Materials Included In Kit

Additional Materials Required

Safety Precautions

Do not allow students to ingest any of the antacid drug samples during this laboratory. The samples are for laboratory use only, have been stored with non–food-grade laboratory chemicals and are not meant for human consumption. Hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes. Methyl orange indicator solution is toxic by ingestion. Universal indicator solution is a flammable, alcohol-based solution. Avoid contact of all chemicals with eyes and all body tissues. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Consult Safety Data Sheets for further safety information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Antacid mixtures may be disposed of down the drain with plenty of water. Excess hydrochloric acid can be saved for later use or neutralized and disposed of according to Flinn Suggested Disposal Method #24b.

Lab Hints

Part 1. Observing an Antacid in Action

• The active ingredient in the MOM tablet is magnesium hydroxide. When MOM reacts with the acid, the solution becomes neutral. Excess MOM continues to dissolve, forming the final basic solution.
• The color changes in Part 1 can be seen more easily if a piece of white paper is placed behind the beaker.
• No buffering occurs with MOM. Tums and generic antacid form buffers during their reaction with HCl. As a result of buffer formation, the pH is maintained at 6 to 7 when all of the tablet is dissolved. The pH will remain at or near this range despite the addition of more acid, within limits. 

Part 2. Titrating a Weak Base (Antacid) with a Strong Acid

• The endpoint of the titration can be detected when the indicator just turns red and remains red, indicating a pH of 3.0. Solutions of antacids, however, have a tendency to turn red and then as more antacid dissolves, return to the yellow color. This is a benefit in the stomach and is due to the relative insolubility of the antacid. Yet it makes titrations difficult in terms of determining the actual endpoint. When the mixture turns red, have students swirl the flask and wait for no longer than one minute. If the red color remains, assume the titration is complete. Don’t worry if the color returns to yellow after 5 minutes; usually the maximum amount of acid that can still be neutralized is no more than 0.5 mL. 

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 prelaboratory 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 15 Phillips’ MOM tablets, one for each group.
• Part 2—The kit contains 15 Tums, 15 Generic antacid, and 15 Phillips’ MOM tablets. Each group should perform two titrations and will use two of the three antacid brands provided. The groups can then share data for the averaging and comparison of all three brands of antacids. This works well by listing all three brands on the board and having students record their results. Note: If time allows, enough antacid tablets are provided for each group to perform all three titrations.
• Manufacturers may change antacid formulations without notice. The sample data were obtained on one set of popular antacids. Student data will vary.
• In-class time needs to be allotted for sharing and averaging results, whereas calculations can be completed outside of class, if necessary.
• Information on price of each bottle of antacid, number of tablets in each bottle, cost per tablet, active ingredient(s), amount of each active ingredient (mg/tablet), and recommended adult dosage is provided in Table 3. You may wish to either provide this information to students, have actual antacid bottles so students can read the information off the bottle, or have students go to the store to research this information on their own.
• During the titration, remind students that they can and should use distilled water to rinse down the sides of the flasks. Adding water does not change the amount of antacid or base and therefore will not affect the titration.
• Table 3 also provides data for Alka-Seltzer^®, a popular combination antacid and pain reliever. You may decide to have students perform titrations on Alka-Seltzer, using perhaps half of a tablet, as well as on other brands of students’ favorite antacids.
• Standard acid solutions in the range of 0.10 M to 1.0 M HCl work well to simulate the human stomach conditions. Using 0.10 M HCl more closely approximates the conditions in the human stomach but will increase the volume of acid used and will lengthen the amount of time required for the experiment.
• Methyl orange indicator was chosen as the indicator of choice as it has its color change endpoint between 3.0 (red) and 4.4 (yellow). When the stomach reaches a pH of 3.0 or below, the “neutralizing power” of the antacid is gone.
• Before the lab, have students generate hypotheses about which antacids will neutralize the most acid and which will be the most cost-effective.
• Try Flinn’s antacid chemical demonstration kit as a perfect introduction to this lab—Upset Tummy? MOM to the Rescue! (Catalog No. AP5934).

