Kids, we all know that aspirin is a medicine, but did you know that it is also a chemical?  It’s name is acetylsalicylic acid.  You have probably heard that it can cause stomach discomfort in some people – maybe even yours, too.  One way to lessen this is to combine the aspirin with an acid buffer — a combination of chemicals that reduces acidity.  This buffered aspirin is a genuine help for this group of people.  But for people who have take aspirin every day (like arthritis sufferers), this is not good enough.  For them, chemists invented specially coated aspirin tablets that pass through the stomach without dissolving.  The coating resists the acid juices of the stomach, but dissolves quickly in the basic environment of the small intestine.   Called “enteric” aspirin, they obviously take a bit longer to work. 

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

In this column you will simulate a stomach to observe the chemistry of aspirin there.  You can see the difference between regular aspirin, buffered aspirin, and enteric aspirin by testing the tablets in neutral, acidic, and basic solutions.   Your stomach is acidic, but your small intestine is basic.  Begin with three clear glasses or plastic cups.  Add 1/2 cup (120 ml) of water to each container.  Note the time, then simultaneously add a regular aspirin tablet to one container, a buffered aspirin to another, and an enteric aspirin tablet to the third.  Note changes in the tablets at 30-second intervals until no further change is evident.  To simulate the acid environment of the stomach, repeat this procedure using vinegar in each container instead of water.  You will see one of the tablets dissolve more vigorously than before.   To simulate the basic intestine, repeat the experiment once more using a solution made by adding 16.3 g (1.5 teaspoons) of powdered baking soda, NaHCO₃, to each 1/2 cup of water.  One of the tablets will dissolve suddenly after a delay of 20-30 minutes.

For the more technical students in the audience, we did a bit of research into buffering agents and coatings.  The typical buffers used to raise the pH of aspirin are MgO and CaCO₃ (magnesium oxide and calcium carbonate).   Enteric coatings appear to have recently changed to an aqueous acrylic resin such as methacrylic acid copolymer, although phthalates of cellulose polymers may still be used.

Here is another interesting aspirin chemistry note.  As acetylsalicylic acid ages, it can decompose to salicylic acid and acetic acid.  If you have a very old bottle of asprin around the house, open it and take a whiff.  It might smell like vinegar, which is dilute acetic acid.  This is one reason why there are expiration dates!

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
January 2005

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Reference:

Gail Marsella and David Robson, ACS ChemMatters, February 1993.

http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/buffered-aspirin.shtml

Enteric coatings:  Pharmaceutical Technology, 2004