inspector071
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Problems purifying ASA
I'm attempting to make some more picric acid. I made a little bit the other night by starting with 50 aspirin tablets and had no issues purifying the
tablets by dissolving them in hot ethanol, filtering, then boiling off the alcohol. The ASA was a clean white powder. Tonight I went out and got some
more 95% ethanol just like I used a few nights ago. I added the aspirin powder of 200 tablets to 650 mL of ethanol, heated to 60°C, filtered and
started to boil off the alcohol. Everything was going fine until it got down to about 150 mL. The liquid became thicker and noticibly yellow. Fine
bubbles appeared in the liquid but it wasn't bubbling as if it was boiling. I let it boil for an hour or so more and hardly any change. It simply
wouldn't boil off the alcohol. I removed it from the water bath and let it cool down. It thickened up even more (corn syrup consistency) and is quite
yellow. What could have gone wrong?
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Rosco Bodine
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Aspirin is not perfectly stable, it usually stinks like vinegar from steady but slow decomposition. Probably excessive heat split off acetic acid and
has left decomposition products which won't crystallize but are forming a depressed melting point mixed product. Instead of boiling off the alcohol
solution of freshly dissolved aspirin, just let it evaporate. The hard but somewhat waxy crystal cake can be broken up with a potato masher.
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inspector071
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Evaporate at room temp? I do have a big evaporating dish I can use for that. Instead of paying $20 for 750 mL of 95% ethanol, what other solvents will
work to dissolve but not decompose ASA? Would any of the denaturing agents in denatured alcohol ruin the ASA?
The thick liquid didnt smell like acetic acid, but it did have an odor a little different from alcohol. I put it in the freezer and it became much
thicker, like I imagine what corn syrup would do if you froze it. I tried to burn a little thinking any alcohol in it might evaporate off but it did
not ignite. The color is almost a highlighter yellow. It was only boiling off at around 100°C and I believe ASA decomposes around 140°C. I guess
what pisses me off most is I ended up wasting $20 of alcohol and 200 aspirin tablets.
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Rosco Bodine
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Denatured alcohol is fine. Easiest would be to simply buy powdered aspirin USP as is sold for veterinary use mainly. It is cheap and can be used as is
because it is pure crystalline aspirin and nothing else.
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inspector071
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Thanks, I'll look into that. You must be the Rosco whose ASA to TNP procedure I am using. Can potassium nitrate be used in place of sodium nitrate if
the moles of nitrate are the same?
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Rosco Bodine
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Yes but it is probably not a direct equivalent substitution and substantially more H2SO4 will be needed for the same molar amount of KNO3 because the
potassium bisulfate byproduct is less soluble in the reaction mixture. Temperature will probably be shifted lower. I haven't done any evaluation of
what changes may be effected. Many experiments I have described are very specific and changing any parameter will affect the result. Changing any
reactant basically
changes other things too and how much the process is changed would have to be charted and analyzed carefully. Nitrations are fickle about conditions.
Changing one thing can change other things also. So I can't give you a detailed answer for not having charted the differences.
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Bot0nist
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From experience I can say it works with potassium nitrate. Just increase the amount of 93% acid a little to keep it manageable, as Rosco says.
U.T.F.S.E. and learn the joys of autodidacticism!
Don't judge each day only by the harvest you reap, but also by the seeds you sow.
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niertap
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Have you considered getting the cheapest kind from a place like cvs, then just washing away the starch binder? ASA is very insoluble, so mixing
~45g aspirin with ~200ml H2O will remove the starch and only remove a fraction of a gram of ASA.
Ignorance is bliss
Outliers in life are modeled by chemical kinetics
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