Honors Chemistry
Lab Activity: Buffering Action of Alka-Seltzer
Background: A buffer resists changes in pH when acid or base is added. Alka-Seltzer
acts as a buffer by providing an antacid, citric acid, to neutralize stomach acidity and
sodium bicarbonate to neutralize basicity.
When Alka-Seltzer dissolves in water these components react:
3NaHCO3 (aq) + H3C3H5O7 (aq)  Na3C3H5O7 (aq) + 3CO2 (g) + 3H2O (l)
The sodium citrate acts as an antacid.
Na3C3H5O7 (aq) + HCl (aq)  H3C3H5O7 (aq) + NaCl(aq)
The sodium bicarbonate acts to neutralize base.
NaHCO3 (aq) + NaOH (aq)  Na2CO3(aq) + H2O (l)
Objective: The purpose of this activity is to test the buffering capacity of Alka-Seltzer.
NaOH and HCl will be used to titrate an Alka-Seltzer tablet and plain water. A
comparison will be made to determine which system will require more acid or base to
achieve an acidic or basic pH.
Materials:
(2)- 50 ml burets and buret stand
1M HCl solution
1M NaOH solution
Methyl Orange indicator
Phenolphthalein indicator
(2) - Alka-Selter tablets
(4) - 250 ml beakers
Stirring rod
Water bottle with distilled water
Procedures:
1. Fill one buret with 50 ml of HCl and the other with 50ml NaOH. Label each
buret.
2. Label four beakers #1-4 and fill with 200 ml of distilled water.
3. Dissolve one Alka-Seltzer in beaker #2 and beaker #4
4. Add approximately 5 drops of methyl orange indicator to beakers # 1 &2
5. Add approximately 5 drops of phenolphthalein indicator to beakers # 3 & 4
6. Titrate beaker #1 with HCl while measuring pH, note the volume of HCl it takes
to bring the water to a very low pH (pH≈1).
7. Titrate beaker #2 with HCl while measuring pH. Record the volume of HCl
required to bring the solution to a very low pH ( monitor with color change).
8. Repeat procedures with beaker #3 & #4 using the NaOH solution and recording
the volume NaOH required to raise the pH of the two samples to a high pH.
Data Table:
Beaker Number
mL HCl
mL NaOH
1 H20
----
2 Alka-Seltzer
----
3 H2 O
----
4 Alka-Seltzer
----
Post Lab Questions: Your answers should be written on a separate piece of paper and
in complete sentences to receive full credit. Be sure to include your data table.
1. Why did the samples containing Alka-seltzer “consume” greater amounts of
acid(HCl) and base (NaOH) than water and why did they eventually reach the
same end point as water?
2. Why were methyl orange and phenolphthalein used as indicators in this study?
3. Based on your results, describe how Alka seltzer functions as a buffer.
4. Identify the compounds responsible for the buffering capacity of Alka seltzer ( i.e.
weak acid/conjugate base).
5. Relate the properties of buffers to your understanding of Le Châtelier’s Principle.
Reference: Bassam Z. Shakashiri (1989) Chemical Demonstrations: A Handbook for Teachers of
Chemistry, Volume 3. p 186-187.