Buffer Capacity of Alka-Seltzer™

Category: 
Demonstration
Topics: 
Acids and Bases

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Different size tablets of Alka-Seltzer™ dissolved in water will neutralize different amounts of acid and base.  Universal indicator solution is dissolved in the water prior to adding the tablets.  A color change is visible when acid or base is added to the solutions.  A whole Alka-Seltzer™ tablet will buffer more acid and base compared to a half  Alka-Seltzer™ tablet.  This demonstration illustrates the concept of buffer capacity.


Curriculum Notes 
Learning Objectives

1. Buffer capacity depends on the concentration of two components (weak acid-conjugate base or weak base-conjugate base) and on the pKa of the acid (or pKb of the weak base) .

2.  The range of a buffer solution should be within ±1 pH unit of the pKa of the weak acid (or pKb of the weak base) .


Discussion 
An Alka-Seltzer tablet contains about 1.9 grams (0.22 mol) of sodium bicarbonate and about 1.0 gram (0.005 mol) of citric acid. When the tablet is placed in water, the citric acid reacts with the sodium bicarbonate, producing sodium citrate in solution and releasing carbon dioxide gas.
$\ce{3HCO3^{-}(aq) + H3C3H5O7(aq) -> C3H5O7^{3-}(aq) + 3CO2(g) + 3H2O(l)}$

When small amounts of acid or base are added to the citrate/bicarbonate solution, the solution resists a major change in pH.
The citrate anion is the anion of a weak acid.  It neutralizes excess acid by forming citric acid:
$\ce{C3H5O7^{3-}(aq) + 3H^{+}(aq) -> H3C3H5O7(aq)}$

The excess sodium bicarbonate reacts with acid, liberating $\ce{CO2}$:
$\ce{HCO3^{-}(aq) + H^{+}(aq) -> CO2(g) + H2O(l)}$
 

Excess  bicarbonate also neutralizes hydroxide ions: 
$\ce{HCO3^{-}(aq) -> CO3^{2-}(aq) + H2O(l)}$
 

 

Preparation 

1. Label four 250 mL beakers A , B., C, and D.

2. To a 1,000 mL beaker, add 825 mL of DI water and add Universal indicator, the pH should be close to pH = 6.5

3.  Add 200 mL DI water with the indicator to each of the 250 mL beakers.


Materials 

  • 0.850 L distilled water
  • 4 250 mL beakers
  • 1 1.00 L beaker
  • Alka-Seltzer™ packets
  • Disposable pipets
  • Universal indicator solution
  • 0.1 M Hydrochloric acid (HCl)
  • 0.1 M Sodium Hydroxide (NaOH)
  • droppers


Procedure 
Procedure for performing the demonstration in front of an audience:

1. Add half of an Alka-Seltzer tablet to beaker A and add a whole Alka-Seltzer tablet to beaker B. Note the colors of the solutions.

2. Add acid drop wise using the disposable pipets to beaker A. Note the number of drops it takes to lower the pH. 
3. Repeat with beaker B. Note it takes more acid to change the pH of the solution in B vs A.

4.  Add half of an Alka-Seltzer tablet to beaker C and add a whole Alka-Seltzer tablet to beaker D. Note the colors of the solutions.

5. Add sodium hydroxide drop wise using the disposable pipets to beaker C. Note the number of drops it takes to raise the pH. 

6. Repeat with beaker D. Note it takes more base to change the pH of the solution in D vs. C.


Footnotes 

1. Shakhashiri, B.Z.; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Wisconsin; Volume 3; 1989; p. 186-187

2. Friedman, N. (1975). A new buffer demonstration. Journal of Chemical Education, 52(9), p. 605. DOI: 10.1021/ed052p605

An Alka-Seltzer tablet in water is used as a buffer system and its resistance to changes in pH are compared to solutions of buffered aspirin and aspirin.