Answers to Prelab Questions

1. What acid is required for proper digestion in the stomach?

   Hydrochloric acid

2. What is the normal pH range of the stomach?

   pH 0.9–2.0

3. Describe two ways in which the stomach prevents acid from damaging its lining (called the mucosa).

   The cells of the mucosa produce mucus and serve as a fatty barrier.

4. Name two substances that can damage the mucosa. Describe how each works.

   Ethyl alcohol and aspirin. Both can damage the fatty barrier formed by the cells of the mucosa because they can dissolve in fat.

5. Why does the reaction between carbonate-containing antacids and stomach acid produce a “burp”?

   The carbon dioxide gas must be released from the stomach, providing some relief.

6. Why do we refer to the reaction between an antacid and stomach HCl as an example of neutralization?

   All of these antacid reactions are neutralizations because they produce salts and water with pH values close to 7. Carbon dioxide(aq) is slightly acidic, causing the pH to be slightly less than 7. However, any acid–base reaction producing carbon dioxide is considered a neutralization reaction because the very low pH of the hydrochloric acid is raised to a pH much closer to 7.

7. Why do some antacids contain both aluminum and magnesium ions?

   Aluminum ions are often constipating while magnesium ions are often laxative. The two are mixed to balance these effects.

8. Why do liquid antacids and chewed antacid tablets increase the neutralization reaction rate when compared to whole tablets?

   Grinding the tablets increases the surface area, thereby increasing the frequency of collisions between the reactants.

9. Under what circumstances are over-the-counter antacids considered safe and effective?

   Occasional, temporary stomach distress caused by overeating or stress.

10. Under what circumstances should over-the-counter antacids be avoided?

    Repeated bouts of indigestion, particularly when accompanied by severe pain or vomiting.

Sample Data

Answers to Questions

Part 1

1. Milk of Magnesia (MOM) is an antacid. How does an antacid work?

   Antacids are basic compounds that raise the pH of gastric juice by neutralizing excess hydrochloric acid produced by the stomach mucosa.

2. Why did the color of the solution in the beaker change when a MOM tablet was added?

   The color changes reflect an increase in pH value.

3. Why does the pH of the solution in the beaker change when MOM is added?

   The pH increases to 7 as the HCl is neutralized by the antacid. The pH becomes basic as excess MOM dissolves in the water.

4. Why does the pH of the beaker containing MOM stop at 10.0, even though some of the tablet remains undissolved?

   The MOM has reached the limits of its solubility in water (at this temperature).

5. How does an antacid such as MOM respond as the stomach continues to secrete acid?

   Any excess active ingredient in the antacid will continue to neutralize HCl secreted by the stomach until the excess antacid is completely consumed by the reaction.

Part 2

6. Is the concentration of the HCl standard solution used in the titration comparable to the concentration of this acid in the human stomach? How do you know?

   Not exactly, but the pH value is close. The pH of human stomach acid ranges from 0.9 to 2.0, which corresponds to a [H⁺] of 0.01 M to 0.13 M. The concentration of HCl used in this experiment is 0.50 M, which corresponds to a pH of 0.30.

7. Why was methyl orange chosen as the indicator in the acid–base titration?

   Methyl orange changes color between a pH of 3.0 and 4.4. When the stomach reaches a pH of 3.0 or below, the “neutralizing power” of the antacid is gone.

8. Which antacid (of those tested) is most effective at neutralizing stomach acid? Answer in terms of “per tablet” and “per gram of antacid.”

   Student answers may vary. The most effective antacid tablet neutralizes the largest volume of acid. The sample data indicates that the Phillips’ MOM provides the largest neutralizing capacity per gram.

9. Which antacid (of those tested) is least effective at neutralizing stomach acid? Answer in terms of “per tablet” and “per gram of antacid.”

