papaya
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Chromium (III) from Cr2O3
Damn, I thought metal oxides are soluble in acids! I wanted to prepare some Cr3+ nitrate from what I have - ammonium dichromate, for which
I ignited latter and collected "volcano ash", washed few times with water to get rid of any dichromate remaining (it's there!), put it in the flask
and dropped of HNO3 (50%), then I dropped more and more.. then I added water for I thought that might help.. ( ) Then I brought it to boiling but it started to eject itself from the flask due to
violent spot boiling, to prevent that I finally transferred it into boiling water bath and after few hours it doesn't want to dissolve! I didn't weigh
the Cr2O3 before because I wanted to dissolve it into just enough volume of nitric by dropping it gradually, now I think I
overdone it.. Still doesn't want to go into solution - it became somehow finer though and went into suspension, but most of it is on the bottom. What
to do?
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woelen
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Many metal oxides are very inert and cannot be dissolved in acids, nor in aqueous solutions of strong bases. Especially when the oxides are calcined
(have been red hot, which is the case with your Cr2O3), then they are really inert and the only way of getting them dissolved is using molten NaOH or
molten anhydrous NaHSO4 at several 100's degrees C. The following oxides are extremely inert: BeO, Al2O3, TiO2, SnO2, Cr2O3, Fe2O3, Fe3O4, Co3O4,
Nb2O5, Ta2O5, CeO2. The following oxides can fairly easily be dissolved in excess aqueous acid: FeO, NiO, MnO, Mn2O3, Cu2O, CuO, Ag2O, ZnO, HgO, CdO,
VO2, most of the trivalent lanthanide oxides. The following oxides can be dissolved in aqueous bases: V2O5, MoO3, WO3.
If you want to make Cr(3+) from your dichromate, then I would suggest you dissolve some in water, add a little HNO3 (not H2SO4, nor HCl) and add
ethanol as reductor, until you get a greyish/purple solution.
To this solution you add excess dilute ammonia, which leads to precipitation of Cr(OH)3. Do not add NaOH, because if you use a little too much, then
the formed Cr(OH)3 redissolves again, giving chromite ion.
Finally, you filter and wash the (slimy) precipitate of Cr(OH)3 and dissolve this in just sufficient dilute HNO3.
Another way to get chromium(III) in soluble form is to dissolve K2Cr2O7 in water and add an excess amount of H2SO4. Allow this solution too cool down
and then slowly add ethanol. Keep the liquid cool (never allow the temperature to rise above 40 C) and use constant stirring. Slowly, the dichromate
is reduced to purple/grey chromium(III). Allow the purple/grey solution to evaporate at room temperature. Dark purple crystals of chrome alum,
KCr(SO4)2.12H2O, separate from the liquid.
If chromium(III) in combination wityh sulfate ion is heated, then the ions react and a sulfato-complex of chromium(III) is formed. The sulfate ions
are so firmly attached to the chromium, that it takes weeks or even months to have them disconnected again. The sulfato-complex is bright green. A
nice demo (for people who have chrome alum or chromium(III) sulfate) is to dissolve some of the solid in water and obtain a purple/grey solution and
then heat this solution. The color shifts to beautiful deep green. On cooling down, the liquid remains deep green and it takes weeks before the liquid
obtains the original purple/grey color of chromium(III).
[Edited on 21-8-13 by woelen]
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papaya
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Thank you woelen, I also thought the alcohol way but didn't go because it's a waste of acids,alcohol and ammonia at least. Now I'm continuing to heat
that mixture, but if I have to do what you said can I use sugar instead of alcohol? btw, why you said that HCL and H2SO4 won't fit?
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woelen
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HCl and H2SO4 form complexes with chromium(III), which are quite inert and it takes days or weeks before these complexes are broken down.
When dichromate ion is reduced, then the chromium(III), which is formed, immediately coordinates to ions or molecules which it finds nearby.
Chromium(III) very easily coordinates to sulfate or chloride, but not to nitrate ion and hence I advice the use of nitric acid.
The use of sugar instead of alcohol is not recommended. Alcohol is converted to simple ethanal or acetic acid, both of which do not coordinate to
chromium(III) and both of which are volatile and easily evaporate. Ethanol itself also does not coordinate to chromium(III) and also is volatile. When
you use sugar, you can expect a lot of messing around when you want to isolate your chromium(III) from the solution and sugar also may be capable of
forming complexes with chromium(III).
