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Topic: bicarbonate with conc. sulphuric--something turned black?  (Read 4204 times)

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Offline wilson

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bicarbonate with conc. sulphuric--something turned black?
« on: May 20, 2008, 05:34:32 AM »
I recall performing the familiar bicarbonate reaction, with conc. H2SO4 (18M). There was phenolphthalein indicator in the H2SO4 used. So it appeared orange.

When both reagents were mixed together, effervescence was noted. But after it was left to stand for some time, the mixture turned black!

Anyone has any idea what happened?
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Offline Borek

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Re: bicarbonate with conc. sulphuric--something turned black?
« Reply #1 on: May 20, 2008, 06:36:26 AM »
Many organic compounds turn black in the presence of concentrated sulfuric acid. It is a very strong dehydrating agent, quite often it leaves only carbon behind.
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Offline wilson

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Re: bicarbonate with conc. sulphuric--something turned black?
« Reply #2 on: May 20, 2008, 06:58:01 AM »
Quote from: Borek on May 20, 2008, 06:36:26 AM
Many organic compounds turn black in the presence of concentrated sulfuric acid. It is a very strong dehydrating agent, quite often it leaves only carbon behind.
That's exactly what I thought when I saw the black stuff--dehydration. However, my tutor said no, it is hard for carbon to "come out". I just did not bother about it then.

Talking about dehydration, I suppose the organic compound you are talking about here is the phenolphthalein indicator. Any chance that the carbon came from the bicarbonate? Although my mind wants to think that the black mixture is caused by dehydration, my heart doubts it. Is dehydration the only possible scenario?
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Offline Borek

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Re: bicarbonate with conc. sulphuric--something turned black?
« Reply #3 on: May 20, 2008, 07:18:08 AM »
Almost any organic impurity will do. Well, some compounds are stable even in concentrated sulfuric acid, but most - especially those more complicated - are not. Carbon from bicarbonate is out of the question, it will just fly away as CO2.

TBH I don't know what was the reason - I am just suggesting the most likely one, doesn't mean that's what happened.
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Offline wilson

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Re: bicarbonate with conc. sulphuric--something turned black?
« Reply #4 on: May 20, 2008, 10:07:20 AM »
Quote from: Borek on May 20, 2008, 07:18:08 AM
Almost any organic impurity will do. Well, some compounds are stable even in concentrated sulfuric acid, but most - especially those more complicated - are not. Carbon from bicarbonate is out of the question, it will just fly away as CO2.

TBH I don't know what was the reason - I am just suggesting the most likely one, doesn't mean that's what happened.
I understand.

It is unlikely that phenolphthalein was dehydrated. If it was, the sulphuric acid reagent itself would have been black. But that wasn't the case...
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