phlogis
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Registered: 12-10-2015
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Why does wet Salicylic Acid turn black?
I prepared some by hydrolysis of Aspirin, and did not filter it very well before leaving it out to dry. When I looked at it later, some of it had
decomposed into "black stuff". I've also noticed this same thing happen to wet Paracetamol when it had accidently been left in the bottom of a wet
glass once.
I know how to stop it from happening: by vacuum filtering it and not letting it stay wet for too long, but I was just curious about what makes this
happen? I notice the similarity between both compounds is that they have a Phenyl ring with a -OH group attached to it, and wondered if that somehow
had anything to do with it?
Googling anything with the terms Salicylic Acid and Black just tells me that teenagers use it treat black heads, and Paracetamol + Water just has lots
of results telling me how to extract Codeine from Paracetamol containing pills. 
Feel dumb even asking about this, so please be gentle ;-)
Thanks
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Sulaiman
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me too
I have no idea of the chemistry involved
but when I re-packaged my commercial / food grade (almost all is made in China) salicylic acid powder in a transparent bottle
after a few days there were three small black spots on the inside of the walls,
I thought some kind of mould/yeast/fungus so reached in with a spatula and removed three small black lumps.
There have been no more black spots in months
so I assumed there were three tiny drops of water left on the inner wall of the bottle due to incomplete drying after washing.
So, a long anecdote just to say
I too think that water + air makes black spots/clumps in salicylic acid powder
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OneEyedPyro
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I highly doubt it's any kind of chemical decomposition.
One of the ingredients in most pills including aspirin is corn starch which is an absolute breeding ground for fungi and bacteria.
My best guess is that it's just mold.
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deltaH
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How did you hydrolyse your aspirin?
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phlogis
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Ooops! Sorry!
OneEyedPyro: You are wrong - it's my brain that is decomposing! I put the salicylic acid onto a pyex dish to dry, but what I didn't notice was the
dirty metal stanley blade that was underneath it until I came back from shopping this morning. The black stuff was just black stuff that was all over
the blade and dissolved in the water. Now I feel stupid  The paracetamol
incident probably was just mold...
deltaH: I hydrolysed the aspirin by refluxing it with NaOH, filtered to remove the tablet fillers, then acidified the filtrate with HCl to precipitate
the Salicylic Acid. I did read about it on a web page somewhere, and I beleive there was a Youtube video as well, but it was a long time ago when I
watched it.
Thanks for the replies! I hope no one wasted too much brain power trying to figure that out for me. I was hoping to try making Phenol by
decarboxylating the Salicylic Acid, but it looks like I should probably start again and be more careful. What a great way to introduce myself to this
forum ;-)
[Edited on 12-10-2015 by phlogis]
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