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Author: Subject: Reduction of Hexavalent Chromium
Poppy
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[*] posted on 28-8-2012 at 22:26


CrO3 when mixed with ethanol reacts vigorously to form Cr (II, III).
I got a small crucible I'll give it a try for sure making some of it.




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[*] posted on 30-8-2012 at 17:11


I tried producing the chromium trioxide by adding anhydrous sodium dichromate (dried in the oven: at first you may say "omg it caked", but as it slowly cools it easily releases from the glass) to 98% sulfuric acid. It dissolved, warming the vessel. I did not measure but I would put the temperature around 60°C, was just hot to touch, guess it could be heated even more au bain marie. Also realised the more excess sulfuric acid you add turns the mess into a better oxydising paste, no better than actual aqueous solution of both ingredients :( . After cooling it becomes a dark red mud.
Analysing the reaction
H2SO4 + Na2Cr2O7 --> CrO3 + Na2SO4 + H2O
The problem obviously is separate the sulfuric acid and water from the rest of the contents. CrO3 decomposes at 251°C, while sulfuric acid boil at more than 300°C. So a vaccum pump mut be put to work as to easily get rid of the sulfuric acid.

Again I insist useing ammonium componds as the product of the dichromate because the procuct ammonium sulfate can evaporate too.

For the synsthesis of ammonium dichromate, do as follows, it has been succesfully tested:
(I) Na2Cr2O7 + 2 NaOH --> 2 Na2CrO4
(just add solid NaOH to conc. solution of dichromate)
(II) Na2CrO4 + CaCl2 --> CaCrO4 (ppt) + NaCl
Make a ten-fold excess solution of calcium chlorid so as to pour the conc. sodium chromate, this will render a lower degree of sodium contamination and also ensure reaction will dislocate as to precipitate CaCrO4 by means of common ion effeft. To this point CaCrO4 precipitates as a hydrate. Filter. Throughly dry in the oven, at 180°C. Now, the anhydrous salts may be washed with boiling water: very few of the now anhydrous CaCrO4 will dissolve, while all other salts will be taken away.
next
(III) 2CaCrO4 + 2(NH4)HSO4 --> (NH4)2Cr2O7 + 2CaSO4
Prepare a saturate solution of ammonium bisulfate at 80°C, pour the anhydrous CaCrO4 and stir to dissolve. This can be next diluted to facilitate filtration.
Now you have the precious ammonium dichromate!
:cool:
Attach: sulfuric acid vapour pressure graph


sulfuric acid vapour pressure.JPG - 30kB

[Edited on 8-31-2012 by Poppy]




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[*] posted on 30-8-2012 at 17:41


Is there any OTC source for ammonium bisulfate?
Once I have the ammonium dichromate, what do I do with it?

EDIT: CrO3 -(197+ C)> Cr2O3 + O2
Why isn't this feasible?

[Edited on 31-8-2012 by elementcollector1]

[Edited on 31-8-2012 by elementcollector1]




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[*] posted on 30-8-2012 at 20:42


The ammonium dichromate carries the chromium content you dealing with all the time!!!

Its a bet you can achieve the high purity light green Cr2O3 and then experiment with CrO,
H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4
Then check what wiki means about the decomposition of CrO at 300°c
Also, check the sticky on the preparation of elemental phosphorus!

Good luck!




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[*] posted on 30-8-2012 at 21:12


I read somewhere on this forum (thread was complete with pictures) that the decomposition of ammonium dichromate does not produce pure Cr2O3, it instead produces a rather dark green powder ripe with impurities
(Ze search function fails me yet again, I can't find ze thread...)




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[*] posted on 30-8-2012 at 21:50


Yeah probably the thread is somewhere, couldn't find it either.
The decomposition method yields a low purity Cr2O3 powder for which there is no much description regarding its behaviour against acids.
I would dream of an equation where CrO desproportionates to Cr and some other chromium oxide!
like
3CrO --> Cr + Cr2O3 !!!!
This could then be purified useing CO to form carbonyl complexes out of the metallic chromium, leaving the oxides unreacted, while Cr(CO)6 might be solvated and extracted :o

[Edited on 8-31-2012 by Poppy]




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[*] posted on 31-8-2012 at 13:40


Washing with distilled, boiling water would probably improve the quality of the chromium oxide, because that would remove all soluble compounds like unreacted dichromate, sodium/ammonium compounds, etc, leaving behind only a mix of chromium oxides (CrO, Cr2O3, and CrO2).
This would probably still work for a thermite.

I've seen several videos of the ammonium dichromate decomposition, and while it is insanely cool, I would probably prefer to just decompose the CrO3 (unless it's an explosion or something).
Is there a reaction for the decomposition of sodium dichromate?




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[*] posted on 31-8-2012 at 14:16


No no, sodium holds everything together for longer. It turns into a melt, and stays like that, specially the dihydrate.



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[*] posted on 31-8-2012 at 22:04


Hmm. Well, how do I turn sodium chromate to pure sodium dichromate? Then I can start the conversion to CrO3, and probably decompose from there.



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[*] posted on 1-9-2012 at 05:22


Forget about decomposing ammonium dichomate thermally, it's not great for pure Cr2O3.