   Student answers may vary. The least effective antacid tablet neutralizes the smallest volume of acid. The sample data indicates that the Phillips’ MOM tablet, while the generic Tums provides the smallest neutralizing capacity per gram.

10. Consider both the cost of the antacids as well as neutralizing ability.
    1. Which antacid tablet is the most cost-effective?

       Student answers may vary. The most cost-effective antacid neutralizes the largest volume of acid for the lowest price. The sample data indicates that Tums neutralizes the largest volume of acid for the lowest price. In fact Tums neutralizes 8.5 mL of acid per penny.

    2. Which antacid tablet is least cost-effective?

       Student answers may vary. The least cost-effective antacid neutralizes the smallest volume of acid for the highest price. The sample data indicates that Phillips’ MOM neutralizes the smallest volume of acid for the highest price. In fact, it only neutralizes 2.4 mL of acid per penny spent.

11. Look at the recommended dosage for each brand.
    1. How did your experimental method differ from expected patterns of usage by consumers?

       Many antacids recommend two to four tablets per dose. In this experiment, we titrated only one tablet at a time. The neutralizing capacity of an individual tablet in the laboratory may be quite different from the neutralizing capacity of multiple tablets in the human body.

    2. How could this difference be corrected?

       We could titrate each brand using the recommended dosage.

References

Krieger, Carla R. pH Laboratory and Classroom Activities: pHysiology, pHarmacology, and other pHantastic pHenomena; Flinn Scientific: Batavia, IL, 1999; pp 41–66.

Introduction

Explore the properties and uses of antacids. Watch an “antacid in action” as it neutralizes simulated stomach acid. Determine the neutralizing ability of common over-the-counter antacids by titration and compare the neutralizing power and costs of various antacids.

Concepts

• Antacids
• Acid–base neutralization

Background

The Role of Hydrochloric Acid in Digestion
The stomach’s digestive juices, or gastric juices, contain hydrochloric acid (HCl), which along with the enzyme pepsin serves to promote digestion of food proteins. During digestion, the stomach lining produces HCl. The average adult produces two to three liters of HCl every day to aid in digestion, with a normal stomach pH ranging from 0.9 to 2.0. While the stomach produces a small amount of acid at all times, it can be stimulated to produce more acid with the presence of food. When a meal is eaten, both hydrogen and chloride ions move from the surrounding blood through the stomach lining into the stomach. Upon reaching the stomach, these ions produce a highly acidic environment. Certain enzymes needed for digestion require this pH in order to be active. HCl also suppresses the growth of bacteria in the stomach and kills a large portion of the bacteria that enter the stomach with the food you eat.

Hydrochloric acid is so reactive that it can corrode metals and is therefore strong enough to react with the stomach’s own lining, known as the mucosa. A normal, healthy stomach, however, does not digest itself because the mucosa’s cells block the action of HCl in two ways. First, the cells of the mucosa form a fatty barrier. Second, the cells of the mucosa secrete a layer of mucus. If the mucosa is damaged, HCl can attack the stomach wall. Certain substances, such as alcohol and aspirin, can damage the fatty barrier because they dissolve in fat. Alcohol alone does not cause permanent damage. However, in combination with stress, alcohol can increase the chance of developing permanent damage. Aspirin also damages the mucosa and causes bleeding. In fact, when the average human takes two aspirin tablets, the mucosa will lose one to two milliliters of blood. People who are aspirinsensitive can lose even more blood. Alcohol taken in combination with aspirin can further increase aspirin’s ability to penetrate the mucosa.

Antacid Use for “Prompt Relief”
Americans spend over a billion dollars per year on antacids. These medications promise prompt relief for stomach distress. Too much food, certain types of food, or high levels of stress may cause the stomach to respond with an outpouring of acid. This excess HCl lowers the pH to a point at which discomfort is felt, commonly called “acid indigestion” or “heartburn.” Antacids are chemicals that promise to provide “prompt relief” for the unpleasant effects of over-acidity by neutralizing excess acid in the stomach and thereby relieving painful symptoms.

