- In the lab, you'll want to take all standard precautions, including wearing goggles and rinsing any skin that comes in contact with the chemicals in question. Sulfuric acid and methanol, for example, can be toxic.
Create a boiling water bath in a beaker; meanwhile combine salicylic acid with acetic anhydride in a ratio of 1 gram salicylic acid for every 2.5 mL of acetic anhydride and every 2 drops of sulfuric acid. Keep the water boiling for 10 minutes.
Add water (1 mL for every gram of salicylic acid), mix, and set aside for five minutes. Add 20 more mL of water for every gram of acid and then stir until you see crystals. Place your flask in a mixture of ice and water to finish the solidifcation, then collect the aspirin. Let it dry for 24 to 48 hours. - Take the dried ASA out and weigh it, preferably on a piece of filter paper. Note the weight in grams and write down the result.
- If you used 1 gram of salicylic acid and used the corresponding ratios of other ingredients (from Step 1), divide the weight by 1.25 and multiply the result by 100. Based on that molarity, you should hypothetically get 1.25g of ASA at 100% purity . If you used 2 grams, divide by 2.5; 3 grams, 3.75; and so on. The result will tell you the quantitative purity of the aspirin.
- To test qualitative purity, add 1 mL of methanol to each of three test tubes. Add salicylic acid, crushed aspirin from a store and your product to the three tubes, respectively. Then, add 1% iron chloride solution (just a drop) to each tube and pay attention to the color changes. If your ASA and the commercial product have the same or closely similar purity levels --- they should both look similar after reacting with the iron chloride solution.
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