Maja
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CrO3
Ok, Having a lot of pyridine and I don't have anything to do it I decide to prepare some PCC from it, but here is the problem... I don't have any
CrO3. I know that is it highly toxic ,but if safety precautions is taken everything is going to be all right. I browsed google, some other forums,
vogel 3 & 5 editions, Modern inorganic chemistry, other books and didn't find anything useful. From one thread(I think it's in COCl2 preparation)
in this forum I came up with this equation :
H2SO4 + K2CrO4 ---> K2SO4 + H2O + CrO3
Will this work ? And what is the details for this reaction ? Thank you very much for your help.
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evil_lurker
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You can purchase it here:
http://store.hvchemical.com/browse.cfm/4,460.htm
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Magpie
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"Chromium oxide" and potassium dichromate are also available from some pottery suppliers. These are very reasonably priced (pottery grade, of course)
and there won't be any hazmat fees.
The single most important condition for a successful synthesis is good mixing - Nicodem
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ethan_c
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| Quote: | Originally posted by Magpie
"Chromium oxide" and potassium dichromate are also available from some pottery suppliers. These are very reasonably priced (pottery grade, of course)
and there won't be any hazmat fees. |
This is when different oxidation states can be deceitful. The compound available in pottery stores is
Cr<sub>2</sub>O<sub>3</sub>, not the extremely corrosive and carcinogenic CrO<sub>3</sub>, better known as chromic
acid. Trust me on that.
Wherever you get it, there WILL be hazmat fees. Trying to make it yourself isn't wise for two reasons; first, obviously, it's carcinogenic and likes
to react very exothermically and suddenly with almost any organic compound. Second, if you indeed find an appropriate reaction to produce it with more
obtainable chemicals (as I did), it is almost impossible to dry- it is probably just about the single most hygroscopic chemical I've had experience
with.
The two main uses for CrO<sub>3</sub> are for chromium plating baths (any rim- or bumper-plating operations nearby?) and as an antiquated
(there are much safer replacements now) yet still frighteningly effective glassware cleaning agent in combination with sulfuric acid. I have
personally worked in two labs that still had it around from its cleaning heyday, so you could certainly find some old surplus if you looked hard
enough. Else, as I said, there ARE going to be hazmat fees.
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neutrino
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The following thread from RS might prove to be useful:
A review of chromium compound synthesis
| Quote: | Chromium trioxide: Add 1 part of a saturated solution of potassium dichromate to 1.5 parts of concentrated sulfuric acid (about 95%). Cool the
solution in an ice bath to precipitate crystals of chromium trioxide. Decant off the acid solution and place the crystals on filter paper to absorb
the remaining acid and moisture. The drying should be done in a desiccator to protect the crystals from moisture. Chromium trioxide can also be
obtained by evaporating a solution of chromic acid to dryness. For high purity chromium trioxide see the procedure for chromium fluoride below.
...
Chromium fluoride: May be prepared by distilling a mixture of lead chromate and fluorspar (calcium fluoride) in concentrated sulfuric acid. The
distillation receiver must be cooled to -100 C to condense the chromium fluoride and the entire apparatus must be protected from moisture. Glass will
decompose chromium fluoride... |
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woelen
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Obtaining K2Cr2O7 (or Na2Cr2O7.2H2O) is MUCH easier than obtaining CrO3, and a mix of the finely crunched dichromate with H2SO4 for most purposes is
equally suitable as pure CrO3. One source of K2Cr2O7 is this: www.artcraftchemicals.com. And for K2Cr2O7 there are NO hazmat fees (at least not from this company), I can tell you from experience.
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Maja
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I think I will try the route with pottasium dichromate, because I have a few hundred grams laying around. Ethan_c, "How" hard is to get it dry and can
you share with us some practical suggestions "how" to get it dry if you are saying that is "VERY HARD TO GET DRY"  . And I think I won't try to dry it ,because PCC reaction doesn't require anhydrous
conditions, so slight excess of it won't hurt. Will post pictures  And yes in
pottery suppliers you can get JUST Cr2O3 ,but not CrO3
'
P.S. I have just searched site suggested by evil_lurker
http://store.hvchemical.com/browse.cfm/4,460.htm
1lb costs just 23$ and 10lb just 65$ maybe I will just order it from that e-shop..
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S.C. Wack
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That price seems rather high. And AFAIK the only hazmat chromate is ammonium dichromate. Other chromates are not on the UPS or CFR hazmat tables.
I'm sure that some of the books that I've scanned have some preparation, but it's really so simple that you don't have to measure anything. The
solubility of CrO3 goes way down with increasing H2SO4 conc. and decreasing temperature. Bisulfate is more soluble. Its not rocket science. The
unrecovered acid is given something to oxidize so as not to freak out the eco-sensitive who somehow missed this thread.