Most antacids are insoluble or only slightly soluble in water. They are designed to dissolve slowly in the acidic juices of the stomach so that carbon dioxide will be given off gradually as the antacid neutralizes excess acid. This property of low solubility in water reduces the likelihood of the antacid being absorbed into the bloodstream. Absorption of antacids into the bloodstream could cause an undesirable effect called alkalosis. Alkalosis is an abnormal increase in the pH of the blood caused by excess base.

Acid–Base Neutralization Titration
The basic compounds in antacids serve to neutralize acid in the stomach by undergoing an acid–base neutralization reaction. The quantitative analytical procedure for determining how much acid can be neutralized by an antacid is called a titration. In this laboratory, a strong acid will be added to an antacid solution until the “neutralizing power” of the antacid is gone.

How will “neutralizing power” be detected? An acid–base indicator, methyl orange, will be added to indicate the loss of “neutralizing power.” Methyl orange indicator undergoes a color change between the pH values of 3.0 (red) and 4.4 (yellow). An antacid, while it still has “neutralizing power,” will raise the pH of the stomach to a near-neutral value. Once the “neutralizing power” of the antacid is gone, the pH of the stomach falls below a pH value of approximately 3.0—the point the antacid is used up. For this reason, a pH of 3.0 is a reasonable pH value to use as an endpoint in an antacid titration; thus, methyl orange is an appropriate indicator.

An effective antacid does not bring the pH of the stomach fluid to complete acid–base neutrality (pH of 7) as this would completely shut down digestion and promote acid rebound, an automatic response which floods the stomach with fresh acid. Instead, an effective antacid neutralizes some of the HCl in the gastric juices—enough to relieve the pain and discomfort, yet still allowing for the continuation of normal digestive processes.

Common Antacids
The active ingredient(s) used in antacids varies from manufacturer to manufacturer. Most common antacids contain weak bases such as sodium bicarbonate, calcium carbonate, magnesium hydroxide, aluminum hydroxide or various combinations of these. Baking soda (a home remedy) and Alka-Seltzer^® contain sodium bicarbonate. Tums^®, Rolaids^® and Di-Gel^® contain calcium carbonate. Phillips’^® Milk of Magnesia (MOM) is largely composed of magnesium hydroxide. Maalox^® and Mylanta^® contain mixtures of aluminum hydroxide and magnesium hydroxide. Each of these substances will neutralize acid in the stomach.

Sodium Bicarbonate Antacids
Antacids differ in their effectiveness and in their side effects. For example, sodium bicarbonate (Alka-Seltzer) provides quick relief by neutralizing HCl to produce a neutral salt, water, and carbon dioxide according to Equation 1.

Alka-Seltzer is a popular antacid and pain relief medicine, containing aspirin, citric acid and sodium bicarbonate (baking soda). When the tablet is dropped into water, the bicarbonate and the citric acid dissolve, producing the familiar “fizz” from the chemical release of carbon dioxide. However, the American Medical Association does not recommend using sodium bicarbonate as an antacid because the sodium ions and bicarbonate ions are absorbed by the blood, causing alkalosis in extreme cases. Furthermore, individuals with high blood pressure or a heart condition are advised to avoid excess sodium and thus may choose to use an antacid without sodium bicarbonate.

Calcium Carbonate Antacids
Using calcium carbonate as an antacid (Tums or Rolaids) avoids many of the problems caused by sodium bicarbonate. Tums contain only calcium carbonate while Rolaids contains a combination of calcium carbonate and magnesium hydroxide. Calcium carbonate is fast-acting, non-absorbable, inexpensive, and reacts with acid according to Equation 2. People with osteoporosis or those who need a calcium supplement may choose an antacid formulated with calcium carbonate. However, if used over long periods of time, calcium carbonate tends to cause constipation, and if taken in large amounts, calcium can promote the development of kidney stones. Some antacids contain a combination of calcium carbonate and magnesium hydroxide. This combination tends to overcome the constipation as magnesium hydroxide produces an opposite laxative effect. On the other hand, any compound containing the carbonate or bicarbonate ions produces CO₂ when in reaction with acids. When CO₂ accumulates in the stomach, it causes the familiar burp, which in itself provides some relief.

