Help From MOM

Introduction

Milk of magnesia (MOM) and other antacids are bases that relieve heartburn by neutralizing the acid found in the stomach’s gastric juices. When hydrochloric acid and universal indicator are added to milk of magnesia, a dramatic rainbow of color changes is observed as the antacid neutralizes the simulated stomach acid. This is a great demonstration to teach concepts of acids and bases, solubility, digestion and “antacid-testing” consumer biochemistry.

Concepts

• Acid–base neutralization
• Digestion
• Antacids

Materials

Safety Precautions

Milk of magnesia is intended for laboratory use only; it has been stored with other non–food-grade laboratory chemicals and is not meant for human consumption. Hydrochloric acid solution is toxic by ingestion, inhalation and is corrosive to skin and eyes. Universal indicator solution contains ethyl alcohol and is a flammable liquid. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all normal laboratory guidelines. 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, and review all federal, state and local regulations that may apply, before proceeding. Neutralize the final solution with sodium carbonate or excess milk of magnesia according to Flinn Suggested Disposal Method #24b. Excess milk of magnesia can be disposed of according to Flinn Suggested Disposal Method #26a solid waste disposal in landfill.

Procedure

1. Measure 20 mL of milk of magnesia (MOM) using a graduated cylinder and pour it into a 1-L beaker.
2. Add a stir bar to the MOM solution and place the 1-L beaker on a magnetic stirrer. (If a magnetic stirrer is not available, use a stirring rod to mix the solution as other reactants are added in steps 3–5.
3. Add water and crushed ice (or ice cubes) to give a total volume of approximately 800 mL. Turn on the stir plate so as to create a vortex in the mixture.
4. Add about 4–5 mL (about 2 pipets-full) of universal indicator solution. Watch as the white suspension of milk of magnesia turns to a deep purple color. The color indicates that the solution is basic.
5. Add 2–3 mL (1 pipet-full) of 3 M HCl. The mixture quickly turns red and then goes through the entire range of universal indicator color changes back to purple.
6. Repeat this process, adding HCl one pipet-full at a time, waiting after each addition until the mixture turns back to blue– purple.
7. The process can be repeated a number of times before all of the milk of magnesia has dissolved and reacted with the HCl. As more acid is added, the color changes will occur more rapidly and eventually the suspended solid in MOM will be completely dissolved. The final solution, when all the MOM has reacted, will be red and clear.

Student Worksheet PDF

10810_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 150 mL of milk of magnesia, 250 mL of 3 M hydrochloric acid solution, 50 mL of universal indicator solution and 15 disposable pipets.
• If a 1-L beaker is not available, use a 600- or 400-mL beaker. Adjust chemical amounts accordingly. Note: The amount milk of magnesia added does not have to be exact in order for the demonstration to work.
• The acid used in the demonstration is 3 M hydrochloric acid (HCl). Actual stomach acid has a pH of 1.4, corresponding to 0.04 M HCl. However, 3 M HCl is used in this demonstration in order to limit the total acid volume and allow the reaction to go to completion with a reasonable volume of acid. If desired, dilute the 3 M acid to 0.04 M and perform the experiment as written. The volume of acid needed will be 75 times greater.
• The reaction is performed on ice in order to slow down the color changes so that all colors in the universal indicator color range can be viewed. The reaction may be performed without the use of ice.
• Consider performing this demonstration at different temperatures—5 °C, 25 °C and 60 °C—to compare the effect of temperature on the rate of reaction.
• An excellent follow-up to this antacid demonstration is Flinn’s Antacid Testing Lab Kit—How Powerful Is Your Antacid? (Catalog No. AP5932).
• Magnesium hydroxide is also a saline osmotic laxative in the intestines. The magnesia suspension causes saline to osmose from the blood into the intestine causing the bowel to distend and expel its contents.

Answers to Questions

Observations

1. What is the initial color of the MOM solution with universal indicator?

   Violet

2. What is the color of the mixture immediately after the acid is added?

   Red

3. Describe the change in the solution after the addition of several milliliters of acid.

   The violet solution becomes red then orange, yellow, green, blue and finally violet.

Questions

1. What is the function of stomach acid?

   Stomach acid converts the inactive enzyme precursor pepsinagen to the active protein enzyme pepsin. Stomach acid also acts to denature proteins and to partially digest the food in the stomach.

2. How does an antacid relieve heartburn?

   Antacids neutralize stomach acid creating a near neutral pH which inactivates the protein digesting enzyme pepsin so the pepsin cannot digest the muscles lining the esophagus.

3. What is the active ingredient in milk of magnesia?

   Magnesium hydroxide, Mg(OH)₂.

4. What type of reaction occurs between the active ingredient in milk of magnesia and stomach acid?

   Neutralization reaction.

