ANATOMY PHYSIOLOGY LAB 2-1 HOW POWERFUL IS YOUR ANTACID? 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. Background: The Role 0f 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 aspirin- sensitive 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. 1
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. NaHCO3(s) + HCl(aq) + NaCl(aq) + H2O(l) + CO2(g) 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 (Turns or Rolaids) avoids many of the problems caused by sodium bicarbonate. Turns 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. CaCO3(s) + 2HCl(aq) + CaCl(aq) + H2O(l) + CO2 (g) 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. Al(OH)3(s) + 3HCl(aq) AlCl3(aq) + 3H2O(l) Equation 3 2
Magnesium-Containing Antacids Magnesium hydroxide (Phillips"" Milk of Magnesia) is fast-acting and relatively long-lasting and reacts with acid to Equation 4. Mg(OH)2(S) + 2HCl(aq) MgCl2(aq) + 2H2O(l) 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 t least 15 minutes to begin working), but is effective in controlling ulcer pain. Both of these magnesium compounds may 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 are: 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 antidepressant 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 a 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 than self-medication. In addition, the label on the antacid container should be read in order to identify the active ingredient to follow the dosage recommendations carefully. Using any antacid in excessive amounts may cause acid rebound, which when the stomach produces more acid than was present initially to overcome the neutralizing effects of antacids. Thus, these remedies should be taken in large amounts or for a prolonged length of time. Each antacid must be taken with cauti dosage information printed on the label should be strictly followed. The need for antacids can be minimized by eating a diet, avoiding stress, and limiting your consumption of coffee, fatty foods, and chocolate. Pre-Laboratory Questions: Read the background information and complete the questions below. Answer on a separate sheet of paper. 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. Define the term antacid. 6. Why does the reaction between carbonate-containing antacids and stomach acid produce a "burp"? 7. Why do we refer to the reaction between an antacid and stomach HCI as an example of neutralization? 8. Why do some antacids contain both aluminum and magnesium ions? 9. Why do liquid antacids and chewed antacid tablets increase the neutralization reaction rate when compared to whole tablets? 10. Define the term acid rebound. 11. Under what circumstances are over-the-counter antacids considered safe and effective? 12. Under what circumstances should over-the-counter antacids be avoided? 3
How Powerful Is Your Antacid? Antacid Testing Lab Chemical Concepts: Antacids Acid-base neutralizations Materials: Antacid tablets Balance: 0.01g readability Part 1 Phillips Milk of Magnesia (MOM), 1 tablet Beaker, 400 ml Part 2 Two of the three tablets listed: Beral-type pipettes, 3 Generic (Tums) Antacid, 1 Erlenmeyer flasks, 125 ml, 3 Phillips Milk of Magnesia (MOM, 1 Graduated Cylinder, 50 ml, or 25 ml Rolaids, 1 Mortar and pestle Hydrochloric acid solutions, HCl, 0.5 M ph indicator strips Methyl orange indicator solution, 0.1% Spatula Universal indicator solution and color comparison chart Stirring rod Water, distilled or deionized 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 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. 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 pipette 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. This 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. 4
Procedure: 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 125mL 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 and record the brand of antacid in Data Table 2. 6. Weigh the tablet on an electronic balance. Record the mass of the tablet in Data Table 2. 7. Use a mortar and pestle to grind the tablet into a powder. 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 solutions 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 (or 25mL) graduated cylinder, obtain approximately 25 or 30 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 solution. (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 15mL 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 for a different antacid using a clean, 125-mL Erlenmeyer flask. 5
How Powerful Is Your Antacid? Antacid Testing Data Data Table 1: An Antacid in Action Step No. Physiological Environment Observations 3 Upset Stomach 5 Antacid Introduced into Stomach 6 Additional Stomach Acid Secretion Data Table 2: Antacid Titration Antacid Brand Antacid Brand Mass of Tablet (g) ph of Antacid Solution (Before Titration) Color of Solution (Before Titration) Starting Volume of HCl (ml) Ending Volume of HCl (ml) Volume of HCl Neutralized (ml) Color of Solution (After Titration) ph of Antacid Solution (After Titration) 6
How Powerful Is Your Antacid? Antacid Testing Data Table 3: Summary of Antacid Data Antacid Brand Antacid Brand Antacid Brand Average Volume of HCl Neutralized Per Tablet (ml) Mass of One Tablet (g) Average Volume of HCl Neutralized Per gram of Antacid (ml/g) Price Per Bottle of Antacid Number of Antacid Tablets Per Bottle Cost per Gram (cent/ml) Cost Per ml of HCl Neutralized (cent/ml) Volume of HCl Neutralized Per Penny (ml/cent) Active Ingredient(s) Amount of Each Active Ingredient (mg/tablet) Recommended Adult Dosage (tablets) Calculations and Summary: Show all work on a separate sheet of paper (for at least one type of antacid) and record your answers in Table 3. Complete Table 3 for all three types of antacids. 1. Compile class data on the board of Volume of HCl Neutralized per antacid tablet for all three types of antacids used by the class Rolaids, Generic Tums, and Phillips Milk of Magnesia. 2. Calculate the average volume of acid neutralized per antacid tablet and the average volume of acid neutralized per gram of antacid. Record this in Table 3. 3. For each of the three antacids tested, record the price per bottle and the number of tablets in each bottle in Table 3. Cost information must be found at the store or provided by your teacher. 4. Calculate the cost per tablet, the cost per gram of tablet, the cost per ml of acid neutralized, and the volume of acid neutralized per penny for each antacid. Record this in Table 3. 7
5. For each antacid, record the active ingredient(s), amount of each active ingredient (mg/tablet), and recommended adult maximum daily dosage in Table 3. This information can be found wither on the bottle or provided to you by your teacher. Part 1 Post-Lab Questions: Answer on a separate sheet of paper. 1. Milk of Magnesia (MOM) is an antacid. How does an antacid work? 2. Why did the color of the solution in the beaker change when a MOM tablet was added? 3. Why does the ph of the solution in the beaker change when MOM is added? 4. Why does the ph of the beaker containing MOM stop at 10.0, even though some of the tablet remains undissolved? 5. How does an antacid such as MOM respond as the stomach continues to secrete acid? Part 2 Post-Lab Questions: Answer on a separate sheet of paper. 6. Is the concentration of the HCl standard solution used in titration comparable to the concentration of this acid in the human stomach? How do you know? 7. Why was methyl orange chosen as the indicator in the acid-base titration? 8. Which antacid (of those tested) is most effective at neutralizing stomach acid? Answer in terms of per tablet and per gram of antacid. 9. Which antacid (of those tested) is least effective at neutralizing stomach acid? Answer in terms of per tablet and per gram of antacid. 10. Consider both the costs of the antacids as well as neutralizing ability. a. Which antacid tablet is the most cost-effective? b. Which antacid tablet is least cost-effective? 11. Look at the recommended dosage fro each brand. a. How did your experimental method differ from expected patterns of usage by consumers? b. How could this difference be corrected? 8