So it figures that if one can get the sodium salt, that would be preferred due to the higher solubility at reasonable temperatures, as adding H2SO4 to
hot solutions can be less fun. The adhering H2SO4 is removed with HNO3 in the books and journals, and by other methods in the patents.
I think that the dangers of Cr+6 have been a little popularly hyped in the past 10 years. I'd worry more about dichromate dust when transferring it
than CrO3; I've been flamed for mentioning this hazard before, though. Cr+6 is more of a nasal hazard than anything else, as long as you don't handle
it particularly recklessly.
(yes I'm aware of the use of Roman numerals)
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ethan_c
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| Quote: | Originally posted by Maja
I think I will try the route with pottasium dichromate, because I have a few hundred grams laying around. Ethan_c, "How" hard is to get it dry and can
you share with us some practical suggestions "how" to get it dry if you are saying that is "VERY HARD TO GET DRY" . And I think I won't try to dry it ,because PCC reaction doesn't require anhydrous
conditions, so slight excess of it won't hurt. Will post pictures And yes in
pottery suppliers you can get JUST Cr2O3 ,but not CrO3
'
P.S. I have just searched site suggested by evil_lurker
http://store.hvchemical.com/browse.cfm/4,460.htm
1lb costs just 23$ and 10lb just 65$ maybe I will just order it from that e-shop.. |
I, too, have a couple hundred grams of some alkali dichromates just sort of sitting around, and I, too, made some chromium trioxide with sulfuric.
Basically, I have no suggestions for you because I sort of failed. I let it sit under a heat lamp, and the driest it got was a viscous liquid, like
old oil-based paint. I put it in a very efficient desiccator at school and that helped just slightly, dehydrating it into sort of more of a paste.
If your reaction doesn't require anhydrous conditions- go for the dichromate oxidation. Dry it as much as is feasible for you (I don't have ether or
anything to spare, and I didn't want to use CH3Cl) and that should be fine.
I actually just got my paws on some fairly dry stuff (its slightly wet but at least its a POWDER) just today…anyways, those prices are GOOD as far
as I've seen- however, the hazmat fee would certainly provide an incentive to buy whatever you might possibly need from that company in the forseeable
future…
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kyro8008
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I cannot vouch for this as it is currently on my todo list, but the other preps from this book are very good:
PREPARATION 62
CHROMIC ANHYDRIDE, CrO3
The addition of sulphuric acid to a solution of either a chromate or a dichromate liberates chromic acid which is very soluble and can exist in
solution in the different forms, H2Cr04, H2Cr207 and CrO3, in equilibrium with each other. With the addition of a large excess of concentrated H2SO4,
water is withdrawn from the hydrated forms and the anhydride separates in the shape of red needles.
Materials:
sodium dichromate, Na2Cr2O7-2H2O, 100 grams = 0.33 F.W.
36 N H2SO4, 400 cc
Apparatus:
8-inch porcelain dish.
glass plate to cover the 8-inch dish,
suction filter with glass marble,
glass-stoppered sample bottle,
tripod.
Bunsen burner.
Procedure:
Dissolve the 100 grams of sodium dichromate in 250 cc. of water and filter from any sediment. Add rather slowly with constant stirring about half of
the concentrated sulphuric acid
until a slight permanent precipitate of CrO3 is formed. Let the mixture cool for half an hour or longer, then add slowly, while
stirring, the rest of the sulphuric acid. Let the mixture stand over night covered with a glass plate in order that the crystal
meal may become somewhat coarser. In such a crystal meal standing in its saturated solution, the smaller grains dissolve and their material deposits
out on the larger crystals. But even now the crystal meal will be rather fine and it will at first run through the filter; if, however, while waiting,
the mixture is heated with stirring to 100° and allowed to cool slowly, and this process is repeated once or twice, a more satisfactory product will
be obtained.
To collect the crystals, use a suction filter, but place a small glass marble in the funnel instead of the usual plate and
paper. If the red crystals at first run past the sides of the marble, pour the liquid in the bottle repeatedly back on to the filter until finally the
filtrate runs clear (see last sentence of Note 3 on page 5). After draining the crystals completely and pressing the surface with the round end of a
test tube, stop the suction and pour 15 cc. of 16 N HN03 so as to wash down the sides of the funnel and cover the surface of the product. Stir up the
product with this washing fluid for a depth of about half an inch. Suck dry and repeat the operation twice with 10 cc. of nitric acid each time.
Finally drain the red crystals as free of liquid as possible, transfer the crystals to a dry 8-inch evaporating dish and place this on a hot plate to
let the nitric acid evaporate. When the product is dry and no longer gives off vapors of nitric acid place it in the glass-stoppered sample bottle.
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S.C. Wack
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I just want to point out the use of the phrase "large excess" in the above. This is a bit too vague (if the directions are not followed) as this can
lead to the release of (ozonized, perhaps) oxygen with too large an excess.
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