Keras
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Chromium VI woes and what to do with an unknown chromium III mixture
So guys,
I attempted yesterday to make some CrO3 crystals starting from chromium III chloride, for my ethylene → benzene synthesis. (Chromium VI
salts, being carcinogenic, have been pulled out in Europe under the REACH directive, so you cannot find, say, potassium dichromate, like you seem to
be able in the US. The only way to get potassium dichromate over here is by synthesis.)
Okay. I dissolve CrCl3 hexahydrate in distilled water, add KOH flakes until the precipitate of Cr(OH)3 redissolves. Then I pour
in some hydrogen peroxide piecemeal, have to put the flask in a cool bath because the reaction is quite exothermic and bubbles a lot. Result: a nice
orange solution of what I assume is potassium dichromate in a slight excess of KOH. I boil it for a while until no bubbling happens to eliminate
excess peroxide, then gravity filtration to eliminate some small impurities → nice orange solution.
At this point, I prepare a cylinder full of concentrated sulphuric acid, pour a little bit into the solution, expecting it to turn red and…
No.
Instead, there’s a large bubbling, the solution turns dark brown, no matter how much sulphuric acid I add. After having almost spent my cylinder, I
put the mixture on a stir/heating plate hoping that by boiling off some water something will precipitate.
No. :p
At the end, I’m left with a solution that has turned back to green it seems (very hard to see: dark coloured). Also, while heating, the solution
fumed. I thought it was just water vapour, but no: from the one or two whiffs I accidentally inhaled, it smelt like bleach. Could it be that HClO was
evolved with the excess of sulphuric acid?
In any case, I assume there was still a large amount of peroxide in the solution, and pouring the sulphuric acid (after the first few drops which
neutralised the excess of KOH) has probably turned the whole solution into Cr II then back to Cr III after air contact.
Drat. 
In hindsight, I’m a bit surprised by two things:
Addition of hydrogen peroxide to my initial solution of CrCl3 + KOH shouldn't have been exothermic – on the contrarty, apparently
one must heat the mixture to get the oxidation going;
I should've obtained a yellow Cr VI solution, since in basic aqueous solution CrO42- predominates and should be yellow,
not orange.
So now I’m left with a ¾ full Erlenmeyer flask with a nice deep green colour. I would like to repeat the process and maybe avoid falling in the
same trap, but the question is: how can I recycle this mixture (sulfuric acid, Cr III, chlorine maybe, potassium also) back to some definite chemical
state, like CrCl3 or some other well-known salt?
If anyone has a clue, I would take it gladly!
Thanks!
[Edited on 2-5-2019 by Keras]
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j_sum1
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Dichromate is a strong enough oxidant to oxidise the Cl- in solution. As soon as you added the H2SO4, that's what happened.
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Keras
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Quote: Originally posted by j_sum1  | | Dichromate is a strong enough oxidant to oxidise the Cl- in solution. As soon as you added the H2SO4, that's what happened. |
Yeah, I suspected that too. So, what you tell me is that before I added sulphuric acid, I was left with a mixture of both potassium dichromate and
potassium hypochlorite, right? Adding the sulphuric acid made chlorine and water bubble out?
What's a bit gloomy here is that you suggest this was no error, and I will obtain the same result anytime. What should I do to get rid of the
Cl- ions before getting to Cr VI?
By the way, this page suggests that using chromium III chloride as a starting point is feasible. TBH, I followed it almost by the book, and yet got out of the
way results (e.g.: how come the oxidation was exothermic while it should’ve been endothermic?).
[Edited on 2-5-2019 by Keras]
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woelen
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Getting K2Cr2O7 in the EU is not a problem. Right now, it is available on eBay, also for EU people.
You can also order this at limac.lv. Getting CrO3 from limac.lv is possible as well, but you need to fill in an end user form for that.
Something is wrong in your initial experiment. The fact that you get an orange solution from H2O2, excess KOH and CrCl3 is not good. I think you
either have very impure reagents, or you only have very dilute solutions, and you have some peroxo complex of chromium(V) in solution (which is brown
at higher concentrations, but can look orange at low concentration, especially if also some yellow chromate is present as well).
A few years ago I did an experiment with oxidation of chromium(III) to chromium(VI) with H2O2 at high pH. I found that the reaction does not run
easily. I needed to boil for quite some time. After that period I had a yellow solution, but it still was not purely yellow, it had a green tinge.
Commercially, chromium(VI) is made by melting Cr2O3 with KNO3 and a little KOH or K2CO3. The route with H2O2 is not very convenient.
