woelen
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Cr2O3 can be dissolved in water easily
By accident I found a nice method of dissolving Cr2O3 (potteries green chrome based pigment) in aqueous solution, without the need to use molten
NaOH/KNO3 or Na2CO3/KNO3 mix, which usually are mentioned as methods for releasing the chromium from the potteries chromium green pigment.
I did the following experiment:
1) Put a spatula of Cr2O3 (fine powder, chromium green pigment from potteries/ceramics store) in a test tube
2) Put twice the amount (two spatulas, no precise measurement) of KBrO3 in the same test tube.
3) Add 3 ml of water.
This does not dissolve the KBrO3. KBrO3 only is sparingly soluble in cold water.
When the liquid is heating until it starts boiling, then the KBrO3 completely dissolves. On cooling down, crystals of KBrO3 settle again at the
bottom, on top of the Cr2O3 layer. The liquid above the Cr2O3 remains colorless.
Next, I added just one drop of 20% H2SO4 and repeated the process. Initially, there is no difference with the previous experiment when the liquid is
boiled, but suddenly, brown vapor of bromine can be observed. After a while, bubbles of gas are produced on the solid Cr2O3 and a clearly visible
brown vapor forms in the test tube (best to do this experiment outside, because you get an irritating amount of bromine vapor). When you keep on
heating for 10 minutes or so, then most of the Cr2O3 has dissolved and you get an orange/brown solution with an intense color and some opalescence. If
you put aside the test tube for a few minutes, then a clear solution is obtained with a thin layer of dark crap at the bottom. Probably this is due to
impurities in the pottery-grade Cr2O3.
I tested this solution as follows:
Take 1 ml of 3% H2O2 and 1 ml of 10% HCl and add 100 ml of water. This makes up a very dilute H2O2/HCl solution. Add a few drops of the orange/brown
solution. At once, the liquid turns deep blue, as if a drop of blue ink is added to the water. This intense color is due to formation of the
CrO5-peroxo complex and this shows that dichromate is formed. The concentration of the dichromate must be fairly high, because almost all Cr2O3
dissolved in the solution and the color was orange/brown. If most of the chromium had dissolved as chromium(III), then one certainly would not have
such a strong orange color.
I think the following reaction occurs:
5Cr2O3 + 6BrO3(-) + 2H2O --> 5Cr2O7(2-) + 3Br2 + 4H(+)
The nice thing is that this opens up another way of making dichromates from materials which can be obtained easily and this method does not require
the use of molten salts and all the associated difficulties of finding safe reaction vessels which are not corrodes by the strongly alkaline and
oxidizing melt.
KBr can be purchased without any eyebrows raised and Cr2O3 also can be purchased without any problems. From KBr one can fairly easily make KBrO3 (I
described this in detail in a page on my website and it is discussed here on sciencemadness as well).
I just wanted to share this with others, because it might be interesting for those people who cannot buy dichromates and would like to have some of
these.
Further work must of course be done on this, such as how to isolate the dichromate from the excess bromate and acid. Maybe using excess Cr2O3 and long
periods of refluxing may result in total destruction of all bromate, but this has to be checked by performing a more thorough experiment.
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kclo4
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Very interesting Woelen, this deffinently seems like an easy method to make/get dichromates. Do you think chlorates would work as well? I would
imagine they would but perhaps they are to stable compared to bromates.
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woelen
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I tried with chlorate as well. This does not work. If only a small amount of acid is added besides the chlorate, then no reaction occurs, if much more
acid is added, then on heating, the chlorate decomposes, but the Cr2O3 does not dissolve. it might be that much longer heating/refluxing does dissolve
the Cr2O3, but if chlorate works, then it works MUCH more slowly and you need more, because a lot of the chlorate simply decomposes.
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nitric
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Does iodate work, it thinks its because bromic acids being produced in the reactoin of bromate and H2SO4
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Jor
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The acid acts as catalyst, because although the reaction does not consume any protons (it even produces protons), the mechanism includes the transfer
of protons . I think these 'remove' the oxygens from bromate, then the Br takes electrons from Cr, and Cr 'consumes' the oxygens from water to form
dichromate. This is ofcourse said in a manner for you to understand, as it doesn't sound very professional 
Another example is the acid catalysed oxidation of iodine with chlorate. This reaction will work at pH=2 and not at pH=7, although the final equation
won't involve any protons.
Iodate would go via the same mechanism, but it won't work probably, because it is not a strong enough oxidiser. But I'm not sure. Anyways, iodate is
much more expensive than bromate, so there is no point.
