Quibbler
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Catechol colours with carbonate
Some time ago I mixed a catechol (1,2-dihydoxybenzene) solution with potassium carbonate (can't remember why) and the solution became green. Even
more confusing when shaken the solution turned red then returned to green after standing. Does anybody have any idea what was going on?
E.b.C: title&spelling
[Edited on 14-9-2005 by chemoleo]
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chemoleo
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Catechol forms with NaOH the catechoxide dianion, which is, due to its resonance structure likely to cause the colour change. I don't know what
this thing about shaking it is about, I imagine that gasses from the air (O2, CO2 less likely) cause it in one form or another.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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neutrino
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This would explain the red color.
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Quibbler
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I've tried it again and its a bit difficult to reproduce, but on about my 30th attempt I got a brown/red colour which turned dark green on
shaking. The process could be repeated a number of times.
Another observation is that (as the link says) adding NaOH gives a dark red/brown colour, but this does not change on shaking. Adding bicarbonate has
no effect.
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Kinetic
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Catechol
Are you still wondering why you observed the colours you did? The link provided by neutrino has essentially answered your question! Bicarbonate is not
a strong enough base to deprotonate catechol, so the anion that readily oxidises to coloured products cannot form, or if it does, only very slowly.
Hence, no colour change is observed.
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Quibbler
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Yes I see what causes the colours the quinone type structure. What I am trying to find out is:
Start : colour red/brown
shake: colour green
stand for 1 minute colour back to red/brown. Repeat as mant times as you like. Its in a sealed conical flask and no noticable pressure/vacuum builds
up during the shaking or rest periods.
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