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Bedlasky
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If you want to prepare chromate from chromium(III) chloride and hydrogen peroxide, you should first precipitating chromium chloride with sodium
carbonate and filter chromium hydroxide. After that chromium hydroxide dissolve in KOH solution (you haven't chloride ions in solution), add hydrogen
peroxide and boiling it. After decomposition of H2O2 you should have yellow souliton of potassium chromate without any reducing agents.
Whats with you Cr(III) solution? Try crystalized it. You should have potassium-chromium sulfate which have relatively low solubility (especially in
conc. H2SO4).
Woelen: I thought that chromium peroxide complex is formed in acidic solution and have dark blue colour. Or exists another complexes in alkaline
solution?
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Keras
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| Quote: Originally posted by Bedlasky | | Whats with you Cr(III) solution? Try crystalized it. You should have potassium-chromium sulfate which have relatively low solubility (especially in
conc. H2SO4). |
Ok, I can try that. Alternatively, I can probably neutralise (carefully) the excess of sulphuric acid with KOH until Cr(OH)3 forms and
precipitates, then filter and wash with distilled water (sodium/potassium sulfate is highly soluble in water, so is KCl, assuming there is any
Cl- ions left in solution). Does that make sense?
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Bedlasky
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Yes, you can try this. But solution will have large volume. You must dilute your chromium(III) solution because of high concentration of H2SO4 and add
slowly solution of hydroxide. Diluting H2SO4 and neutralization are very exothermic. And you should wear goggles. Neutralize with hydroxide only to pH
cca 7. After that precipitate Cr(OH)3 with sodium carbonate - excess of sodium carbonate can't dissolve Cr(OH)3. Add carbonate slowly because of CO2
evolution.
[Edited on 3-5-2019 by Bedlasky]
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woelen
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@Bedlasky: Chromium forms a whole bunch of peroxo complexes. I wrote webpages about them:
The following are about complexes made at high pH:
http://woelen.homescience.net/science/chem/exps/K3CrO8_synth...
http://woelen.homescience.net/science/chem/exps/Cr_ammine_pe...
http://woelen.homescience.net/science/chem/exps/raw_material...
http://woelen.homescience.net/science/chem/exps/explosive_ch...
At low pH you get the deep blue complexes:
http://woelen.homescience.net/science/chem/exps/cr_peroxo_py...
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Bedlasky
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Thanks, it's very interestring. I've read about tetraperoxochromate once, but just briefly. The blue complex I tried prepare few times, the best
result I have with sec-amyl alcohol. I thought to try it with ethyl acetate, but I don't want to buy one litre of ethyl acetate just only for this.
Btw you have nice web 
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Keras
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I’m somehow resuscitating this thread because I got back to my deep green Erlenmeyer flask yesterday, that I'd left more than a month ago sealed
with a rubber stopper. When I opened it, I got a strong whiff of hypochlorite (smelled like concentrated bleach), which seems to indicate there was
some hydrogen peroxide left which reacted with the chlorine ions to form ClO.
At the bottom of it was a layer of big transparent flakes of potassium (bi)sulfate, I suppose (since I originally used KOH to basify the solution of
CrCl3).
So, my plan was to neutralise the amount of sulphuric acid present in the mix, then with excess NaOH precipitate Cr(OH)3, wash it, and redo
from start to get Na2CrO7 or K2CrO7.
But something strange happened.
1. Carefully dropping NaOH beads into the solution produced a small effervescence when the beads hit the solution, then nothing. However, a couple of
minutes later, unexpectedly, the whole beaker started to hiss or seethe (but no frothing, no bubbles visible on the surface) – might be heat
generated by the neutralisation process on the surface of the beads locally boiling the water which would then immediately condense back into liquid.
After a while (and putting the beaker in an ice bath), it subsided.
2. I added more pellets of NaOH with stirring, but nothing happened. More surprisingly, when I filtered the solution off, I found that most pellets
(if not all?) had not dissolved at all. Despite stirring, they were still left intact at the bottom of the beaker.
I’m a bit baffled – how come NaOH pellets won’t dissolve in a 250 ml water solution of various ions? Could it be that the solution was fully
saturated? (Needless to say, I got not gelatinous chromium III hydroxide precipitate.)
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woelen
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it might be that the pellets get covered by some insoluble layer of Cr(OH)3, which covers the interior of these pellets and prevents dissolving and
further reaction.
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Keras
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| Quote: Originally posted by woelen | | it might be that the pellets get covered by some insoluble layer of Cr(OH)3, which covers the interior of these pellets and prevents dissolving and
further reaction. |
I hadn't thought about that. Well done! Next time I can get to it, I’ll dissolve the pellets in fresh water and then pour the fresh NaOH solution
into my green mix.
Thanks for the hunch!
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