woelen
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Riddle with chromium(III)
I stumbled upon a remarkable observation with chromium(III) and sulphuric acid.
I wanted to make a solution of CrO3 in H2SO4 and I proceeded as follows:
- Add a small spatula of K2Cr2O7 to 2 ml of 96% H2SO4
- Heat, until all of the solid has dissolved.
This results in a clear dark orange/brown solution, as expected. After use, I had some of this solution left. I wondered what would happen if this is
heated strongly. I expected the CrO3 to decompose, and formation of chromium(III) sulfate. This indeed happens. When the acid become so hot, that
white fumes are produced, then gas bubbles are produced (oxygen) and the liquid turns green (chromium in +3 oxidation state).
On continued heating, however, I found that the liquid became turbid, and the green color became less intense. This is remarkable. What could cause
the turbidity?
I let the liquid cool down and poured all in a beaker with water, expecting the turbidity to dissolve quickly. But it didn't! A grey precipitate
slowly settled at the bottom, with a green clear liquid above it.
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Based on this, I decided it was time to investigate this strange phenomenon in more detail. I took a spatula of chromium(III)sulfate, Cr2(SO4)3.xH2O.
This is a dark purple solid, which dissolves in water, giving purple/violet solutions. Very similar to chrome alum, but without the potassium ions.
I added some of this chromium(III)sulfate to concentrated H2SO4. The solid partly dissolves and a blue/violet solution is obtained. On heating, the
solution quickly turns bright green and all solid dissolves. On much stronger heating, such that the acid starts fuming, the green color becomes less
bright, it turns a little brownish/green, and the solution becomes very turbid. Heating for a longer time does not change very much. The turbidity
remains.
I let this liquid cool down, and added 10 times its volume of water and swirled all of it in order to mix thoroughly (carefully, splashing of water,
due to reaction with concentrated acid). This results in a green solution which is very turbid. A few hours later, a grey precipitate settled at the
bottom, and a green clear solution is above it.
So, it really is the chromium(III) in combination with concentrated sulphuric acid, which shows this reaction.
As a counter experiment, I dissolved some chromuim(III)sulfate in water with 10% H2SO4 in it. Heating of this solution causes a shift from violet/blue
to deep green, but no precipitate is formed. The shift of color can be explained by exchange of water ligands by sulfate ligands.
All chemicals used in this experiment are high quality chemicals, so no drain cleaner H2SO4, and no chrome mordant, but true lab chemicals.
Any idea what the grey precipitate could be?
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not_important
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It sounds like the hydroxide or a basic sulfate. A distant possibility is the anhydrous sulfate which, like the chloride, will not dissolve in water
unless a trace of Cr(II) is present.
Might run a slightly larger batch, and try to isolate the well washed precipitate. Test it for solubility in HCl and OH-, and in water with a spec of
zinc added.
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unionised
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I don't generally expect hydroxides or basic salts to form in conc sulphuric. The anhydrous sulphate looks like a good explanation.
The ligand exchange reactions of Cr(III)can be very slow. Perhaps you made some sulphato complex that's not souble and it won't dissolve until it
hydrolyses back to the aquo complex.
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not_important
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The colour matches the hydroxide, not the sulfate, that's why I wondered about the basic sulfate. It's not concentrated when he actually gets the ppt,
that's a fairly dilute solution.
Making enough to be able to wash it well, then treat with base or HCl, followed by testing for SO4 would confirm it being a sulfate. Testing some
with water or very dilute acid and a speck of zinc would help confirm the anhydrous sulfate.
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woelen
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I did get the precipitate already in the concentrated sulphuric acid. The addition to water does not change it, but it really was formed in the
concentrated acid.
Right now it is quite late already (23:15, I just arrived from work), but tomorrow I'll try what happens when the precipitate is added to a solution
of NaOH (I have some of this, under water). Right now, the precipitate is more than 24 hours under water, and still nothing has dissolved, at least
not visibly.
If it is the anhydrous sulfate, then I could also try heating the dry powdered hydrated violet sulfate. This should result in formation of a grey
powder. I also will try that tomorrow evening and come back on this.
What really stroke me in this experiment is the formation of such an insoluble precipitate from concentrated acid.
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woelen
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Here are some results of the experiments, I described in my post of yesterday.
http://woelen.scheikunde.net/science/chem/exps/raw_material/...
The result is not fully conclusive, I think that anhydrous chromium(III) sulfate is a good candidate, but still, the result is quite remarkable and
unexpected. I probably will try the test with a reductor in order to make it dissolve in water (analogy of anhydrous chromium(III) chloride).
