How to Demonstrate a Buffer Solution
The pH of a solution represents the amount of acid or base the solution contains. In general, a pH of 7 represents a neutral solution with neither acidic nor basic character. A pH less than 7 indicates an acidic solution and pH greater than 7 indicates a basic solution. Buffer solutions resist changes in pH. Pure water, for example, exhibits a pH of 7. The addition of small amount of strong acid or base to pure water will drastically change its pH. The addition of the same amount of acid or base to a buffer solution, however, will result in only a minimal change in pH. Science instructors can easily demonstrate buffer solutions with the aid of pH meters or colorimetric pH indicators.
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Things You'll Need
- Distilled water
- 250-mL beakers, 2
- pH 7 buffer solution
- 2 pH meters with probes
- 10-mL graduated cylinder
- 6 M hydrochloric acid (HCl)
Instructions
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1
Pour about 200 mL of distilled water into a 250-mL beaker. Label the beaker "water." Pour 200 mL of pH 7 buffer solution into a second 250-mL beaker labeled "pH 7 buffer."
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2
Calibrate two pH meters according to the manufacturer's instructions. Insert one pH probe into each beaker and turn the meters so that anyone observing the demonstrations can see the pH meters' readings. Both meters should give pH readings of about 7.
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3
Measure 1 mL of 6 M hydrochloric acid (HCl), using a 10-mL graduated cylinder. Pour the HCl into the beaker containing the distilled water. Use the pH probe to gently swirl the water. Its pH should rapidly decrease to less than 2.0, indicating a strongly acidic solution.
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4
Measure out another 1 mL of 6 M HCl and add it to the beaker containing the buffer solution. Swirl the buffer with the pH probe. The pH should decrease only a few tenths of a pH unit.
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5
Explain to any observers that the small decrease in pH occurred because of the presence of the buffer. Emphasize that both solutions contained the same volume of liquid and the same amount of acid was added to each solution. To help emphasize the point, add additional hydrochloric acid to the buffer solution while continuing to monitor its pH.
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Tips & Warnings
If you do not have 6 M hydrochloric acid, you can prepare it by adding concentrated hydrochloric acid to distilled water in a 1:1 ratio. Make certain, however, that you add the acid to the water, and not the water to the acid, to avoid splattering.
If the distilled water does not give an initial pH of 7, then it has probably absorbed carbon dioxide from the atmosphere. Boiling the distilled water and allowing it to cool should eliminate the carbon dioxide and return the pH to 7.
Always wear appropriate eye and hand protection when handling acids and/or bases.
