Apolo7
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How to make chromium (III) oxide (Cr2O3)
I want to make potassium dichromate, but I need a source of chromium oxide for that. As of what I've seen, chromium oxide is quite expensive (25g are
about 5$), but chromium metal is not (100g are about 6$). To my knowledge, chromium is quite unreactive, so I don't think that hydrogen peroxide will
be able to turn it into its oxide. What method would you recommend for producing chromic oxide? I have access to quite a few oxidants (35% H2O2,
KMnO4, KNO3, KClO3) and acids (HCl, HNO3, H2SO4). I was thinking of dissolving the chromium with nitric acid, but I can't find any documents about the
reaction of the metal and the acid. I will then react the nitrate with sodium oxide and hopefully get chromium oxide (and useful NaNO3 as a side
product). You might say that it will be cheaper to just buy the oxide directly, but HNO3 is very cheap where I live (0.36 dollars a liter of
azeotropic nitric acid), and I can easily make Na2O by decomposition of Na2CO3.
[Edited on 15-11-2017 by Apolo7]
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UC235
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I think you're searching wrong. Look on ebay for Cr2O3. I see a pound for $15, half pound for $9
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RogueRose
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Quote: Originally posted by Apolo7  | I want to make potassium dichromate, but I need a source of chromium oxide for that. As of what I've seen, chromium oxide is quite expensive (25g are
about 5$), but chromium metal is not (100g are about 6$). To my knowledge, chromium is quite unreactive, so I don't think that hydrogen peroxide will
be able to turn it into its oxide. What method would you recommend for producing chromic oxide? I have access to quite a few oxidants (35% H2O2,
KMnO4, KNO3, KClO3) and acids (HCl, HNO3, H2SO4). I was thinking of dissolving the chromium with nitric acid, but I can't find any documents about the
reaction of the metal and the acid. I will then react the nitrate with sodium oxide and hopefully get chromium oxide (and useful NaNO3 as a side
product). You might say that it will be cheaper to just buy the oxide directly, but HNO3 is very cheap where I live (0.36 dollars a liter of
azeotropic nitric acid), and I can easily make Na2O by decomposition of Na2CO3.
[Edited on 15-11-2017 by Apolo7] |
Where do you find HNO3 for that price? Is it used locally for something? What country?
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ninhydric1
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If nitric acid is cheap for you (definitely not for me), allow the metal to sit in the acid until no metal remains. Boil down the solution and
decompose the chromium nitrate at above 100 degrees Celsius into produce chromium(III) oxide, nitrogen dioxide, and probably water due to the water of
hydration. Convert the chromium(III) oxide to dichromate using alkali hydroxide and hydrogen peroxide. It would be easier to buy chromium(III) oxide
directly and cheaper too.
The philosophy of one century is the common sense of the next.
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woelen
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You will have a very hard time dissolving chromium in nitric acid. The metal passivates and dissolving will be VERY slow. It dissolves more easily in
concentrated hydrochloric acid, giving CrCl2 and CrCl3. If you allow the solution to stand in contact with air, then quickly all chromium(II) is
oxidized to chromium(III). The solution will be dark green. Keep in mind that dissolving it in conc. HCl requires a large excess amount of acid,
otherwise the reaction also will be very slow in hydrochloric acid. Heating the acid somewhat does help.
Once you have the metal dissolved, neutralize the acid with excess ammonia. This will cause precipitation of mainly Cr(OH)3. Only very little of the
metal ions remain in solution as a purple/red ammonia complex. You can filter this precipitate, rinse with distilled water to get rid of chloride ions
(but you will not get it totally free of chloride) and then heat it to make Cr2O3.
Do not use NaOH or KOH to precipitate the chromium(III). You will introduce sodium ions, which are a hard to remove contamination, and if you add too
much hydroxide, then the chromium(III) dissolves again, as the green soluble sodium chromite. With excess ammonia you do not have this problem, the
chromium remains precipitated in that case, so it is easier to have nearly all chromium precipitated, just add a lot of ammonia, so that all acid is
neutralized, plus some extra.
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ninhydric1
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The OP says that he wants to make potassium dichromate, so wouldn't alkali hydroxides be OK for the procedure?
The philosophy of one century is the common sense of the next.
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Fantasma4500
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is there no other way than heating chromium hydroxide? i checked up on it and it was i believe... 650*C, which is.. somewhat feasible to reach- its
def on the end where electronic heating equipment fails if its not designated for reaching that temperature
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S.C. Wack
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Of course there are (from higher oxidation states), but not everyone rushes to answer question threads in the wrong subforum from 1-posters who are
clueless or trolling. (at least the subject was spelled correctly)
Measuring the temperature is unnecessary. Obviously, heating the hydroxide or buying the pigment isn't something that one would want to do at all if
the goal is Cr+6...not that it's impossible...just more of a challenge.
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Fantasma4500
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im realizing ammonium dichromate decomposes into Cr2O3- but if you wanna recycle 100g of Cr2O3 for a reaction that probably gets silly eventually-
supposing that its a well exothermic reaction that will yield some chrome and, even dichromate vapors, and then you'd have to also produce the
ammonium dichromate in the first place
as for chromate the chromium hydroxide can simply be reacted with bleach, i'd typically mix it with salt and run electrolysis on it, but KNO3 can also
be used for it, yielding potassium nitrite and potassium chromate and thats where i see it as interesting
heating to 650*C is very doable, using a hotplate that can go to high temperature, ceramic wool and a propane gas torch will get you far - even though
i can buy 1kg at 30 euros, its preferential to just have 100 grammes that you recycle
could dichromate salts maybe be reduced in a less hostile manner than decomposition of ammonium dichromate to yield Cr2O3?
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Texium
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Thread Moved
9-3-2022 at 07:15 |
S.C. Wack
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Directions are in at least half of the 8 full inorganic lab manuals I scanned, and many references are given in my scan of the 362 page chapter 60 of
Mellor. They all involve heat.
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