Sciencemadness Discussion Board

Cr(III) to dichromate

reactofurnace - 26-6-2019 at 09:22

Guys I recently did a reaction involving the oxidation of 2-propanol with acidified dichromate. (the image below shows an oxidation reaction with potassium dichromate using ethanol)

Because chromium is not particularly safe in the environment, i wanted to recycle all my chromium(III) waste. The procedure i adapted involved reacting the Cr(III) with hydrogen peroxide in the presence of sodium hydroxide.
i did the reaction in a test tube and it seems to work perfectly as there was a colour change from green to yellow/orange. If i added too much NaOH the solution would turn slightly more yellow(due to the formation of chromate).

I wanted to make a youtube video on this procedure but i'm not 100% sure of how the reaction works.
This is the balanced equation I came up with:
Cr2(SO4)3 + 10 NaOH + 3 H2O2 --> 8 H2O + 2 Na2CrO4 + 3 Na2SO4
Is this the reaction happening?

Also is it possible to let the solution dry in air?(ik sodium salt have a tendency to be hydroscopic).




Thanks to Bedlasky on the forum for suggesting this procedure




[Edited on 26-6-2019 by reactofurnace]

Bedlasky - 26-6-2019 at 10:01

Hi.

Yes, this reaction is happening. If you get pure sodium chromate from solution, evaporate water (but not all water) and cool it down to 0°C. After one or two days sodium chromate will be crystalized on the bottom. If you decant solution from crystals and dissolve some ammonium sulfate and again cool it. After day will be crystalized ammonium chromate. Both chromates wash with small amount of cold distiled water.

During oxidation of chromium(III) be careful because of chromium(VI) aerosol. Cover the beaker with paper tissue or overturned funnel.

[Edited on 26-6-2019 by Bedlasky]

Keras - 26-6-2019 at 11:31

If you turn chromium III into chromium VI, you turn a cation which is mildly annoying to something which is a known pollutant and carcinogen. I'd suggest you find a way to reduce Cr III into Cr (0) metal, like, e.g. aluminium foil or vitamin C, maybe?


Ubya - 26-6-2019 at 11:35

Quote: Originally posted by Keras  
If you turn chromium III into chromium VI, you turn a cation which is mildly annoying to something which is a known pollutant and carcinogen. I'd suggest you find a way to reduce Cr III into Cr (0) metal, like, e.g. aluminium foil or vitamin C, maybe?




he wants to recycle his Cr III turning it back to dichromate, not for disposal, but for reuse

Keras - 26-6-2019 at 21:53

Quote: Originally posted by Ubya  

he wants to recycle his Cr III turning it back to dichromate, not for disposal, but for reuse


Oh, okay. My bad :p
Then yeah, hydrogen peroxide is definitely the way to go.
Alternatively, you could use sodium percarbonate. Sodium percarbonate is basic so you don't have to use sodium hydroxide.