Foeskes
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Chromium(III) oxide from K2Cr2O7
I need some Cr2O3 for a thermite reaction however I only have some potassium dichromate. According to wikipedia it can be reduced with sulfur.
Although I'm not sure how messy it would be.
[Edited on 3-9-2017 by Foeskes]
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j_sum1
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It can be reduced with just about anything. I commonly convert waste Cr(VI) to Cr(III) for storage and later recovery. I use whatever is at hand:
often ethanol or pieces of zinc. You can then precipitate out the Cr(III) by adjusting the pH. Roasting will give you the oxide.
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Foeskes
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Is there a way to tell if it's completely reduced since the green solution is super dark? Anyways does ethanol take a long time?
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j_sum1
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I always use a bit of excess and of couse keep it acidic as well as giving it some time.
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JJay
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Ethanol generally does take a long time to react with dichromate unless it's acidic.
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ninhydric1
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Sodium dithionite (Na2S2O4) is my favorite because you can immediately see the color change from hexavalent to trivalent chromium. The insoluble
chromium(III) hydroxide can be filtered out. I'm pretty sure anything that can reduce works, but sodium dithionite works the fastest IMO.
[Edited on 9-5-2017 by ninhydric1]
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Foeskes
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Does chromic acid actally oxidize glucose and other organic reducing agents(ascorbic acid) to CO2 and water?
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woelen
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Dithionite is a fast reductor, but not very clean. It gives decomposition products, which may lead to contamination with solid finely divided sulphur.
A better reductor is sodium sulfite, acidified. It acts instantaneously on dichromate at low pH. You get a dark green solution.
Procedure:
- Compute stoichiometric amounts of potassium dichromate, sulphuric acid and sodium sulfite.
- Take appr. 1.25 times the stoichiometric amount of acid and dilute with water so you get 10% acid or so.
- Dissolve potassium dichromate in the dilute acid.
- Take appr. 1.1 times the stoichiometric amount of sodium sulfite and dissolve in water.
- Carefully mix the solutions. The final solution will have a smell of sulphur dioxide.
- Boil for a while. This drives off sulphur dioxide.
Allow solution to stand for several weeks. The color shifts from green to ark blue/grey. This period allows the green sulfato-complex of chromium(III)
to decompose.
You may skip this period of a few weeks and use the green solution as is, but it will lead to less pure Cr2O3 in the next phase. The material then
will contain some residual sulfate which is bound strongly to the chromium ions. For thermite this probably is not a real issue, but I want to mention
it anyway.
Add excess ammonia to the green (or grey/blue) solution. A lot of precipitate is formed. This is Cr(OH)3 and if the solution is not allowed to stand
for some weeks, then it will also contain some Cr(SO4)(OH).
Filter the solid material.
Heat the solid material to a few hundred degrees. It will lose nearly all water and become Cr2O3. If it contains some sulfate, then it will be mainly
Cr2O3, but some oxygen ions will be sulfate ions instead.
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If you don't want the long waiting period, then use dilute nitric acid and ethanol as reductors. You get a bluish/grey solution right away and then
you can add ammonia to that. Sulfate ions lead to the formation of green solution. Chloride is even worse and will never be removed. You then get a
mixed chloride/oxide material on addition of ammonia.
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Foeskes
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So is the reduction using sulfur a viable way?
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woelen
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I would not do that. The reaction is very incomplete and certainly does not work smoothly. Just try it. Mix a little sulphur with K2Cr2O7 and heat the
mix. You just get some smouldering and if heating strongly enough the sulphur may burn. Compare this with mixes of sulphur with e.g. KNO3, KClO3 or
KMnO4. You'll see a huge difference.
The reaction does work, but you'll have a lot of work separating the solid Cr2O3 formed in the melt, mixed with unreacted sulphur and other partially
soluble solid materials.
I think that with alkali dichromate salts the only way to Cr2O3 is aqueous reduction to chromium(III) and then precipitating with ammonia (not with
NaOH or KOH, because a small excess will cause your chromium(III) to go in solution as chromite).
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Foeskes
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I did the sulfur method and got 20g (theoretical: 25g), I boiled out most of the sulfur. And removed most of the sulfate. Anyways it should be good
for thermite.
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S.C. Wack
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Schlessinger:
7. Chromium (III) Oxide
(I)
K2Cr2O7 + 2NH4Cl -> Cr2O3 + 2KCl + N2 + 4H2O
Thirty-seven grams of potassium dichromate are mixed intimately with an equal weight of ammonium chloride in a clay crucible and heated with slowly
rising temperature over a Bunsen flame in the hood until no more vapors are given off. The contents are cooled and powdered. The material is washed by
decantation with successive portions of boiling water until no more test for chloride is obtained, then filtered and dried for several hours at
105°-120°C. The filter cake is reground and dried again overnight. The product is a loose green powder which is not attacked by acids or alkalies,
especially after ignition.
Yield almost theoretical.
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