Materials Included In Kit

Additional Materials Required

Safety Precautions

Do not ingest any of the aspirin samples during this laboratory. The samples are for laboratory use only, have been stored with other non–food-grade laboratory chemicals and are not meant for human consumption. Sodium hydroxide solution is corrosive to skin and eyes. Hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes. Phenolphthalein solution is a moderately toxic, flammable, alcohol solution. Avoid contact of all chemicals with eyes and all body tissues. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. Aspirin mixtures may be disposed of down the drain with plenty of water. Excess solutions can be saved for later use or disposed of according to the appropriate Flinn Suggested Disposal Method.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students. Both parts of this lab can reasonably be completed in one 50-minute class period. The pre-laboratory assignment should be completed before coming to lab, and the data compilation and calculations can be completed the day after the lab.
• Part 1—The kit contains 16 of the three types of aspirin tablets to be used for this part—generic, buffered and enteric-coated. Each group needs one tablet of each type.
• Part 2—The kit contains 16 of the two types of aspirin tablet to be used for this part—generic and Bayer. Each group should perform titrations of both types of aspirin. The groups can then share data for the averaging and comparison of the results. This works well by listing the aspirin brand on the board and having students record their results.
• Notice that generic aspirin is used in both parts. However, Bayer rather than buffered aspirin is used in Part 2. The reason for this is that the weak bases in the buffered aspirin causes inaccurate titration results (see Table 3 in Sample Data). The pH of the buffered aspirin solution becomes too high to use the indicator to detect the endpoint. Thus an accurate titration to determine percent of aspirin is not possible with the techniques used in this lab. Titration data is provided in Table 3 for buffered aspirin for teacher reference or to share this information with students.
• During the titration, remind students that they can and should use distilled water to rinse down the sides of their flasks. Adding water does not change the amount of aspirin or base and therefore will not affect the titration.
• In-class time needs to be allotted for sharing and averaging results, whereas calculations can be completed outside of class.
• Information on the mass of aspirin in each tablet and other active ingredient(s) is provided in Table 3. You may wish to either provide this information to students, have actual aspirin bottles so students can read the information off the bottle, or have students go to the store to research this information on their own.

Further Extensions

How could you determine which type of aspirin is the most cost-effective? Describe the procedure you would use.

Answers to Prelab Questions

1. How was salicylic acid first discovered?

   2,300 years ago, it was noted that people who chewed willow bark could relieve pain and fever. Willow bark contains salicylic acid.

2. List the physiological effects of aspirin.

   Aspirin relieves fever, pain and inflammation. It enlarges blood vessels and reduces blood clotting. It penetrates the fatty lining of the stomach and causes at least some bleeding. Aspirin can prolong both pregnancy and labor and can increase bleeding during menstruation as well as before and after delivery of a newborn.

3. How can you tell if aspirin is too old to use? What chemical reaction explains this change?

   The aspirin is too old to use if it smells like vinegar. The decomposition of acetylsalicylic acid to acetic acid (vinegar) and salicylic acid causes the odor.

4. Suppose you accidentally ingested some very old aspirin. What effects would you expect it to have on your body?

   The increased level of acid may cause acidosis. The salicylic acid is likely to irritate the stomach lining, causing bleeding and heartburn.

5. How does aspirin relieve pain?

   Aspirin prevents the formation of a certain group of prostaglandins that are responsible for producing fever, pain and inflammation. (Aspirin does this by interfering with the action of one enzyme, prostaglandin H₂ synthase, in the chain of prostaglandin production.)

6. What happens to the body during an aspirin overdose?

   The blood pH drops precipitously. If the body cannot reduce the acidity, the excess acid will damage the brain, liver, and other tissues. The symptoms of salicylate poisoning include ringing in the ears, extremely rapid respiration rate, nausea and fever.

7. What do willow bark, Chamomile tea and aspirin have in common?

   All can be used to relieve pain but can also cause salicylate poisoning in large enough doses.

8. What is the most common side effect of aspirin? Describe two modifications of aspirin tablets designed to minimize this effect.

   The most common side effect of aspirin is stomach distress, caused by bleeding from the lining. Buffered aspirin contains a buffer that reduces the acidity of the aspirin and increases the rate of its absorption into the bloodstream. Enteric-coated aspirin has a special coating that prevents the release of the aspirin until the tablet has passed into the intestine.

