Lesson: Acid-catalyzed hydrolysis of acetylsalicylic acid to salicylic acid and acetic acid
What Happened: We dissolved the active ingredient of aspirin in water, separated it from the binder, then purified the drug using sulfuric acid as a catalyst.
The World of Chemistry video series, which you can watch online at Annenberg Media, has been serving as our spine lately. We were up to the episode on catalysts this week, so I found a demonstration from The Joy of Chemistry (actually from the chapter on organic chemistry) which used dilute sulfuric acid (sold as aquarium pH lowering solution) as a catalyst to purify aspirin. FYI, another example of a catalyst at work were the pineapple enzymes we used to dissolve Jello.
According to Wikipedia, the end product of this demonstration, salicylic acid, is what aspirin metabolizes into in the liver. Its name comes from the Latin word for the willow tree, Salix, from whose bark it can be obtained. Interestingly, it can also be derived from methyl salicylate (oil of wintergreen). In 1897, Felix Hoffmann, a chemist at Friedrich Bayer & Co., obtained acetylsalicylic acid by a reaction of salicylic acid and acetic anhydride; this is the basis for Bayer's claims to the discovery of aspirin.
10-15 aspirin (plain or buffered)
½ cup (120 ml) rubbing alcohol (70% isopropyl)
2-4 large glass containers (we used a Pyrex bowl and old honey jars and canning jars)
Coffee filters and rubber bands
Sturdy plastic spoon
Aquarium lowering solution (dilute sulfuric acid)
Pipette or straw
- Place aspirin in glass.
- Pour in alcohol, a little more than needed to cover the aspirin.
- Heat the glass in the microwave on 50% power for 30 seconds until warm but not boiling. The acetylsalicyclic acid will dissolve in the alcohol, leaving the starch binder.
- Gently crush remaining residue with spoon to extract as much acid as possible. Let sit 15-30 minutes.
- Take coffee filter and spread it over top of second glass. Push it down slightly so it resembles a funnel. Secure with rubber band.
- Carefully pour the solution through the filter. The liquid that drips through is called the “mother liquor.” The acetylsalicylic acid has dissolved in the water. What's left on the filter is the starch binder that holds the drug in the pill shape.
- Wearing gloves, dispose of coffee filter. Don’t touch the wet part.
- Run a small stream of cold tap water. Take the glass with the mother liquor and add water until it is about ¾ full. Small white flakes of acid should begin falling out of solution. Let sit for a couple hours.
- Set up another filter on another glass. Pour mixture through filter to separate out the crystals. Filter 2-3 times if needed, letting solution sit for 1-2 hours in between.
- Allow to dry overnight, away from breezes. The crystals will become fluffy.
- Take ¼ of wet or dry crystals and put into glass. Add aquarium solution dropwise with a pipette or straw until the entire sample is completely covered. The sulfuric acid is the catalyst and remains at the end, so be careful with the liquid.
- Heat the mixture in the microwave for no more than 15 seconds at 50% power. It may start to steam immediately.
- Remove glass. You should smell vinegar (acetic acid) evaporating. If not, wave your hand over the glass to waft the fumes towards your nose. The sludge that remains is salicylic acid.
- Dispose of solids in the trash and liquids in the toilet.
NOTES: We ended up doing the demonstration twice -- although, as it turned out, we probably didn't need to -- because the shopping list at the beginning of the book didn't specify that the alcohol needed was 70% concentration. I found an old bottle of the right concentration, and we did everything over. However, we discovered that letting the first solution sit and filtering it several more times yielded enough crystals to do the demonstration.
In the end, we had twice as much acetylsalicylic acid as we needed. We probably used too much in the final step (as well as too much sulfuric acid, which I tried to pour slowly out of the bottle instead of using a pipette) because when we put it in the microwave, it immediately started steaming! I turned it off a few seconds short of 15 and the vinegar smell was overwhelming.