Instead dissolve it in water, add some acid and add (slowly!) methylated spirits, then gently heat if necessary. The alscohol reduces the Cr (VI) to Cr (III) (green/blue). Now alkalise the solution with ammonia and Cr(OH)3.nH2O precipitates. Filter this off, wash with copious amounts of water and dry/calcine to Cr2O3.

Instead of alcohol, hydrogen peroxide can also be used.

[Edited on 1-9-2012 by blogfast25]




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[*] posted on 1-9-2012 at 21:54


Er, has anyone heard what I've said? About CrO3 decomposing to Cr2O3? I've gotten no feedback on that so far, and it would be great if I could just skip most of the steps to that part.

Blogfast, what if I used sodium dichromate instead?




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[*] posted on 2-9-2012 at 05:16


Quote: Originally posted by elementcollector1  
Er, has anyone heard what I've said? About CrO3 decomposing to Cr2O3? I've gotten no feedback on that so far, and it would be great if I could just skip most of the steps to that part.

Blogfast, what if I used sodium dichromate instead?


CrO3? Basically I'd forget about that if I were you.

All dichromates reduce alcohols and H2O2: it's a function of the anion, not the accompanying cation (K, NH4, Na,...)




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[*] posted on 2-9-2012 at 19:42


Aww. I thought that might work, given how there would be no extra stuff in CrO3, just more oxygen.
Well, I switched to the chemical method a while back,and have a mix of my stainless steel chlorides and bleach reacting and filtering away. Last I saw, it was just beginning to turn yellow.




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[*] posted on 2-9-2012 at 21:19


Could you describe what's that you performing with the ss you have, becaue a bunch of SS is sometimes difficult to dissolve
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[*] posted on 2-9-2012 at 21:22


1. Dissolved the stainless in a bunch of concentrated HCl. After a while, the solution darkened from clear to emerald green to opaque green. No reflux was needed, just some time.
2. Added bleach. Ferric hydroxide precipitated out in large quantities, this is busy being filtered out while the bleach additionally reacts with the chromium (III) hydroxide precipitated to form sodium chromate.




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[*] posted on 2-9-2012 at 21:27
keep the acid


If your HCl goes weak you should attach a HCl generator lined into the batch vessel, the use of a non expensive silicon rubber tube should suit for a couple runs
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[*] posted on 6-9-2012 at 19:37


Well, I got what I thought was a large quantity of sodium chromate, but upon adding acid it did not change color to the orange dichromate, so... Any thoughts?



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[*] posted on 6-9-2012 at 19:46


You aware how many acid you must put to displace the reaction? haha
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[*] posted on 6-9-2012 at 20:00


Not much: http://www.youtube.com/watch?v=zP9qEiaL4kQ



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[*] posted on 7-9-2012 at 05:16


Quote: Originally posted by elementcollector1  
Not much: http://www.youtube.com/watch?v=zP9qEiaL4kQ


At a very minimum, check pH after acid addition. And if the chromate/dichromate concentration is relatively low the colour change may be subtle...

[Edited on 7-9-2012 by blogfast25]




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[*] posted on 7-9-2012 at 08:45


It's not even subtle, still at that same bright yellow. Unless that's iron?
Here's an idea, why not run electrolysis of the chromate. That would get rid of any iron present as the insoluble hydroxide, and in all likelihood add more chromium to the solution from all that acid. Granted, I'll have to filter the mess again, but I'll boil it down this time to concentrate whatever chromate is present. Is there any way to remove trace iron that somehow made it into solution? And why didn't the concentrated HCl have any effect on the chromate?
I don't have anything in the way of pH checking.




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[*] posted on 8-9-2012 at 05:02


Quote: Originally posted by elementcollector1  
It's not even subtle, still at that same bright yellow. Unless that's iron?


If it's iron it would have to be quite a lot to match the intense yellow of the CrO<sub>4</sub><sup>2-</sup> ion. Test for Fe (III) with KSCN (red FeSCN<sup>2-</sup>;) and for chromate with lead nitrate (yellow PbCrO4 precipitates) or silver nitrate (reddish Ag2CrO4 precipitates).

But going by your description there should be no iron insolution because Fe(OH)3 is extremely insoluble.

Check pH of your chromate/dichromate solution.

You can also check for dichromate as follows:

* to a small sample add an excess of clear 'denaturated spirits' (ethanol + methanol), warm up: the yellow should dissapear and make way for a light green/blue (Cr3+)

* in the above substitute the alcohol with H2O2.



[Edited on 8-9-2012 by blogfast25]




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[*] posted on 9-9-2012 at 11:54


Upon standing for three days, the solution now seems to have a metallic substance floating on top of it. I don't think this is metallic chromium, but what else could it be?




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[*] posted on 10-9-2012 at 06:46


Quote: Originally posted by elementcollector1  
I read somewhere on this forum (thread was complete with pictures) that the decomposition of ammonium dichromate does not produce pure Cr2O3, it instead produces a rather dark green powder ripe with impurities
(Ze search function fails me yet again, I can't find ze thread...)


Plante1999 killed some spoons here:
http://www.sciencemadness.org/talk/viewthread.php?tid=15839

Look at these too:
http://www.sciencemadness.org/talk/viewthread.php?tid=19202
http://www.sciencemadness.org/talk/viewthread.php?tid=13347
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[*] posted on 10-9-2012 at 08:52


Excellent! That second thread was the one I was talking about.
Anyway, some of that strange metallic float disappeared, but it still remains, oddly enough, in the already filtered flask.




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