9. Should any of the aspirin tablets used in this laboratory be ingested? Why or why not?

   em>No, aspirin samples should not be ingested. The samples are for laboratory use only, have been stored with other non–food-grade laboratory chemicals and are not meant for human consumption.

Sample Data

Data Table 1. The Simulated Stomach

Data Table 2. Aspirin Titration

*Not provided in kit for Part 2.

Calculations

1. Compile class data on the board of “Volume of NaOH” added per aspirin tablet for all three types of aspirin used by the class—Generic, Bayer^® and Enteric-coated.
2. Calculate the average volume of base added per aspirin tablet.

   Solution: Results for five trials were 16.5 mL, 17.2 mL, 14.3 mL, 17.8 mL and 16.9 mL of NaOH. Since the value of 14.3 mL is significantly different, it will be thrown out of the averaging. (Note: If there is not one value that is significantly different, average all values or throw out the highest and lowest values.)

   {13361_Data_Equation_2}
3. Record the molarity (concentration) of NaOH solution in Table 3.
4. Calculate the amount of NaOH (in moles) that was neutralized by each aspirin tablet according to the following equation.

   Molarity of NaOH (in moles/L) x Volume of NaOH (in L) = moles of NaOH

   Solution: 0.1 M x 0.0171 L = 0.00171 moles NaOH

5. Determine the amount of aspirin (in moles) that was in each tablet. (Remember: One mole of aspirin neutralizes one mole of base.)

   Solution: 0.00171 moles NaOH = 0.00171 moles of aspirin

6. Calculate the mass of aspirin in each tablet according to the equation below. (MW of aspirin = 180 g/mole)

   Moles of aspirin x Molecular Weight of aspirin (in g/mole) = Mass of aspirin (in grams)

   Solution: 0.00171 moles aspirin x 180 g/mole = 0.308 g aspirin

7. Record the mass of aspirin in each tablet (as claimed on the bottle) in Table 3.
8. Calculate the percent difference for each type of aspirin. This is the difference between the mass of aspirin experimentally determined to be in each tablet and the mass of aspirin the company claims is in the tablet. Use the following equation.
   {13361_Data_Equation_3}
9. Record the actual mass of one full tablet (as weighed in grams) in Table 3.
10. Calculate the actual aspirin content of each tablet (using the mass of aspirin experimentally determined to be in each tablet rather than the company’s claim). Use the following equation.
    {13361_Data_Equation_4}
    Table 3. Summary of Aspirin Data
    {13361_Data_Table_3}

    *Not provided in kit for Part 2.

Answers to Questions

Part 1

1. Which type of aspirin—generic, buffered or enteric-coated—will most likely provide the most rapid relief? Explain.

   The regular aspirin will provide the most rapid relief because it dissolves most quickly in stomach acid, releasing its active ingredient.

2. Which type of aspirin will provide the most protection for your stomach? What evidence do you have for this?

   The enteric aspirin will provide the most protection for the stomach because it will not release its active ingredient until it reaches the basic conditions in the intestine. This is evidenced by the fact that it did not dissolve in the water or in the acidic environment.

3. How does buffered aspirin work to protect your stomach?

   The weak base in the buffer reduces the acidity of the aspirin, as noted by the pH of the buffered aspirin in the “stomach acid.” As a result, buffered aspirin reduces the stomach distress caused by aspirin and provides some stomach protection.

Part 2

4. How many milligrams of aspirin (acetylsalicylic acid) does each plain aspirin-based pain reliever claim to contain?

   Regular-strength aspirin tablets contain 325 mg (the recommended dosage) of aspirin per tablet.

5. Did your calculations for the amount of aspirin in each tablet match the amount of aspirin claimed on the bottle? Discuss possible reasons for discrepancies in the aspirin content.

   There is very little likelihood that the measured aspirin content exactly matches the 325 mg claimed on the bottle. Discrepancies are most likely due to experimental errors or variability associated with the manufacturing process of the aspirin. Decomposition of the aspirin to salicylic acid and acetic acid will also affect the titration (see Question 10).

6. Buffered aspirin also claims to contain 325 mg of aspirin per tablet, yet it was not used in the titration. Why not? Predict what the results would have been if buffered aspirin (rather than plain aspirin-based) tablets were used in the titration.

   Buffered aspirin contains weak bases (buffers) to raise the pH of the stomach. Thus, in a titration the solution would reach the endpoint much sooner than it should (as the pH would be high enough to cause an indicator change). Thus, less base would be added and aspirin content results would be well below 325 mg.

