How do antacids work? Titration of an Antacid

Titration of Mg(OH)₂ with CH₃COOH

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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       

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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)₂ 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.)

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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

CLEANUP:

• 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.

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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.