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Topic: Preparing Chrome Alum and Inhibiti Hydrogen peroxide  (Read 7652 times)

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Offline DQD

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Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« on: November 09, 2011, 06:30:06 AM »
I prepare Chrome Alum from K2Cr2O7, pure H2SO4 and Ethanol 95o
And the question is
What is the Ethanol's effect in process ?

Which substance can Inhibiti Hydrogen peroxide while storing ?
And its mechanism ?
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Offline Borek

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #1 on: November 09, 2011, 07:59:20 AM »
What you make is not a chrome alum. Chromic acid if anything.
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Offline DQD

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #2 on: November 09, 2011, 10:19:18 AM »
It's impossible, solution after reacting has dark green color
and I have let it for 1-2 week(s), it appear crystal ???
I do this experiment from my text book
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Offline Borek

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #3 on: November 09, 2011, 12:15:36 PM »
Hm, perhaps it makes sense - it guarantees correct Cr:K ratio. Interesting.

Think: why does the solution become green?
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Offline DQD

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #4 on: November 09, 2011, 07:19:42 PM »
i wonder
May ethanol cause ?
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Offline Borek

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #5 on: November 10, 2011, 02:38:52 AM »
Yes, it is because of the ethanol presence, but it is not ethanol that becomes green.

What is green in this solution? Initially the solution should be red/brown. What do you know about dichromate, especially in low pH?
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Offline DQD

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #6 on: November 10, 2011, 07:14:13 AM »
Green in this solution, I guess mixture of Cr2(SO4)3 and K2SO4 cause this color
not only Cr2(SO4)3, which has pink color

And in low pH, I dont be sure to know about dichromate :)
Plz explain this problem
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Offline Borek

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #7 on: November 10, 2011, 11:11:16 AM »
If memory serves me well Cr(III) in solutions is green as long as there are no chlorides.

I am not going to give you a direct answer. First paragraph of the wikipedia page on dichromate contains an important hint.
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Offline DQD

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #8 on: November 10, 2011, 08:52:10 PM »
Um
So in low pH, It means in acid solution the unstable blue peroxo complex Chromium(VI) oxide peroxide, CrO(O2)2, is formed
CrO(O2)2 is a substance which I have never learn it  ::)
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Offline Borek

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #9 on: November 11, 2011, 05:28:23 AM »
Another hint: redox process.
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Offline DQD

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #10 on: November 12, 2011, 11:40:53 PM »
I think i dont have any anwser :D
btw, I hear my friend explain about it
due to the strong acid hydrolysis of dichromate, the ethanol presence is inhibiti this hydrolysis
According you, Does this thing right ?
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Offline Borek

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Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
« Reply #11 on: November 13, 2011, 06:09:37 AM »
It is not about hydrolysis. It is about reducing Cr(VI) to Cr(III).

I already told you almost everything, start to follow hints and learn what a redox process is.
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