7. Why is it important to obtain an average of the class results for the three types of aspirin before performing calculations?

   Scientific investigation and reliability requires repeatability of measured data. Performing multiple trials and averaging the results provides assurance that the results are reasonably accurate and that there are not undetected experimental errors.

8. How could the aspirin content have been determined more accurately?

   Accuracy may be increased by performing more trials; by using more quantitative laboratory titration equipment such as a buret or volumetric pipet to measure the volumes; by weighing the aspirin on an analytical balance.

9. If aspirin tablets are not 100% aspirin, what else is in the aspirin? What is the purpose of other added ingredients in aspirin tablets?

   Aside from acetylsalicylic acid (aspirin), aspirin tablets contain binders to prevent crumbling or fillers for bulk. Some brands contain sugars; others contain starch; some brands contain caffeine; buffered aspirin contains weak bases (buffers).

10. If aspirin decomposes to salicylic acid and acetic acid, how will this affect the titration?

    If the aspirin has undergone decomposition, two moles of base are required per mole of aspirin—one for the salicylic acid and one for the acetic acid. Thus, the experimental amount of aspirin will be higher than it should.

References

Krieger, Carla R. pH Laboratory and Classroom Activities: pHysiology, pHarmacology, and other pHantastic pHenomena; Flinn Scientific: Batavia, IL, 1999; pp 145–165.

Introduction

How much aspirin is actually in an aspirin tablet? Are all types of aspirin equal in pain relieving ability? Experimentally determine the aspirin content of various types of aspirin by titration. Compare the effects of regular, buffered and enteric-coated aspirin tablets on a “simulated stomach.”

Concepts

• Aspirin
• Acid–base neutralization

Background

Aspirin is one of the oldest and most frequently used (and misused) drugs in the treatment of illness or injury. Some 2,300 years ago, Hippocrates noted that people could chew willow bark to relieve pain and fever. Willow bark contains salicylic acid, which is an antipyretic (fever reducer) and analgesic (pain reliever). Unfortunately, salicylic acid is very irritating to the stomach’s lining. This irritation is so distressful that many patients preferred aches and fever to the severe heartburn caused by this remedy.

In 1897, Felix Hoffman discovered a chemical reaction that altered salicylic acid just enough to reduce this miserable side effect, while still retaining its antipyretic and analgesic properties. Hoffman reacted salicylic acid with acetic acid to produce acetylsalicylic acid, the active ingredient in aspirin, according to the reaction shown in Equation 1. Interestingly, acetylsalicylic acid is a natural component of Chamomile tea, which had also been used as a folk remedy for fever and pain.

In 1899, the Bayer Company began to produce acetylsalicylic acid and marketed it under the common or trade name of “aspirin.” This name was derived from a- for acetyl and -spirin for Spiraea, one of the natural plant sources of salicylic acid. Today, aspirin is one of the least expensive drugs, mainly due to the huge volume of aspirin tablets produced. Americans consume nearly 29 billion tablets of aspirin annually.

Aspirin acts as an analgesic (pain reliever), an antipyretic (fever reducer) and an anti-inflammatory agent (inflammation reducer). Aspirin is a fairly acidic compound which reacts slowly with moisture from the air to undergo hydrolysis, the reverse reaction from Equation 1, forming salicylic acid and acetic acid (vinegar). Thus if aspirin is stored for several months, it may smell of vinegar. In order to slow this decomposition, aspirin bottles should be tightly capped and only purchased in small quantities. Taking aspirin that has begun to decompose should be avoided because it is even more acidic. This increased acidity can be very irritating to the lining of the stomach and intestines, causing side effects such as upset stomach, gastrointestinal bleeding, and various allergic reactions. Furthermore, aspirin as with any drug, must get into the bloodstream in order to do its work. The time it takes for the acetylsalicylic acid in the tablet to enter the blood is limited by the rate at which the tablet disintegrates in the stomach, which is dependent on pH. The higher the pH, the faster the tablet breaks up and dissolves.

