Synthesis and Analysis of Aspirin: A Comprehensive Experimental Approach

Acetylsalicylic acid, commonly known as aspirin, is a widely used pain reliever. The synthesis of aspirin involves salicylic acid, an ancient pain reliever extracted from willow tree bark. However, salicylic acid has a bitter taste and can cause stomach irritation with frequent use. To address this, the milder acetylsalicylic acid was successfully synthesized by German chemist Felix Hoffmann in 1893.

The primary objectives of this experiment are to synthesize and analyze aspirin. The chosen synthesis method involves reacting acetic anhydride with salicylic acid in the presence of phosphoric acid as a catalyst.

Two tests will be conducted to verify the aspirin synthesis and assess its purity. The first test involves measuring the melting temperature of the product using a Melt Station. The second test utilizes a Spectrophotometer to analyze the absorbance of salicylic acid impurity in the aspirin sample complexed with Fe3+ from iron (III) nitrate solution.

Objectives of the experiment include:

• Synthesizing acetylsalicylic acid (aspirin).
• Calculating the percent yield of the synthesis.
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• Measuring the melting temperature of the aspirin sample.
• Conducting a spectrophotometric analysis of the aspirin sample.

Part I Synthesis Materials

Part II Melting Temperature

Part III Spectrophotometric Absorbance

1. Safety Measures:
   • Wear goggles, a long-sleeve lab coat, and gloves.
   • Perform the reaction in a fume hood.
2. Prepare Salicylic Acid:
   • Measure 2.0 g of salicylic acid into a 50 mL Erlenmeyer flask.
3. Reaction Mixture:
   • Add 5.0 mL of acetic anhydride and 5 drops of 85% phosphoric acid.
   • Swirl the mixture, rinsing any solid bits with distilled water.
4. Synthesis:
   • Create a 70–80°C hot-water bath using a 250 mL beaker.
   • Submerge the flask in the water bath.
   • Heat for 15 minutes, occasionally stirring.
   • Add 2 mL of distilled water after 10 minutes.
5. Crystallization:
   • Remove the flask when vapors cease.
   • Add 20 mL of distilled water.
   • Cool and transfer to an ice bath.
   • Crystals of aspirin should form.
6. Wash Aspirin:
   • Set up vacuum filtration.
   • Filter the mixture, wash crystals with cold water.
   • Air-dry the filter paper with the product.
7. Weigh and Record:
   • Weigh the dried product on the filter paper.

Part II: Test Melting Temperature

8. Prepare Sample:
   • Pulverize 0.2 g of aspirin and fill a capillary tube.
9. Melt Station Setup:
   • Connect Melt Station to power and LabQuest.
   • Start data collection for up to 20 minutes.
10. First Trial:

• Observe melting process, mark temperature.

11. Accurate Determination:

• Turn control knob to Rapid Heat near expected melting temperature.
• Mark temperature during melting.

12. Repeat Trials:

• Repeat until the melting temperature range is determined.

Part III: Spectrophotometric Absorbance

13. Stock Salicylic Acid Solution:

• Prepare a solution with 0.20 g of salicylic acid, diluted to 250 mL.
• Record the molar concentration.

14. Prepare Standard Solutions:

• Prepare five standard solutions with varying concentrations using the stock solution and Fe(NO3)3.

15. Calculate Concentrations:

• Calculate molar concentrations of the five standard solutions.

Follow the remaining steps in the procedure for testing the absorbance of an aspirin sample, including the preparation of salicylic acid standard solutions and developing a Beer’s law plot.

Disconnect the Melt Station and connect the Spectrophotometer to the USB port of LabQuest or a computer. Open the data-collection program and select New from the File menu.

19. Calibrate the Spectrophotometer. a. Prepare a blank cuvette by filling it 3/4 full with 0.025 M Fe(NO3)3 solution. Place the blank cuvette in the Spectrophotometer. b. Choose Calibrate from the Sensors menu of LabQuest or the Experiment menu of Logger Pro. c. After the warmup period, select Finish Calibration and then OK.
20. Determine the optimal wavelength for the standard curve and set up data collection. a. Empty the blank cuvette and rinse it twice with ~1 mL of the 100 mL volumetric flask solution of salicylic acid (Trial 1). Fill it 3/4 full, wipe the outside, and place it in the Spectrophotometer. b. Start data collection. Stop when the absorbance stabilizes. The wavelength of maximum absorbance (λ max) is automatically identified. c. Change the mode to Events with Entry and select a wavelength for analysis: • In LabQuest App, tap the Meter tab, then Mode, and select Events with Entry. Enter Name (Concentration) and Units (mol/L). • In Logger Pro, click Configure Spectrometer, choose Abs vs. Concentration as Collection Mode, and the λ max will be selected.
21. Collect data for five standard solutions. a. Keep Trial 1 cuvette in the Spectrophotometer and start data collection. b. When absorbance stabilizes, select Keep, enter molar concentration, and select OK. c. Discard cuvette contents as directed. Rinse cuvette with the second 100 mL volumetric flask solution, fill it, and follow the same process. d. Repeat for remaining solutions, stop data collection, and record absorbance and concentration values. e. Choose Curve Fit from Analyze menu, select Linear as Fit Equation, and select OK.
22. Prepare synthesized aspirin sample. a. Measure about 0.4 g of aspirin, add 10 mL ethanol, swirl to dissolve, add 150 mL distilled water, and transfer to a 250 mL volumetric flask. Rinse beaker with distilled water. e. Transfer 5 mL of aspirin solution to a clean 100 mL volumetric flask. Add 0.025 M Fe(NO3)3 solution to make precisely 100.0 mL. Mix thoroughly.
23. Measure and record absorbance of treated aspirin sample within 10 minutes. a. Rinse and fill cuvette 3/4 full with the sample. Cap and place in Spectrophotometer. b. Monitor absorbance value in Logger Pro or LabQuest. Record if within the range of standard solutions.
24. Determine salicylic acid concentration in aspirin sample. a. Choose Interpolate from Analyze menu. b. Click/tap along the regression curve to find absorbance closest to Step 23. Record salicylic acid concentration.
25. Discard all solutions as directed.

DATA TABLE ...

DATA ANALYSIS

1. What is the theoretical yield of aspirin in your synthesis? The mole ratio is 1:1 between salicylic acid and acetic anhydride in this reaction.
2. Using a literature source or the internet, find the accepted melting temperature value of pure acetylsalicylic acid. How does the melting temperature test of your aspirin compare to the accepted value?
3. Based on the results of the absorbance testing with the Spectrophotometer, what is the percent purity of your sample of aspirin? Does this percent purity compare well with the results of the melting temperature test? Explain.
4. Use your percent purity calculations to determine the percent yield of your synthesis of aspirin.