Aluminum-Containing Antacids
Aluminum compounds (aluminum hydroxide) are effective in reducing stomach acidity and are not absorbed into the blood. These compounds are safer than calcium-containing compounds for people with impaired kidneys or circulatory functions. Unfortunately, aluminum compounds also tend to produce constipation. They also interfere with the body’s absorption of important chemicals, including the phosphorus needed for healthy bones. Furthermore, there may be a connection between ingesting aluminum and developing Alzheimer’s disease. Unlike sodium bicarbonate and calcium carbonate, aluminum hydroxide produces no CO₂ gas when it neutralizes stomach acid. Rather, aluminum hydroxide produces a salt and water upon reaction with HCl according to Equation 3. Magnesium-Containing Antacids
Magnesium hydroxide (Phillips’ Milk of Magnesia) is fast-acting and relatively long-lasting and reacts with acid according to Equation 4. Magnesium hydroxide, however, has a lower neutralizing capacity and can cause problems for people with kidney impairment. Magnesium trisilicate is also used in some antacids. It has a lower neutralizing capacity, is slow-acting (generally taking at least 15 minutes to begin working), but is effective in controlling ulcer pain. Both of these magnesium compounds may produce diarrhea. Therefore, while magnesium-based antacids tend to be laxative, aluminum-based antacids tend to produce constipation. For this reason, some medications, such as Maalox and Mylanta, contain both aluminum and magnesium salts. Furthermore, overuse of magnesium-containing medications can cause magnesium poisoning. The symptoms of this type of poisoning include clumsiness, weakness, paralysis, drowsiness, confusion and coma. The elderly, longtime diabetics, people who have had digestive surgery and those who are taking medications that slow the digestive system (such as narcotics and some antidepressants) are particularly susceptible to magnesium poisoning. Taken as directed, however, magnesium-based antacids are safe.

Cautions About Antacid Use
Antacids are available in both liquid and tablet forms. When using antacid tablets, the tablet should be chewed thoroughly, not swallowed whole. By grinding the tablet with the teeth, the speed of the neutralization reaction between the antacid and HCl in the stomach is maximized. For this reason, liquid antacids are faster-acting than are tablets.

A person with frequent indigestion must take caution when choosing self-medication. Occasional indigestion caused by overeating or stress can be safely and temporarily treated with over-the-counter remedies. However, symptoms of indigestion or heartburn (a burning sensation in the chest caused by stomach acid leaking into the esophagus) can indicate more serious problems. Repeated bouts of indigestion, particularly when accompanied by severe pain or vomiting, require medical attention rather than self-medication. In addition, the label on the antacid container should be read in order to identify the active ingredient(s) and to follow the dosage recommendations carefully. Using any antacid in excessive amounts may cause acid rebound, which occurs when the stomach produces more acid than was present initially to overcome the neutralizing effects of antacids. Thus, none of these remedies should be taken in large amounts or for a prolonged length of time. Each antacid must be taken with caution and dosage information printed on the label should be strictly followed. The need for antacids can be minimized by eating a healthy diet, avoiding stress, and limiting your consumption of coffee, fatty foods, and chocolate.

Materials

Prelab Questions

1. What acid is required for proper digestion in the stomach?
2. What is the normal pH range of the stomach?
3. Describe two ways in which the stomach prevents acid from damaging its lining (called the mucosa).
4. Name two substances that can damage the mucosa. Describe how each works.
5. Why does the reaction between carbonate-containing antacids and stomach acid produce a “burp”?
6. Why do we refer to the reaction between an antacid and stomach HCl as an example of neutralization? 
7. Why do some antacids contain both aluminum and magnesium ions?
8. Why do liquid antacids and chewed antacid tablets increase the neutralization reaction rate when compared to whole tablets?
9. Under what circumstances are over-the-counter antacids considered safe and effective?
10. Under what circumstances should over-the-counter antacids be avoided?