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Per
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May it could be an interesting attempt just to use HCl instead of the sulfuric acid because it can be removed much easier and should be also strong
enough.
By using equimolar amounts of bromate and Cr2O3 (with the acid as catalyst) it should be possible to get fairly "pure" potassium dichromate by just
evaporating the liquid by heating. Would be my idea.
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chloric1
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Damn! I sold all my Cr2O3 on ebay !!
I will have to get more when I order V2O5 or Fe3O4. In warmer times I am a thermite flunky and that was what I originally bought the Cr2O3 for.
Also, it is a good color for cold process soap! i have no need for dichromate as I buy reasonably pure potassium dichromate in 25 pound bags so any
interest in this for me is purely academic. I have the strong urge to oxidize Cr2O3 with sodium persulfate I bought at the electronics shop. I know
that is excessively strong as an oxidizer because it can convert chlorates to perchlorates!
Fellow molecular manipulator
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kclo4
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| Quote: | | Also, it is a good color for cold process soap! |
Chromium compounds in soap? That sounds like a bad idea, doesn't it?
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woelen
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Quote: Originally posted by Per  | May it could be an interesting attempt just to use HCl instead of the sulfuric acid because it can be removed much easier and should be also strong
enough.
By using equimolar amounts of bromate and Cr2O3 (with the acid as catalyst) it should be possible to get fairly "pure" potassium dichromate by just
evaporating the liquid by heating. Would be my idea. |
This does not work. HCl immediately reacts with
bromate, giving Cl2 and Br2 and no acid remains.
I also tried the reaction with persulfate, but this does not work neither. Persulfate is a strong but slowly acting oxidizer. It is capable of
oxidizing chromium(III) ions in solution, but only very slowly. Oxidizing the solid Cr2O3 is extremely slow and the persulfate decomposes (giving
oxygen) fairly quickly when its aqueous solution is boiled.
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Jor
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And what happens if you add a little AgNO3 together with the persulfate? Ag(I) catalyses oxidations by persulfate.
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chloric1
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| Quote: Originally posted by kclo4 | | Quote: | | Also, it is a good color for cold process soap! |
Chromium compounds in soap? That sounds like a bad idea, doesn't it?
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No ill effects on me yet! Besides if you use soap with green, blue-green, or turquios color it has chromium in it! The mass produced soap use
chromium hydroxide. Cr(III) is not dangerous for external use like Cr(VI). The only thing is it is best to buy cosmetic grade chrome oxide and NOT
actually use the pottery grade Cr2O3 for impurities sake. I have a separate batch of Cr2O3 that is FDA approved cosmetic grade pigment. Cost more
but worth the peace of mind.
Fellow molecular manipulator
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Per
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| Quote: | | This does not work. HCl immediately reacts with bromate, giving Cl2 and Br2 and no acid remains. |
So may dilute nitric acid could do the job, but since I don´t understand the mechanism taking place it´s just an assumption.
[Edited on 18-4-2009 by Per]
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woelen
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Yes, nitric acid will do the job. This acid is not reduced by bromate.
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woelen
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Peroxodisulfate with Ag(+) as catalyst also works
As suggested by Jor, I also tried the experiment with acidified sodium peroxodisulfate to which a pinch of silver nitrate is added. This also works.
The Cr2O3 dissolves fairly easily, but one must use a large excess amount of Na2S2O8, a large amount decomposes giving oxygen.
The exact description of the experiment is given below:
- Add one spatula of Na2S2O8 to 3 ml of water
- Add a tiny pinch of silver nitrate
- Swirl, until all of the solid has dissolved. The liquid now is dark grey and turbid, due to formation of Ag2O2 (which is a mixed
silver(I)silver(III) oxide).
- Add a few drops of dilute H2SO4
- Add a pinch of Cr2O3
- Swirl and heat.
After some time, most of the Cr2O3 has dissolved and the liquid has become yellow/brown. It remains somewhat turbid, probably due to silica-based
impurities in the Cr2O3 and the formation of solid Ag2O2.
Again, when tested with very dilute H2O2/HCl solution, one obtains the deep blue color of CrO5, which shows that dichromate is formed.
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Taoiseach
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How about dissolving Cr2O3 in bromic acid obtained from H2SO4+Ba(BrO3)2? This should give CrO3 upon evaporation (unless it forms some sorta bromyl
compound), a chemical very hard to obtain/make for the home chemist.
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12AX7
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That's quite interesting.
FYI, I find potassium chlorate works quite well in a stainless steel vessel. Easy to find, no harsh alkali required, and the dichromate is the direct
product.