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papaya
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I'm going to try reduction withsucrose (actually I already did a small test), the literature is scarce on this :
http://en.cnki.com.cn/Article_en/CJFDTOTAL-HGYJ200301012.htm
and a google book "A Critical Study of the Reduction of Potasium Dichromate by Sucrose" that I cannot find now :O
Anyone knows what is the right molar ratio?
The test I did: a crop of ammonium dichromate placed into test tube and LOTS of sugar was added to it dissolved all in small volume of water. First
the liquid was of normal dichromate color, then I put tube into boiling water bath and after half an hour it remained the same color, but became
turbid. It was apparent that reaction is too slow for that reason I added 1-2 drops of 50% nitric acid an placed it back to water bath. I noted that
in the course of time bubbles are formed and slowly gas is given off - most likely CO2. Color went more bluish then and after some 30 minutes when I
came back it was violent colored and TRANSPARENT. I filteret the liquid and nothing was left on filter - great news, whatever Cr3+ it is completely
SOLUBLE. In my oppinion the oxidation of sucrose generated organic acids and this is why only drops of nitric were needed to initially start reaction
- all needed acid is generated from sugar during reaction and no precipitate is formed(self catalyzing?). Anyway, I added a solution of NaHCO3
(instead of NH4OH) to what I obtained and fluffy precipitate was obtained, the color is try to describe, I think it's mainly "hydroxide" of chromium
with organic contaminants. But one surprize here - the supernatant liquid didn't completely loose it's violet color - still contains some chromium, it
preserved even if I added NaOH to it (must be super strong soluble complex). During all the operations I never smelled of ammonia - where it goes
btw.?
I'm going to try this again, even if it's a "dirty" thing to do, any suggestions are welcomed!
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woelen
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Chromium(III) coordination chemistry is quite special in the sense that coordinated ligands are bound to it very tightly and once connected are very
hard to remove again. This property is shared with cobalt(III).
Coupling of chromium to a ligand in many cases is achieved by making use of redox chemistry. Chromium(VI) is reduced to chromium(III) in the presence
of the ligand. As soon as the chromium(III) is formed, the ligand coordinates to the chromium. This method usually works better than the method of
ligand exchange by adding a large amount of the ligand to a chromium(III)-containing solution.
In your situation, I expect that part of the chromium(III) is coordinated to the sugar or some oxidation product of the sugar and that this ligand is
attached very firmly to the chromium(III), so much, that it does not precipitate in the presence of OH(-) ions.
Why don't you want to use a simple alcohol instead of sugar for your redox reaction? If ethanol is hard to obtain or expensive, you can also use the
cheaper methanol or 1-propanol (both should be easy to obtain for decent cost, where I live, ethanol is EUR 50 per liter due to high taxes, methanol
is around EUR 10 per liter and 1-propanol is around EUR 20 per liter). Even 2-propanol works, it is oxidized to acetone and this really is dirt-cheap
(EUR 7 per liter). The big advantage of using a lower alcohol is that the reaction products can easily be driven off by heat and do not coordinate to
the chromium(III) as long as the liquid is kept acidic.
The fact that you don't smell any ammonia surprises me, unless you have a very dilute solution. How much ammonium dichromate did you use and in how
much liquid is this finally dissolved when you add the hydroxide?
[Edited on 22-8-13 by woelen]
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papaya
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I did this from curiosity, as well as in the test I described the acid needed was only few drops (does this work with alcohol?) to start the reaction,
it maybe worth to investigate if really works (final product here I expect to get is chromium hydroxide precipitated by bicarbonate from complexed
solution). I'm doing now a greater scale test - 5gr ammonium dichromate + 8gr sugar (a molar ratio about 1:1.4 referring to paper abstract I pointed)
and only 0.5ml of 50% nitric started reaction rather quickly, at 80°C there's noticeable bubbling of mixture, but it turned to thick and black mass
compared to previous test where I got clear violet solution (it was dilute, this may be reason). Anyway, I want to see what I can get.
Btw. the Cr2O3 + nitric after 2 days of heating turned violet color, but still contains lots of precipitate, very resistant stuff! (also I noted that
diluted acid seem to dissolve it better, any explaination?)
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