Sciencemadness Discussion Board

How to make chromium (III) oxide (Cr2O3)

Apolo7 - 15-11-2017 at 13:23

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]

UC235 - 15-11-2017 at 17:40

I think you're searching wrong. Look on ebay for Cr2O3. I see a pound for $15, half pound for $9

RogueRose - 15-11-2017 at 20:12

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?

ninhydric1 - 15-11-2017 at 20:33

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.

woelen - 16-11-2017 at 00:03

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.

ninhydric1 - 16-11-2017 at 07:36

The OP says that he wants to make potassium dichromate, so wouldn't alkali hydroxides be OK for the procedure?

Fantasma4500 - 8-3-2022 at 11:47

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

S.C. Wack - 8-3-2022 at 14:56

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.

Fantasma4500 - 9-3-2022 at 05:37

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?

S.C. Wack - 9-3-2022 at 15:09

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.