Tim
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woelen
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Potassium chlorate is not easy to find where I live. Potassium bromate (or sodium bromate) can be purchased from multiple sources, but chlorates are
very hard to obtain (unless you make them yourself). This is due to the fact that pyrotechnics is totally forbidden in the Netherlands. Bromates could
be used for that purpose as well, but they are much less suitable for that purpose and also somewhat expensive. So, for making dichromates from Cr2O3,
the use of bromate is an easier alternative, but on the other hand, dichromates can be purchased over here without too much difficulty.
@Taoiseach: Making CrO3 is not that difficult. K2Cr2O7 is easy to obtain and not expensive and from this and concentrated H2SO4 it is easy to make
CrO3. Isolating this from the H2SO4 is another matter, you need a glass filter and a vacuum pump to get rid of the H2SO4. Using a paper filter of
course does not work, the paper will be charred by the oxidizing mix of CrO3 and H2SO4.
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sonogashira
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Mr. woelen, what chlorate salt did you use please (in the third message)?
I only know of a source for sodium chlorate weed-killer... but bromates and potassium chlorate I don't think I've seen. Maybe it is only the potassium
salt that works? Or maybe the magic touch of Tim's saucepan?!
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Taoiseach
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@woelen
CrO3 precipartes as extremely fine crystals from dichromate+H2SO4. Its extremely troublesome to remove the acid from these crystals... washing with
conc. HNO3 is said to remove most but how to remove the HNO3 then? As you said, filter paper will be charred or even burst into flames.
Still this is a fascinating chemical. What do you think about destilling chromyl chloride into some cold water - it should hydrolize and give chromic
acid + HCl. The HCl I guess could be carefully evaporated. Do you think this could work? I dont know if chromic acid and HCl would form chromyl
chloride again, the water should prevent it but evaporating the final portion of HCl (where little water is left) could be problematic. Dessicating
over NaOH could do the trick tough.
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woelen
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I have an old description of how to make CrO3. Basically it is as you describe. Add concentrated H2SO4 to a concentrated solution of K2CrO7 or
Na2Cr2O7. Filter the crystal meal on a glass fritte, using vacuum to get rid of most H2SO4. Then rinse with conc. (68%) HNO3 on the same glass fritte,
using vacuum again. This removes the sulphuric acid, while leaving most of the CrO3 undissolved. After rinsing, the material must be pressed between
pieces of glasswool wadding to get rid of most of the HNO3. Finally, the product must be dried in a desiccator over NaOH (for removal of vapor of
HNO3) plus H2SO4 (for removal of vapor of water) for several days. This procedure is rather elaborate, but when you have the right equipment it is
feasible.
I have some commercial CrO3 (25 grams from an old photography supply). This material is dark brown and easily dissolves in water, giving an orange
solution of dichromate. It is VERY hygroscopic and forms sticky lumps of solid matter. It reacts violently with alcohol, especially if a small amount
of water is added to the alcohol (e.g. mix of 10% water + 90% alcohol) dripped on solid CrO3 gives a very violent reaction with hefty boiling. Pure
alcohol does not react that violent, but when a drop of water is added to such mixtures then an extremely violent reaction sets in, sometimes with
material ejected with violence from the reaction mix.
Using CrO2Cl2 to make CrO3 sounds interesting, but it seems not to work. I once tried this on a microscale, but the problem is that the solution of
CrO2Cl2 in water turns dark when it is heated. I think that the hexavalent chromium is partially reduced to chromium(III). The solution did not become
green, but it became very dark brown, even at fairly high dilutions. Not something useful.
[Edited on 4-12-09 by woelen]
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12AX7
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Incidentially, is there such a thing as chromic(III) chromate(VI)? Would it disproportionate to something like CrO2?
Tim
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Taoiseach
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The other idea I had was to make perchromic acid by adding H2O2 to a suspension of Cr(OH)3 and leaving this to decompose. I once tried this on a small
scale - Cr(OH)3 does dissolve in ice cold H2O2 (altough slowly) forming a dark brown solution and upon warming it starts bubbling (O2) and turns
bright orange. I tried evaporating the solution to yield CrO3 but I overheated and most of the crystals turned green (Cr2O3 I guess).
The problem is that only freshly prepared Cr(OH)3 seems to work - the dry stuff from lab supplies is useless. So you have to precipate the hydroxide
from a soluble Cr(III) salt by adding ammonia. This produces a very fine green-blue, almost gel-like precipate of Cr(OH)3 which is a pain in the ass
to filtrate. It has to be washed several times with hot water until it no longer smells of NH3. Most of the Cr(OH)3 is lost during the process and
thus the procedure turned out to be pretty useless.
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