Aspirin is the common name of the pain reliever chemically named acetyl salicylic acid.
Aspirin is one of the most commonly used -- and abused -- drugs on the market. In thisexercise we want to see how to make this drug and to test its purity.
II. PREREQUISITES 1. As required In GENERAL INFORMATION - LAB, be ready to turn in a COPY of your: 2. Be ready to answer the following questions: B. The chemical used in the preparation of aspirin which tends to take off skin is C. Why is 85% phosphoric acid used in the preparation of aspirin? D. What may happen when water is added to the hot reaction mixture? E. What compounds might be found in impure or decomposed aspirin tablets? F. How will you know if a sample of aspirin passed your test for purity? G. Sketch and describe the use of the equipment and apparatus used in this When you finish with this exercise you should be able to: 1. Explain how aspirin can be manufactured from salicylic acid.
2. Explain how to test for the presence of excessive amounts of salicylic acid.
3. Tell why salicylic acid might be found in aspirin.
All excess reagents, aspirin, and filtrate from the vacuum flask should be placed inthe "Organic" container.
Acetyl salicylic acid (aspirin) can be prepared according to the equation in Figure 1.
Use the following procedure to prepare your aspirin: SPECIAL NOTE: You will need to prepare a hot water bath and an ice bath for thisexercise (and others). Choose a beaker large enough to hold the container to beheated or cooled. Use 600 ml beakers (or larger) for today's exercise. Fill the beakerabout half full of tap water. For a hot water bath, heat the water to boiling. For an icebath, add enough ice to bring the level of the mixture up to about the three quarters fullmark on the beaker.
Place about 2 gm salicylic acid POWDER into a dry 250 ml flask. CAUTION:Salicylic acid takes off skin! Handle with care! IN THE HOOD, measure out 5 ml of acetic anhydride and put it in with the salicylic acid.
Add 5 drops of 85% phosphoric acid as a catalyst. (A catalyst is a chemical whichmakes the reaction go faster but which is not converted to a product. Concentratedsulfuric acid is sometimes used instead of phosphoric acid.) Now take the flask back to your desk and float it in a hot water bath.
Keep the water boiling gently for 15 minutes. Stir occasionally if needed to dissolve allof the salicylic acid.
While this is heating, place 50 ml of water into a flask and cool in an ice bath.
At the end of the 15 minutes, remove the flask from the hot water bath, place it in thehood, and then add 5 ml of water (not from the water cooling in the ice bath) to theflask. BE CAREFUL: it may splatter when water is added. And don't breath the hot acetic acid vapors which come out as the water decomposes the acetic anhydridewhich is left.
After 2 minutes in the hood, return to your desk and add 20 ml of the cold water.
Cool the flask in the ice bath for about 10 minutes.
At the end of this time, check with the instructor to make sure that the crystals areforming correctly. These crystals are the aspirin.
Filter the crystals by vacuum filtration. Your instructor will assist you with the set up. Ifneeded, use the remainder of the cold water to rinse your aspirin from the flask.
Save the solid, your home-made aspirin, for use in determining purity.
Since it was manufactured from salicylic acid and acetic anhydride, you might suspectthat your impure aspirin contains some very small amounts of both of thesecompounds. However, acetic anhydride is very unstable in water, and will all have beenchanged to acetic acid by now. The acetic acid is not harmful (It's what is in vinegar).
There might still be some salicylic acid in your impure aspirin, and this is harmful.
Commercially made aspirin is purified several times before being made into tablets, andso should not have any salicylic acid left in it. However, moisture from the air candecompose the aspirin, reforming salicylic acid and acetic acid. So it is possible thataspirin tablets which were not purified properly or, more likely, which have been aroundfor a while and have undergone some decomposition may contain some salicylic acidand acetic acid.
According to the United States Pharmacopeia, aspirin should not contain more than0.1% of salicylic acid. We will use the test specified in that book to see if thehome-made aspirin and some commercial brand meet this standard of purity.
Use the following procedure to determine the purity of aspirin: Place 1.25 gm of your aspirin into the first flask.
Grind four commercial aspirin tablet of one brand into a fine powder with a mortar andpestle and place in the second flask.
Add 12 ml of ethyl alcohol to each flask.
Your aspirin should dissolve completely. The commercial tablet contains a binder whichwill not dissolve in this time, although the aspirin in it will dissolve.
Don't worry about the binder, but allow it to settle a little before going on to the nextstep.
Label three large test tubes as follows: "A: home-made aspirin""B: the Brand Name""C: 0.1% salicylic acid" Use a pipet (and make sure to use a different pipet for each solution) to add: 1 ml of ferric ammonium sulfate to Tubes A, B and C 1 ml of your home-made aspirin to Tube A.
1 ml of the Brand Name aspirin to Tube B.
1 ml of 0.1% salicylic acid to Tube C.
Compare the color of each with the color of the solution in Tube C (the tube whichdefinitely will contain the equivalent of 0.1% salicylic acid).
A sample passes the test for purity if it is less colored than the 0.1% salicylic acid tube(Tube C). It fails the test only if it is more colored than Tube C.
Name each aspirin tested and tell if it passed or failed the test for excess salicylic acid: Explain why salicylic acid might be found in aspirin tablets, even though they haveundergone extensive purification by the manufacturer:


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