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Buffered Solutions

Summary

The buffering capacity of two solutions are compared with small additions of acid and base.

Chemicals and Solutions

  • Buffered solution at pH 7
  • Unbuffered HCl solution at pH 7
  • Universal indicator
  • Universal indicator key
  • 0.1 M HCl
  • 0.1 M NaOH

Materials

4 standard hydrometer cylinders

Procedure

  1. Fill 2 cylinders half full of unbuffered solution.
  2. Fill the remaining 2 cylinders half full of buffer solution.
  3. Arrange solutions as two pairs of unbuffered and buffered solution.
  4. Use small additions of HCl to show buffering capacity of buffered solution vs. unbuffered water. Do the same using NaOH.

Disposal

After neutralization solutions can be poured down the drain. Demo usually generates 300 mL neutralized 2.5 x 10-5 M HCl and 300 mL 0.5M sodium acetate/acetic acid buffer.

Method 2

Chemicals and Solutions

  • Buffered solution at pH 4
  • Unbuffered HCl solution at pH 4
  • Methyl purple indicator, dropper bottle
  • Thymol blue indicator
  • Tap water (NOT deionized water)
  • 0.1 M HCl
  • 0.1 M NaOH

Materials

6 standard hydrometer cylinders

Procedure

  1. Fill 3 cylinders half full of unbuffered solution.
  2. Fill the remaining 3 cylinders half full of buffer solution.
  3. Arrange solutions as three pairs of unbuffered and buffered solution.
  4. Add an indicator and dilute with a mixer according to the table below. Observe the color changes between the buffer solution and the unbuffered HCl solution when each mixer is added. (As an alternative, universal indicator solution can be used instead of methyl purple and thymol blue for all six cylinders)
Solution Indicator Mixer Color Change
pH 4 buffer methyl purple tap water no change
pH 4 HCl methyl purple tap water green
pH 4 buffer thymol blue 0.1 M HCl no change
pH 4 HCl thymol blue 0.1 M HCl pink
pH 4 buffer thymol blue 0.1 M NaOH no change
pH 4 HCl thymol blue 0.1 M NaOH blue

After neutralization solutions can be poured down the drain. Demo usually generates 300 mL neutralized 2.5 x 10-5 M HCl and 300 mL 0.5M sodium acetate/acetic acid buffer.

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