Demonstrations › Acids, Bases, Buffers, Titrations › 16.4


Acids, Bases, Buffers, Titrations

16.4 Buffering capacity of Alka-Seltzer™

Subjects: Acids/Bases, buffers

Description: The buffering volume of Alka-Seltzer is examined.


1 l distilled water 4 250 mL beakers 2 150 mL beakers (for hold HCl and also NaOH) Alka-Seltzer™ packet Disposable pipets global indicator‡ 0.1 M Hydrochloric acid (HCl)‡ 0.1 M salt Hydroxide (NaOH)‡ Optional: pH probes or Vernier pH probes with Go-Link user interface (requires Logger agree or Logger Lite software)*

‡HCl and also NaOH are located in the cabinets under the hood. global indicator is located in the flammables cabinet.

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*Probes are situated in the drawers the contrary the bin warehouse shelves.

Pre-class Preparation:

1. Label the four 250 mL beakers A-D. 2. Include 200 mL dI water to all four beakers. 3. Add the very same amount of global indicator to every beaker so that they are about the exact same color. 2. Label the two 150 mL beakers with HCl and also NaOH 3. Pour samples of HCl and also NaOH into the two 150 mL beakers.


1. Add one Alka-Seltzer tablet to beaker B and also the other Alka-Seltzer tablet to manufacturer D. Note the colors of the solutions. 2. Include acid autumn wise using the disposable pipets come beaker A. Keep in mind the number of drops it takes to lower the pH. 3. Repeat v beaker B. Note it takes much more acid to change the pH. 4. Repeat measures 2 and 4 through beakers C and D, adding NaOH until the pH increases.


An Alka-Seltzer tablet computer contains 1.9 grams (0.22 mol) of sodium bicarbonate and also 1.0 gram (0.005 mol) of citric acid. As soon as the tablet is put in water, the citric acid reacts through the sodium bicarbonate, producing sodium citrate in solution and also releasing carbon dioxide gas.

3HCO3-(aq) + H3C3H5O7(aq) –> C3H5O73-(aq) + 3CO2(g) + 3H2O(l)

Sodium citrate behaves like an antacid, due to the fact that citrate is the anion of a weak acid. The neutralizes excess mountain by developing citric acid:

C3H5O73-(aq) + 3H+(aq) –> H3C3H5O7(aq)

The excess salt bicarbonate additionally acts together an antacid, liberating CO2:

HCO3-(aq) + H+(aq) –> CO2(g) + H2O(l)

Excess bicarbonate also neutralizes hydroxide ions, helping the systems to stand up to a change in pH.

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HCO3-(aq) + OH-(aq) –> CO32-(aq) + H2O(l)

Alka-Seltzer now additionally contains aspirin, i m sorry is a weak acid. Exactly how does this affect the buffering capacity?


The solutions deserve to be flushed down the drainpipe with water.


1. B.Z. Shakhashiri; cg-tower.comical Demonstrations: A Handbook for Teachers of cg-tower.comistry; Wisconsin; Volume 3; 1989; p. 186-187 2. N. Friedman; J. Cg-tower.com Education; 52:605 (1975)

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