, but I got another, orange-brownish insoluble mass, so that wasn't the way to go either
(not to mention one of my Pt-Ir crucibles reacted it seems... Very strange, I thought these things could take Na2O2...)
KNO3/Na2CO3 => Works like a charm
First I tried to do it in the ratio (Cr2O3/KNO3/Na2CO3) 1/5/5, and firstly I heated it on the bunsen burner... And I only saw on the edge formation of
K2Cr2O7 (orange mass), so I put it in the muffle at 950°C and woohoo, it reacted like mad (a bit too harshly)...
In my second attempt (wanting to optimize (i.e. minimize) the salt concentration) I used the ratio 1/2/2, and not wanting the potassium as
contaminant, I used sodiumnitrate... And started the reaction in a muffle oven @ 775°C (1-2 min) and once it was off, just put it in another muffle
oven @ 550°C to move the flux around a couple of times... (+/- 10 min). This gives a brightly orange melt, when cooled off looks like yellow soap.
To dissolve the melt, I used
nitric and chloric acid and plain water. The water gave me a brightly yellow solution, while both acidic solutions were dark orange.
But my mission has succeeded I dissolved Cr2O3, and with lesser salt contents than I suspected (4/1 salt ratio isn't too bad )
Well, I hope my ICP's don't give me any problems anymore, though I'm seriously worried about contamination of some elements, more
specific Ca and Ba... Eh, worries for tomorrow
Thanks people for all the info
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