How do antacids work? Titration of an Antacid

materials for antacid kitchen chemistry

This is a procedure for a titration experiment suitable for use both at home and in supervised high school or first-year college laboratory settings. Other than an inexpensive pH meter, all materials should be easily available for students to purchase at local stores. A suitable pH meter may be obtained online for $12-15, or the experiment may be modified to use only the indicator obtained from red cabbage. Clear instructions, photos, a video, and a report form template with suggested answers are available. This should be useful for high school laboratories, distance learning, or anywhere that large costs are prohibitive, or where the availability of traditional laboratory or personal protective equipment is questionable.

Time required: 
  • Preparing the cabbage juice indicator (30 minutes)
  • Titration of Milk of Magnesia (90 minutes)
  • Writing the report (30 minutes)

For preparation of cabbage juice indicator 

  • Red cabbage
  • Heat-proof container, such as a Pyrex bowl or saucepan
  • Sharp knife (optional)
  • Deionized or purified water
  • A funnel or coffee filter basket
  • Coffee filters

For Titration of Mg(OH)2 with CH3COOH 

  • Milk of magnesia, 30 mL (the regular version, not “concentrated”)
  • Deionized or purified water
  • White vinegar, about 150 mL
  • 1-cup (250-mL) plastic measuring cup
  • Measuring spoon, 1 Tbsp (optional)
  • Plastic spoon or wood craft stick
  • Plastic syringe, 5 or 10 mL (available at pharmacies)
  • Paper towels
  • Cabbage juice indicator (see above), 15 mL
  • pH meter (available online or where swimming pool or aquarium supplies are sold)
  • Cabbage juice indicator, 15 mL
  • Digital camera or smartphone

Safety Concerns: The solutions are all food grade, but they could cause irritation and tissue damage to eyes. Goggles should be worn when making and pouring solutions. All solutions may be safely disposed of down the drain.


Milk of magnesia is an aqueous suspension of magnesium hydroxide, Mg(OH)2, used as an antacid, and sometimes as a laxative. Excess hydrochloric acid (HCl) excreted by the cells in the stomach lining can lead to indigestion, heartburn, and acid reflux. As an antacid, milk of magnesia is dosed from about 1000-5000 mg. In the stomach, the hydroxide ions in Mg(OH)2 combine with the hydrogen ions in HCl to neutralize excess acid.

In this experiment, students will use cabbage juice pH indicator and a pH meter to investigate how milk of magnesia neutralizes the extra acid in the stomach by performing a titration.

My students complete a report form available as a Google Doc. Students can either make a copy of the document and complete it or they can download it as a Word document and save it as a pdf after completing it. Either way, they submit it to me electronically.

You can see the procedure provided to students below. You can also download these student directions and other teacher documents from the Supporting Information below. Log into your ChemEd X account for access. Don't have an account? Register for free! 



Titration of Mg(OH)2 with CH3COOH

1. Take a photo of the ingredients label of the Milk of Magnesia. Shake up the bottle thoroughly. Carefully measure 30 mL (1 oz, 2 Tbsp.) of Milk of Magnesia (MoM) into a clear plastic measuring cup. Rinse the container that you used to measure the MoM with an equal volume of water and add to the measuring cup. Stir with a spoon or craft stick. You are creating a suspension, not a true solution, because not all the Mg(OH)2 dissolves, producing a milky suspension of white solid. 

2. Add 1 tablespoon (about 15 mL) of cabbage juice indicator. This indicator turns red on the acidic (low) end of the pH scale and green on the alkaline (basic) end. The exact amount of indicator solution is not important, so if you do not have measuring spoons, just get close.

3. Use a spoon or craft stick to thoroughly mix the liquids. You'll see that the suspension turns green, indicating that it is somewhat basic due to the small amount of Mg(OH)2 that is dissolved. Record the color, and measure the pH with your meter. The pH should be above 9. Take two photographs. You need one photograph showing both yourself and your experimental setup ("selfies" are fine, or have a friend take your photo). Be sure that your photo shows good safety practices (wearing goggles, organized work area, etc.) You need a second, closeup photograph showing your pH meter in the Milk of Magnesia with the pH reading visible in the photo.


Video 1: Step 4 of titration of Mg(OH)2 with CH3COOH       


4. Use your syringe to add 5.0 mL of vinegar, and stir. You should observe a rapid color change to pink/red near where the acetic acid in the vinegar dissolves into the mixture, reacting with and neutralizing the small amount of hydroxide ion from the Mg(OH)2. As you keep stirring, more of the Mg(OH)from the suspensions gradually dissolves into solution. The acid in the vinegar is used up (neutralized), and eventually the solution becomes basic again.  Wait until the color and pH stabilize (15 seconds or so). Record the color and measure the pH with your meter again. Video 1 shows what this might look like.

5. Continue adding vinegar 5 mL at a time, stirring, recording the final color, and measuring the pH with the meter. Eventually, all of the magnesium hydroxide will be used up by the vinegar (acetic acid) and the solution will remain purple, and eventually turn pink or red. (Note: the color and pH will not change much for up to 20-25 readings. Don’t worry about this! Note in Figure 2 below how the pH didn’t go below 8.8 until 100 mL of vinegar were added.)

6. Once the solution is clear (not cloudy or milky), continue adding 5 more 5 mL portions of vinegar and recording your results until the color is violet to pink and has remained so for several additions. The pH should be below 5 at this point.

7. Record your data into a table that has three columns: pH, mL vinegar added, and color of mixture. See Table 1 below for an example.


Table 1: pH and color of MoM mixture after addition of various amounts of vinegar

 9.87    0     green
 9.31   5     green
 9.24   10     green



  • Rinse off the pH electrode with water and dry with a paper towel. Replace the cap.
  • Pour the remaining solutions down the drain.
  • Rinse syringe and cups with water and set aside to dry or dry with paper towels.
  • Wipe work area clean with a sponge or damp paper towel.
  • Wash hands.

Figure 2: Titration curve for Milk of Magnesia with Vinegar


Thinking About the Data

Students type the pH and mL vinegar into a spreadsheet such as Google Sheets or Microsoft Excel. Then a scatter plot is created. See an example in Figure 2. 


The cabbage juice indicator can be prepared in advance and stored in the refrigerator.

Procedure for preparing cabbage juice indicator

1. Using the knife, coarsely chop about 1 cup of red cabbage into small pieces, and add to the heat proof container. (Alternatively, you can simply tear the cabbage into small pieces.)

2. Heat about a cup (250 mL) of purified water to boiling. Pour boiling water over the cabbage until the pieces are well submerged (about ½ cup should be sufficient). Allow to steep for one hour.

3. Pour the water with the boiled cabbage leaves through the coffee filter to remove the pieces of cabbage, collect the filtered juice in a clean glass, and let it cool; it should appear purple.

4. If you will not use it immediately, cover and refrigerate the cabbage juice indicator until you’re ready to use it.

Post a copy of the Report Form or make paper copies. The REPORT FORM is on Google Drive but you can also find a pdf in the Supporting Information. Don't see the Supporting Information? Log in. Not a member? Register for free now!


Teacher and student documents can be found in the Supporting Information! Don't see the Supporting Information below? Log in. Not a member? Register now for FREE! 


General Safety

For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).  

For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.

Other Safety resources

RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies



Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at and further resources at


Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Assessment Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.


Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.