One of the most common side effects of aspirin is stomach upset. Aspirin in solution has a pH of about 3. This is near the pH of the stomach. At this pH, aspirin molecules can penetrate the fatty lining of the stomach and cause bleeding. For most people, this bleeding is trivial (0.5 to 2 mL after two tablets). However, a small percentage of aspirin users may lose hundreds of milliliters of blood, causing them to require hospitalization. One strategy for reducing stomach irritation is to take a buffered aspirin, which commonly contains aspirin and one or more weak bases such as calcium carbonate, magnesium oxide, and magnesium carbonate. The combination of aspirin and weak bases increases the pH, thus helping with the disintegration of the tablet and hence the absorption of the acetylsalicylic acid into the bloodstream. The basic compounds in the buffered aspirin reduce the acidity that results from the hydrolysis of aspirin in the acidic juices of the stomach; thus, buffered aspirin is found to be genuinely more effective in preventing irritation of the stomach lining than aspirin.

Aspirin relieves pain and reduces fever by blocking the body’s production of prostaglandins. The body’s response to illness, the flu, or a back strain for example, is the making of prostaglandins. These hormone-like chemicals are one reason that fever, headaches and pain are experienced. They also regulate digestion, kidney output, reproduction, blood circulation and some nervous system functions. Aspirin also enlarges blood vessels and reduces the clotting of the blood by preventing the production of certain kinds of prostaglandins. This is one of the reasons that physicians believe that aspirin may help prevent some types of heart attacks and strokes if taken regularly in small amounts. However, females should be cautious of using aspirin during menstruation, because it reduces blood clotting, thereby increasing the amount of blood loss. Furthermore, taking aspirin during the last three months of pregnancy can prolong both pregnancy and labor and can cause increased bleeding before and after delivery. Therefore, pregnant women should avoid taking aspirin, unless directed to do so by their physicians.

If a person takes an overdose of aspirin, the salicylic acid absorbed by the stomach and intestine lowers the pH of the blood. If the dose of aspirin is extremely large, the body cannot compensate, and the excess acid in the blood begins to damage the brain, liver, and other tissues. This acid–base imbalance, known as acidosis, may be fatal. Salicylate poisoning causes ringing in the ears, extremely rapid breathing, and, ironically, fever. In small doses, aspirin is not toxic to adults, but as little as one tablet can poison a baby.

Some people, such as arthritis sufferers, are under a doctor’s orders to take aspirin around the clock every day. Stomach irritation may pose a serious problem for these patients, unless they use enteric-coated aspirin. These aspirin tablets are specially coated to pass through the stomach without dissolving. The coating is resistant to the acid in the stomach but dissolves readily in the slightly basic environment of the small intestine. These tablets effectively eliminate stomach irritation but cannot begin working until the stomach passes its contents to the intestine, which may not occur until several hours after the tablet is ingested.

Materials

Prelab Questions

Read the Background information and complete the following questions. Answer on a separate sheet of paper.

1. How was salicylic acid first discovered?
2. List the physiological effects of aspirin.
3. How can you tell if aspirin is too old to use? What chemical reaction explains this change?
4. Suppose you accidentally ingested some very old aspirin. What effects would you expect it to have on your body?
5. How does aspirin relieve pain?
6. What happens to the body during an aspirin overdose?
7. What do willow bark, Chamomile tea, and aspirin have in common?
8. What is the most common side effect of aspirin? Describe two modifications of aspirin tablets designed to minimize this effect.
9. Should any of the aspirin tablets used in this laboratory be ingested? Why or why not?

Safety Precautions

Do not ingest any of the aspirin samples during this laboratory. The samples are for laboratory use only, have been stored with other non-food-grade laboratory chemicals, and are not meant for human consumption. Hydrochloric acid and sodium hydroxide are corrosive to skin and eyes. Phenolphthalein solution is a moderately toxic, flammable, alcohol solution. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. The Simulated Stomach

A. Calculating the Percent of Aspirin in Each Tablet

1. Using a laboratory balance with 0.01 g readability, weigh a single tablet of each of the pain reliever tablets to be tested—generic aspirin, buffered aspirin, and enteric-coated aspirin. Record the mass of each tablet in Data Table 1.
2. Record the mass of aspirin in each tablet (as claimed on the bottle) and other listed ingredients in Data Table 1.
3. Calculate the percentage of aspirin of the total mass in each tablet. Record the values in Data Table 1.