Safety Precautions

Do not ingest any of the antacid drug samples during this laboratory. The samples are for laboratory use only, have been stored with non–food-grade laboratory chemicals and are not meant for human consumption. Hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes. Methyl orange indicator solution is toxic by ingestion. Universal indicator solution is a flammable, alcohol-based 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. Observing an Antacid in Action

1. Fill a 400-mL beaker with approximately 200 mL of distilled or deionized water.
2. Add 2–3 mL (1 pipet full) of universal indicator solution to the water.
3. Add 6 drops of 0.5 M HCl. This solution represents an “upset” stomach. Record the color of the solution in Data Table 1. Record an approximate pH by comparison with the universal indicator color comparison chart.
4. Use a mortar and pestle to grind one Phillips’^® MOM tablet into a powder. (Note: If a mortar and pestle are not available, crush the tablet between two sheets of paper.) This action represents “chewing” the antacid tablet before swallowing.
5. Add the powdered antacid to the beaker and stir. Observe for several minutes. This represents the antacid tablet being introduced into the acid stomach. Record all observations in Data Table 1.
6. After the solution has finished changing color, add 2–3 more drops of 0.5 M HCl. This demonstrates how an antacid reacts as your stomach continues to secrete acid. Record observations in Data Table 1.
7. Rinse the solution down the drain with plenty of water.

Part 2. Titrating a Weak Base (Antacid) with a Strong Acid

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.
2. Add 20 drops of 0.5 M HCl to the flask.
3. Add 10 drops of methyl orange indicator solution and swirl the flask.
4. Note the color of the acidic solution. Set this flask aside and use it as a color comparison (a control) during the titration.

B. Preparing the Antacid Tablets

5. Obtain an antacid tablet from your instructor. Record the brand of antacid in Data Table 2.
6. Weigh the tablet on an electronic balance with a readability of 0.01 grams. Record the mass of the tablet 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 Base–Strong Acid 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 antacid tablet to the flask and stir with a stirring rod. The antacid will disperse but not fully dissolve.
10. Determine the pH of the starting solution by touching the wet end of the stirring rod to one half of a strip of pH paper. Record the starting pH in Data Table 2.
11. Add 10 drops of methyl orange indicator solution and swirl the flask. Record the color of the solution in Data Table 2.
12. In a 50-mL graduated cylinder, obtain 40 mL of 0.5 M HCl from the stock bottle. Read the exact starting volume of 0.5 M HCl in the cylinder and record this value in Data Table 2.
13. Using a Beral-type pipet, begin adding 0.5 M HCl 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.) For the first 15 mL of HCl, add acid in increments of 5 mL at a time, swirling the flask with each addition. (Note: Try to get as much antacid powder off the sides of the flask as possible when swirling. Use distilled water to rinse the sides of the flask.)
14. After 15 mL of HCl has been added, continue to add acid in increments of 1 to 2 mL until a pH of 3.0 or below is reached. This will be indicated by a color change in the methyl orange indicator. (Remember: If the stomach pH falls below 3.0, the antacid is considered “used up”.) Toward the end of the titration, add HCl a few drops at a time and swirl. The endpoint is near when the red color remains longer and longer before turning back to yellow (as the antacid continues to neutralize the acid).
15. When the color of the solution in the flask remains the same as the control (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 acid or “overshoot” the endpoint.) The antacid/acid mixture will be cloudy while the control is a clear solution. Only compare the color of the mixture.
16. Return any remaining acid from the Beral-type pipet to the graduated cylinder. Read the exact ending volume of 0.5 M HCl 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 the other half of the strip of pH paper. Record the final pH in Data Table 2.
18. Rinse the final solution down the drain with plenty of water.
19. Repeat steps 5–18 using a different antacid and a clean, 125-mL Erlenmeyer flask.

Student Worksheet PDF

13359_Student1.pdf