Analysis of Aspirin Part 1

Transcription

1 Experiment 7 Analysis of Aspirin Part 1 Introduction Aspirin, acetylsalicylic acid, (C 9 H 8 O 4 ) is a very simple yet effective drug. Aspirin, acetylsalicylic acid Aspirin has been in use as a drug for over 100 years, and its precursor, salicylic acid, has been in use as a drug for approximately 250 years. Aspirin is used as a common treatment for pain and fever reduction but has also found widespread use as a preventative medicine for heart disease, and more recently, evidence suggests that it may also deter formation of colon cancer. Aspirin can readily be synthesized by the reaction of salicylic acid with acetic anhydride forming acetylsalicylic acid and acetic acid. + + Salicylic acid acetic anhydride acetylsalicylic acid acetic acid

2 Commercially available aspirin tablets are a combination of aspirin and a binder (typically starch), which allows for the formation of a solid tablet. Because of the presence of the binder, determining the amount of aspirin in a tablet cannot be done by determining the mass of the tablet. In the experiments this week and next week, you will be using two methods to analyze the amount of aspirin in commercially available tablets; spectrophotometry and titrimetry. Analysis of Aspirin by Spectrophotometry Acetylsalicylic acid when in a strongly basic solution containing the iron (III) ion forms a complex ion with the iron that is violet in color. Complex formed from combination of acetylsalicylic acid and iron (III) ion in strongly basic solution According to the Beer-Lambert law, the amount of light that the complex ion absorbs at a particular wavelength is related to the concentration of the salicylic acid iron complex that is in the solution. A=εbc Beer-Lambert Law If one knows the relationship between the absorbance and concentration (in other words, if one know the value of εb) one can determine the concentration of the complex ion by measuring its absorbance at a particular wavelength. In this experiment, you will take an aspirin tablet with an unknown amount of aspirin in it and determine the amount through spectrophotometry. You will first determine the value of εb in the same manner that you determined it for the equilibrium experiment; by making a calibration curve with known aspirin concentrations. You will then determine the absorbance of a solution made using the aspirin tablet, and using your calibration curve, calculate the concentration of aspirin in your solution. (For a more complete description of Spectrophotometry, see the equilibrium experiment.)

3 Procedure Spectrophotometry Preparation of the standard solutions 1. Combine 0.200g (it does not need to be exactly 0.200, but whatever it is must be recorded in your data!) of acetylsalicylic acid and 10 ml of 0.5 M NaOH in a 125 ml Erlenmeyer flask and heat to boiling. 2. Transfer the solution to a ml volumetric flask. Rinse the Erlenmeyer flask several times with deionized water and add this to the volumetric flask to ensure the complete transfer of the solution. 3. Fill the volumetric flask to the mL mark with deionized water. Cap and mix the solution. 4. Using a graduated pipette, transfer ml of the standard solution to a ml volumetric flask and dilute it with the 0.02M buffered iron (III) chloride solution. Transfer the solution to a test tube. 5. Make 4 more samples with ml, ml, ml, and ml of the standard solution. Keep track of which solution is in which tube. Preparation of commercial aspirin solution 1. Combine one aspirin tablet and 10 ml of 1 M NaOH in a 125 ml Erlenmeyer flask and heat to boiling. 2. Transfer the solution to a ml volumetric flask. Rinse the Erlenmeyer flask several times with deionized water and add this to the volumetric flask to ensure the complete transfer of the solution. 3. Fill the volumetric flask to the ml mark with deionized water. Cap and mix the solution. 4. Using a graduated pipette, transfer ml of this solution to a ml volumetric flask and dilute it with the 0.02M buffered iron (III) chloride solution. Transfer the solution to a test tube. Measuring Absorbances. 1. Plug in LabQuest to AC outlet. 2. Turn on the LabQuest, 3. Plug in the Spectrometer into the USB outlet the LabQuest will show USB: Abs 4. Fill a cuvette ¾ full with the reference (0.02M iron (III) chloride) solution and place in cuvette holder 5. Touch the red area on the Labquest and choose Change Wavelength 6. Enter 530 nm in the space for the wavelength. Choose OK 7. Touch the red area on the Labquest and choose Calibrate 8. When warm up is complete choose Finish Calibration, after a few seconds choose OK. 9. Fill a cuvette ¾ full with your first solution. Make sure the outside of the cuvette is clean and dry. 10. Place the cuvette in the cuvette holder and press the collect button. 11. After a few seconds, press the stop button (or collect button again). 12. Record the absorbance from the box on the right hand side of the screen. (Double check that the wavelength is at or near 530nm in the box at the lower right) 13. Place your next sample in the cuvette holder and press collect to determine its absorbance. Choose discard when prompted to save or discard data. Repeat the process to determine the absorbances of each solution.

4 Data Exp. 7 Record all data neatly. Include units and be careful with the accuracy of your measurements.

5 Analysis 1. Determine the concentration of the acetylsalicylic acid in the ml volumetric flask containing the solution from which the standards were made. 2. Determine the concentration of the acetylsalicylic acid in each of the ml calibration solutions. This is a simple dilution as long as you use the correct volumes!

6 3. Make a scatter plot (on the computer again) of absorbance versus concentration for the calibration samples. On the plot, add a linear trendline and display the equation on the chart. Take your time to label the axes. Attach the chart to the end of this experiment. 4. From your plot, what is the value of εb? 5. For the aspirin sample, calculate the concentration of acetylsalicylic acid present using the value of εb that you found. Absorbance [Acetylsalicylic acid] Aspirin Solution 6. The concentration from analysis question 5 represents the concentration in the ml sample that was prepared in the volumetric flask using an aliquot of the solution in the ml volumetric flask. Calculate the concentration of acetylsalicylic acid in the ml volumetric flask. This is a simple dilution as long as you use the correct volumes! 7. Using your result from the previous question, determine the mass of acetylsalicylic acid in the tablet.