B. Simulating the Stomach’s Response to Aspirin

4. Obtain three test tubes and place them in a test tube rack. Label the tubes with each aspirin type—generic aspirin, buffered aspirin or enteric-coated aspirin.
5. Add approximately 2–3 mL (1 pipet full) of distilled or deionized water to each tube.
6. Note the time, and simultaneously add a generic aspirin tablet to tube 1, a buffered aspirin tablet to tube 2, and an enteric-coated aspirin tablet to tube 3. Note the changes in the tablets until no further change is evident. Record observations in Data Table 1.
7. Determine the pH of the aspirin solutions or mixtures. To do this, stir each mixture with a clean stirring rod and touch the wet end of the stirring rod to one half of a strip of pH paper. Record the pH in Data Table 1. Note: If any of the tablets did not dissolve in the water, do not try to crush it with the stirring rod.
8. Add 2–3 mL (1 pipet full) of 0.1 M HCl “simulated stomach acid” solution to each test tube. Stir the mixtures with a stirring rod and determine the pH of each solution using the other half of the pH paper strips. Record this as the “simulated stomach” pH in Data Table 1.
9. Did any of the three tablets not dissolve or disintegrate in the water or stomach acid? If so, add 5 mL (2 pipets full) of 0.1 M NaOH “simulated intestine” solution to that tube only. Note the changes in the tablet until no further change is evident. Record observations in Data Table 1.

Part 2. Titration of a Weak Acid (Aspirin) with a Strong Base

A. Preparing the Control

1. Use a graduated cylinder to measure approximately 25 mL of distilled or deionized water into a 125-mL Erlenmeyer flask. Add 1–2 drops of 0.1 M NaOH to the flask.
2. Add 2–3 drops of phenolphthalein indicator solution and swirl the flask. Note the color of the basic solution. Set this flask aside and use it as a color comparison (a control) during the titration.

B. Preparing the Aspirin Tablets

3. Obtain an aspirin tablet from your instructor. Record the brand of aspirin in Data Table 2.
4. Using a laboratory balance with 0.01 g readability, weigh the tablet on an electronic balance. Record the mass of the tablet in Data Table 2.
5. Record the mass of aspirin in the tablet (as claimed on the bottle) in Data Table 2.
6. Calculate the percentage of aspirin of the total mass of the tablet. Record the value in Data Table 2.
7. Use a mortar and pestle to grind the tablet into a powder. (Note: If a mortar and pestle are not available, crush the tablet between two sheets of paper.)

C. Performing the Weak Acid–Strong Base Titration

8. Use a graduated cylinder to measure approximately 25 mL of distilled or deionized water into a 125-mL Erlenmeyer flask.
9. Use a spatula to transfer the powdered aspirin tablet to the flask and stir with a stirring rod. The aspirin will disperse but may not fully dissolve.
10. Determine the pH of the starting solution by touching the wet end of the stirring rod to a strip of pH paper. Record the starting pH in Data Table 2.
11. Add 2–3 drops of phenolphthalein indicator solution and swirl the flask. Record the color of the solution in Data Table 2.
12. In a 25-mL (or 50-mL) graduated cylinder, obtain approximately 15–20 mL of 0.1 M NaOH from the stock bottle. Read the exact starting volume of 0.1 M NaOH in the cylinder and record this value in Data Table 2.
13. Using a Beral-type pipet, begin adding 0.1 M NaOH in 1–2 mL increments to the Erlenmeyer flask, swirling the flask after each addition. (Hint: This process is made easier if the pipet is held in one hand and the flask swirled gently in a circular motion with the other. Try to get as much aspirin powder off the sides of the flask as possible when swirling. Use distilled water to rinse the sides of the flask.)
14. Continue adding base in small increments of about 1–2 mL until the aspirin is just neutralized by the base. This will be indicated by a color change in the phenolphthalein indicator. Toward the end of the titration, add NaOH a few drops at a time and swirl. The endpoint is near when the pink color remains longer and longer before turning back to colorless.
15. When the color of the solution in the flask turns faint pink and remains pink (even after sitting for a few minutes), the titration is complete. This is considered the endpoint of the titration. (Note: Be careful not to add too much base or “overshoot” the endpoint.) Compare the color of the solution to the control (from step 2) to determine when you are done.
16. Return any remaining base from the Beral-type pipet to the graduated cylinder. Read the exact ending volume of 0.1‑M NaOH in the cylinder and record this value in Data Table 2.
17. Determine the pH of the final solution by touching the wet end of the stirring rod to a strip of pH paper. Record the final pH and solution color in Data Table 2.
18. Rinse the final solution down the drain with plenty of water.
19. Repeat steps 3–18 for a different aspirin using a clean, 125-mL Erlenmeyer flask.

Student Worksheet PDF

13361_Student1.pdf