plante1999
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Partial oxydation of acetic acid at high pressure
Yesterday, I was cleaning the stuff I had lying in my storage. Glassware, old chem. Writing down chem needing a new label until I found a 25ml bottle
which had been caped for at least a year. It contained a sol. of ammonium dichromate in 10% acetic acid.
I was not able to open the bottle with my bare hand, so I tried with hot water and later a rubber mat, and it still didn't work. So I put two rubber
mat and two 1' long pliers (if it is the name) and finnaly oppened it. It made a loud of gasing sond. Much like a small gun shot. And quite a large
amount of wind was made. I was quite impressed by the small 25 ml bottle. Usually, when pressure build up, the cap break open.
When I poured the solution, it contained a semi-soluble green compound. No smell at all was observed I washed the interior a few times and found a
black layer on the bottom of the bottle. So I used a steel rod to break it to pieces and removed them. They were highly glassy, of a very dark black
color. I took one piece and pressed it in my (pvc gloved) hand, and it broke making cut in the glove and in my hand. By looking at very sharp edges, I
can see a very dark purple color.
I think the pressure made such a reaction, but what is this thing? Possibly ammonium dichromate and acetic acid reacted under pressure to make
acetamide, water and chromic acid. Then somehow, the high pressure, and partially oxidizing condition turned the organic compounds to this insoluble
''glass''.
Do not try to replicate the result if you are not experienced in chemistry, with the sound and wind it made, it could have been several hundred PSI of
pressure inside the bottle.
Any tough on this?
I never asked for this.
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DraconicAcid
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I'd guess that the dichromate oxidized the acetic acid to carbon dioxide (causing the pressure) or the ammonium ion to nitrogen; the green/purple
stuff is probably some chromium(III) compound.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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Yes, I think DA's explanation is the most likely one. The only slightly more powerful oxidiser KMnO4 easily oxidises oxalic acid to CO2. In forcing
conditions perhaps dichromate can do the same to acetic acid, given enough time?
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plante1999
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But what is the black/purple insoluble glass brittle solid?
I never asked for this.
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DraconicAcid
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Probably a chromium(III) salt- possibly acetate, maybe a basic acetate. Merck says that the monohydrate of chromium(III) acetate is only slightly
soluble (the other hydrates are freely soluble). I wouldn't expect that you would get an ammonia complex in a solution full of acetic acid.
If you let a piece sit in water for extended periods of time, does it dissolve, or is it completely insoluble? It may be soluble, but very slow to
dissolve.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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plante1999
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PS: I knew that the semi-soluble green thingy was chromium.
The solid is very very black, like totally opaque, only sharp edge are very dark purple. The solid is completely insoluble, after 24h in water, it is
still the same as it was.
I never asked for this.
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papaya
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Chromium metal?
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blogfast25
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Virtually impossible in the given conditions. It takes a lot to reduce Cr VI to Cr 0.
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