Discussion

Hydrochloric acid secreted in the stomach gives the gastric juice a pH of between 1.5 and 3.5, depending upon the amount of food within the stomach. The pH of an empty stomach is usually less than 2. The acidic stomach environment is necessary for two reasons. First, an acidic environment is necessary to convert pepsinogen, an inactive enzyme precursor, into the active enzyme pepsin. Pepsin, which digests protein, is the most abundant enzyme in the stomach. Second, the acidic environment is responsible for denaturing proteins and partially digesting the food within the stomach. The mixture of partially digested food and gastric juice forms a thick liquid called chyme, which then enters the small intestine for further digestion.

When gastric juices contact the esophagus, the acid may irritate and partially digest the esophageal lining. The resulting heartburn is often treated with antacids. Antacid literally means “against acid.” Antacids are weak bases which work by neutralizing the acidic environment of the stomach. This demonstration uses the antacid milk of magnesia (MOM). The active ingredient in milk of magnesia is magnesium hydroxide, Mg(OH)₂. MOM works well for the demonstration because the magnesium hydroxide forms a suspension in water due to its very low solubility—0.0009 g/100 mL in cold water and 0.004 g/100 mL in hot water.

Magnesium hydroxide is classified as a weak base due to its very limited solubility in water. This limited solubility makes it an ideal compound for use in commercial antacids because, rather than dissolving all at once, it slowly dissolves as it reacts with and neutralizes stomach acid. As the “excess stomach acid” neutralizes the dissolved magnesium hydroxide, more magnesium hydroxide enters the solution from the solid suspension state until the solution is neutralized or all of the reactants are used up. (This is an example of Le Chatelier’s principle.)

Universal indicator is added to the MOM suspension to help visualize the neutralization reaction as it occurs. The initial color of universal indicator in the MOM suspension is violet, corresponding to a basic solution (pH ≥ 10). (See the Universal Indicator Color Chart, Table 1.) When hydrochloric acid (the simulated “stomach acid”) is added, the mixture quickly turns red (pH 4) as the acid disperses throughout the beaker and neutralizes a small amount of dissolved magnesium hydroxide. There is still some unreacted acid present, however, so the solution remains red.

However, the unreacted acid causes more magnesium hydroxide from the suspension to gradually dissolve and react. As more of the magnesium hydroxide goes into solution, the excess acid is neutralized and eventually the solution turns blue or violet (pH 9–19) again, indicating excess magnesium hydroxide is present. The addition of universal indicator allows this process to be observed. During the process, the color of the mixture cycles through the entire universal indicator color range—from red to orange to yellow to green to blue and finally back to violet. By adding more “stomach acid,” the process can be repeated several times before all of the magnesium hydroxide dissolves and is neutralized. The final solution, after all the MOM has dissolved and reacted with HCl, is red and clear.

The neutralization reaction is the reaction between Mg(OH)₂ (a weak base) and HCl (a strong acid). The overall reaction is summarized in Equation 1. In aqueous solution, the reacting species for the strong acid, HCl, is the hydrogen ion, H⁺. The products are a H₂O molecule and a Mg²⁺ ion in solution. The chloride ion, Cl^–, from HCl is considered a “spectator” ion (it is not involved in the reaction), and thus is not included in the net ionic equation.

While Mg(OH)₂ is practically insoluble in water, a very small amount of Mg(OH)₂ does dissolve and dissociate into ions (Mg²⁺ and OH^–) when put in water. In the demonstration, the initial milk of magnesia suspension in water contains very few Mg²⁺ and OH^– ions before the acid is added. As HCl is added to the beaker containing milk of magnesia, the H⁺ ions from the HCl react with the OH^– ions (those that are actually in solution from the Mg(OH)₂ according to Equation 2. The reaction between H⁺ ions (stomach acid) and OH^– ions (antacid) to form water uses up some of the OH^– ions, causing more Mg(OH)₂ to dissolve and dissociate into ions according to Le Chatelier’s Principle. As more acid is added, the Mg(OH)₂ continues to dissociate until all of it is dissolved. The final solution in the milk of magnesia demonstration will thus be clear and acidic (red in color from the universal indicator), indicating that the Mg(OH)₂ has been fully “consumed.” At this point, the “antacid power” or “acid-neutralizing ability” of the milk of magnesia is depleted.

References

Special thanks to Annis Hapkiewicz, Okemos High School, Okemos, MI, and to Penney Sconzo, Westminster School, Atlanta, GA, for separately bringing this demonstration to our attention.

Summerlin, L. R.; Borgford, C. L.; Ealy, J. B. Chemical Demonstrations: A Sourcebook for Teachers, Vol. 2; American Chemical Society: Washington, DC. 1988; p 173.