The Synthesis of Aspirin
Purpose: To synthesize aspirin and to determine the yield or
fraction of the theoretical amount which can be made. The purity of the
product is confirmed by measuring its melting point range.
Safety Considerations: This experiment uses salicylic acid, acetic
anhydride and phosphoric acid. The salicylic acid and aspirin may cause
irritation to your skin or eyes, but are basically not hazardous. An excess
of these can be disposed of in the sink or if packaged, in the trash. If
you spill some, wipe it up with a wet paper towel and throw the towel in
the trash. The acetic anhydride and phosphoric acid can cause bad burns.
Use them in the hood. Be sure to wear gloves and safety goggles when using
these chemicals. Excess chemicals must be disposed of in the plastic tub
of water. This will convert the acetic anhydride to vinegar and dilute
the phosphoric acid. If you spill a lot of either of these, notify your
- Weigh out 3.0 g of salicylic acid and place in a 250 ml Erlenmeyer
- Measure out 6.0 ml of acetic anhydride and add this to your flask.
Be sure to do this in the hood and wear your goggles.
Don't let the acetic anhydride contact your skin and don't get the vapors
in your eyes.
- Carefully add 5 to 10 drops of 85% phosphoric acid, a catalyst, to
the flask and swirl to mix everything thoroughly.
- Still in the hood, heat the mixture for about 10 min. in a beaker of
warm water (70-80 oC).
- After heating, cautiously add 20 drops of distilled water.
- Next add 20 ml of distilled water and cool in an ice bath. You can
do this at your bench. If crystals do not appear, you can scratch the walls
of the flask with a stirring rod to induce crystallization.
- Filter the solid aspirin through a piece of pre-weighed filter paper
using a Buchner funnel and the aspirator. Wash the crystals with 2-3 ml
of chilled water. The liquid is mostly water and can be washed down the
sink. Allow the air to be drawn through the solid and filter paper for
15 minutes. Be sure to record the filter paper weight in your notebook.
- Place the filter paper with the product in a watch glass and put it
in the oven at 100 oC for about 30 min. until dry.
- Put the dry aspirin and the filter paper into a pre-weighed plastic
bag and weigh again.
- Measure the melting point range with the Meltemp Apparatus (your instructor
will demonstrate) and compare to the value for pure aspirin of 138-140
- Calculate the weight of your product by subtracting the weight of the
paper and bag from the total. The theoretical (maximum) yield is 3.9 grams.
What percent of this did you get? This is your percent yield. Record it
in your notebook and turn in your product to the Stockroom.
This activity has been copied, with permission, from the California State University at Stanislaus
server to ours, to allow faster access from our Web
site. We encourage you to explore the original site
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