Practical Investigation: Preparing a Buffer and Demonstrating Its Properties
Dot-Point 7: conduct a practical investigation to prepare a buffer and demonstrate its properties (ACSCH080)
Buffers are essential solutions in chemistry and biology, known for their ability to resist significant changes in pH when small amounts of acid or base are added. This practical investigation involves preparing a buffer solution and comparing its pH stability to that of a non-buffered solution.
What is a Buffer?
A buffer is a solution containing a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal concentrations. This equilibrium system minimises pH changes when small amounts of H⁺ or OH⁻ ions are introduced.
Buffer Mechanism:
When acid is added: The increase in H₃O⁺ causes the equilibrium to shift left, consuming A⁻ and neutralising the added acid.
When base is added: OH⁻ reacts with H₃O⁺ to form water, reducing H₃O⁺. The equilibrium shifts to release more H₃O⁺, neutralising the added base.
Practical Procedure: Preparing an Acetic Acid Buffer
Materials Required:
Acetic acid (CH₃COOH) solution (0.1 M)
Sodium acetate (NaCH₃COO) solution (0.1 M)
pH probe
Universal indicator (UI)
Beakers, stirring rod, measuring cylinder
Steps:
Prepare the Buffer Solution
Mix 20 mL of 0.1 M acetic acid with 60 mL of 0.1 M sodium acetate in a clean beaker. Stir thoroughly.
Measure and Calibrate pH
Measure the buffer’s pH using a pH probe. Adjust the pH to 4.0 by adding more acetic acid (to lower pH) or sodium acetate (to raise pH) as needed.
Divide the Buffer Solution
Separate the prepared buffer into three 20 mL portions in clean beakers. Label them:
Buffer Control
Buffer with Acid
Buffer with Base
Prepare a Non-Buffered Solution
Prepare 60 mL of demineralised water adjusted to the same pH as the buffer (e.g., add HCl dropwise to achieve pH 4.0).
Divide the Non-Buffered Solution
Separate the non-buffered solution into three 20 mL portions in clean beakers. Label them:
Non-Buffer Control
Non-Buffer with Acid
Non-Buffer with Base
Add Universal Indicator
Add 2-3 drops of universal indicator to all six beakers and note the initial colour of the solutions.
Add Acid and Base
Add 5 drops of HCl to the “acid” beakers and 5 drops of NaOH to the “base” beakers. Observe and record the final colour and pH of all solutions.
Results
Analysis
The buffer system demonstrated strong resistance to pH changes when acid or base was added. The pH of the buffer remained stable at approximately 4.0, with only minor deviations.
The non-buffered solution experienced significant pH changes. The addition of acid caused the pH to drop to 2.0 (red UI colour), while the addition of base raised the pH to 6.0 (yellow-green UI colour).
Conclusion
This investigation demonstrates the critical role of buffers in resisting pH changes. The acetic acid-sodium acetate buffer system effectively neutralized the effects of added acid and base, maintaining a stable pH. In contrast, the non-buffered system showed significant pH fluctuations under the same conditions. Buffers are essential in both natural and laboratory settings where pH stability is crucial.
