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Date_______________
Acid Neutralizing Capacity of an Antacid
Background
Stomach acid contains hydrochloric acid which aids in food digestion. Excess stomach acid produces a
condition known as acid indigestion or acid reflux. Commercial antacids containing one or more bases
are available to treat this condition by neutralizing the excess acid in the stomach. The acid neutralizing
capacity (ANC) of an antacid is the amount of acid that it can neutralize. This ANC can be best measured
in the laboratory by a process known as back titration. This involves dissolving the antacid in an excess
of acid and then titrating the acidic solution against a known concentration of base until the endpoint is
reached. The moles of acid neutralized equals the difference between the moles of acid added and the
moles of base required for the back titration.
For this investigation:
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑎𝑐𝑖𝑑 𝑛𝑒𝑢𝑡𝑟𝑎𝑙𝑖𝑧𝑒𝑑 = 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑 – 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑁𝑎𝑂𝐻 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
= (𝑉𝑜𝑙𝑢𝑚𝑒𝐻𝐶𝑙 𝑥 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦𝐻𝐶𝑙 ) – (𝑉𝑜𝑙𝑢𝑚𝑒𝑁𝑎𝑂𝐻 𝑥 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦𝑁𝑎𝑂𝐻 )
𝑎𝑐𝑖𝑑 𝑛𝑒𝑢𝑡𝑟𝑎𝑙𝑖𝑧𝑖𝑛𝑔 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑝𝑒𝑟 𝑔𝑟𝑎𝑚 𝑜𝑓 𝑎𝑛𝑡𝑎𝑐𝑖𝑑 =
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻𝐶𝑙 𝑛𝑒𝑢𝑡𝑟𝑎𝑙𝑖𝑧𝑒𝑑
𝑔𝑟𝑎𝑚𝑠 𝑜𝑓 𝑎𝑛𝑡𝑎𝑐𝑖𝑑
Objectives
1. After completing this laboratory investigation, the student will be able to determine the acid
neutralizing capacity of an antacid.
2. After completing this laboratory investigation, the student will be able to compare the acid
neutralizing capacity of various antacids.
Materials/Equipment
(per team)
Antacids, tablets of selected commercial antacids
Beakers, 250 mL (2)
Burettes, 50 mL (2)
Distilled water
Double burette clamp
Electronic balance
Erlenmeyer flask, 125 mL (2)
Funnel (1)
Hydrochloric acid (0.15 M HCl, 100 mL)
Mortar and pestle
Phenolphthalein indicator solution (2 mL)
Ring stand
Sodium hydroxide solution (.2 M NaOH, 100 mL)
Stirring rod
Name_______________________________________
Date_______________
Procedure
1. Choose 4 antacids to investigate. Record their names and the names and amounts of the active
ingredients in each.
2. Use a ring stand with the double burette clamp and two burettes to set up the titration
apparatus. Label one burette acid and the other base. Place a 250 mL beaker under each
burette tip.
3. Add 5-10 mL of 0.15 M HCl to the acid burette to rinse the burette and drain into the 250 mL
beaker. Complete the same process with the base burette using the unknown concentration of
NaOH. Discard the acid and base.
4. Obtain an antacid tablet and measure its mass and record in the data table. Using a mortar and
pestle, crush the antacid tablet.
5. Place a clean 125 mL Erlenmeyer flask on the electronic balance and zero or tare the balance.
Add approximately 0.25 grams of the antacid tablet to the 125 mL Erlenmeyer flask and record
the mass added to the flask in the data table.
6. Fill the acid burette with the 0.15 M HCl carefully until the acid is above the zero mark.
Dispense acid from the burette into a discard beaker until all air is removed from the burette tip
and the level of acid is within the graduated portion of the burette. Record the burette level as
the initial burette reading of HCl. Use the bottom of the meniscus for your burette level
reading. Complete the same process with the base burette using the 0.2 M NaOH.
7. Into the 125 mL Erlenmeyer flask containing the powdered antacid, dispense approximately 25
mL of the acid. Add approximately 50 mL of deionized water and 3-5 drops of phenolphthalein
solution to the flask and swirl to mix. Let stand for 10 minutes.
8. Titrate slowly by adding the base into the flask containing the acid with stirring until a pink color
starts to persist in the beaker. Decrease the flow of base to a slow drop by drop process and
continue until the pink color persists for more than a few seconds.
9. If you overshoot the titration endpoint, slowly add acid drop by drop until the pink color
disappears. Then add base again drop by drop until a faint pink color persists. Repeat this
titration process until one drop of base will cause the faint pink color to remain.
10. Record the final acid and base burette readings in the data table.
11. Repeat steps 1-9 with three more different antacids.
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Date_______________
Data Table
Molarity of HCl solution __________________
Antacid 1
Molarity of NaOH solution _________________
Antacid 2
Antacid 3
Antacid 4
Name of antacid
Name and mass of
active ingredients
Mass of antacid
tablet (g)
Mass of antacid
added to flask (g)
Initial burette
reading for HCl (mL)
Final burette
reading for HCl (mL)
Initial burette
reading for NaOH
(mL)
Final burette
reading for NaOH
(mL)
Data Analysis
1. Calculate the volume of HCl dispensed and NaOH required from each trial and place in the
calculations table.
2. Using the molarity formula, calculate the moles of HCl dispensed in each trial and place in the
calculations table.
3. Calculate the moles of HCl neutralized by the antacid.
4. Calculate the neutralizing capacity of the antacid per gram of antacid.
5. Calculate the neutralizing capacity of the antacid per tablet of the antacid.
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Calculations Table
Antacid 1
Antacid 2
Antacid 3
Antacid 4
Volume of HCl
dispensed (mL)
Volume of NaOH
required (mL)
Moles of HCl
dispensed
Moles of NaOH
required
Moles of HCl
neutralized by
antacid
Neutralizing capacity
of antacid per gram
(moles HCl
neutralized/ g
antacid)
Neutralizing capacity
of antacid per tablet
Conclusions
1. Which antacid neutralized the most acid per gram of antacid tablet?
2. Which antacid neutralized the most acid per tablet?
3. Identify a method to calculate the mass of base neutralized, calculate the amount of base
neutralized, and compare it to the manufacturer’s value.
4. Compare any claims by the labels on the acids like “extra strength” or “maximum strength” to
your results for the antacids tested. Do these terms match your results?