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guy
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I checked the CRC handbook; anhydrous chromium III sulfate is a red-brown solid.
Chromium hydroxide can be a whitish color, so I would think this is definitely a a chromium oxide sulfate. I'm not sure why Cr(OH)3 is whitish but
Cr2O3 is very green; perhaps it depends on the crystal structure
[Edited on 8/29/2007 by guy]
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not_important
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Nice set of photos.
I'm sure that you made a basic sulfate or even the oxide or hydrated oxide. Chromium sulfate should be unstable to hear as are the other transition
metal sulfates, as you weren't heating it to extremes the basic sulfate would seem to be likely. However Cr2O3 can really hold onto water, it is
possible that what you see is the formation of a lower hydrate (i know that mild heating doesn't drive all the water off the hydrated chromium
sulfates), then the loss of SO3 but retention of OH.
You might try heating a little of the brown solid, or fresh sulfate, in a test tube and take it slowly up really hot - just starting to glow red.
Somewhere along the temperature climb chrome oxide often loss the last water and does a phase change, releasing heat and glowing.
[Edited on 30-8-2007 by not_important]
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12AX7
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Does it dissolve in anything? E.g., HCl, F-, aqua regia, KOH, NH3, etc.?
Tim
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Antwain
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As an interesting coincidence i am trying to make several 'chrome alums' myself atm from reduction of CrO3.
First point... CrIII is extraordinarily non-labile so it takes days to months for the ligands to exchange.
Second point... The green solution encountered on boiling chromium sulfate solutions is one or two sulfate ligands 'stuck' onto the chromium. In my
experience they have a half life (changing back to the purple cold solution) of about 2 weeks.
I would speculate that in conc. H2SO4 you could get [Cr(SO4)3]3-
If you started with dichromate, there are not 3:1 K+ ions to Cr atoms/ions, hence protons would make up the rest of the deficit. If you dumped this
into water, it could hydrolise in weird and wonderful ways, depending on the temperature of reaction.
Try leaving some of the precipitate (in water or otherwise) for a few weeks, or boiling the shite out of it 
An interesting thought for soluble Cr ligated salts is to try to precipitate the ligands (say SO4 2- of Cl-) and see if you get the amount you would
expect. do you have the analytical ability to ppt BaSO4, or do you have bodgy 0.1g resolution scales like most of the rest of us
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woelen
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| Quote: | | If you started with dichromate, there are not 3:1 K+ ions to Cr atoms/ions, hence protons would make up the rest of the deficit. If you dumped this
into water, it could hydrolise in weird and wonderful ways, depending on the temperature of reaction. |
This could be, but keep in mind, I also had this precipitate already, while I was heating the concentrated sulphuric acid with the chromium(III) in
it. So, the precipitate is already formed in conc. H2SO4, but this requires temperatures close to 300 C.
@12AX7: I tried all of those, except the fluoride. Nothing makes it dissolve. So, you get a precipitate in conc. H2SO4, and once it exists, nothing
can dissolve it. Maybe molten NaOH?
I changed the webpage somewhat, the text was not very clear, now I focus more on the initial experiment with formation of a precipitate from H2SO4,
and less on the heating of chromium(III)sulfate. It might indeed be that the precipitate from H2SO4 is a basic chromium(III) sulfate (color matches
the solid, obtained on heating anhydrous chromium(III) sulfate), but wouldn't that be very weird: a basic salt, precipitated from a
concentrated acid?
[Edited on 30-8-07 by woelen]
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guy
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I think you should try and do an analysis on the change in mass. This could be helpful in determining what it is made out of.
Also see if it would dissolve in by adding some reducing agents.
[Edited on 8/30/2007 by guy]
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woelen
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I did the test with reducing agents. Adding the solid to HCl and then adding Na2SO3 does not result in dissolving the material. Next, I added a
stronger reductor, being powdered Zn metal and vigorous shaking, such that it comes in contact with the metal and the gas bubbles. Again, no result,
besides the dissolving of the zinc and the formation of H2.
I'll think about making some of this material and then rinsing and drying and see what happens if it is heated strongly. With this experiment I can
measure mass before and after heating. Unfortunately, I do not have the equipment to do a mass analysis of wet material. The chromium sulfate I have
has not a nicely specified water contents (it is sold to me as Cr2(SO4)3.xH2O, unknown x). So, mass determination is very hard. Also, precipitation is
not complete. I always have some of the green material left in the concentrated acid.
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