Buffers | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive guide on buffer solutions in chemistry, focusing on their composition, functionality, and applications. Understanding buffers is crucial for various fields, especially in biological systems where pH regulation is essential.

Step 1: Understand What a Buffer Solution Is

• A buffer solution minimizes changes in pH when small amounts of acid or base are added.
• It is composed of:
  • A weak acid (HA) and its conjugate base (A-).
  • Alternatively, a weak base (B) and its conjugate acid (BH+).
• Buffers cannot completely prevent pH changes but can limit them significantly.

Step 2: Explore Acidic Buffers

• Acidic buffers consist of a weak acid and its conjugate base.
• Key components:
  • For example, acetic acid (CH3COOH) and sodium acetate (CH3COONa).
• Functionality:
  • When a small amount of acid is added, the increased [H+] ions are neutralized by the conjugate base, maintaining equilibrium.
  • When a small amount of alkali is added, the conjugate acid reacts with the excess OH-, stabilizing the pH.

Step 3: Learn About Basic Buffers

• Basic buffers are made from a weak base and its conjugate acid.
• Common example: Ammonia (NH3) and ammonium chloride (NH4Cl).
• Functionality:
  • Adding acid uses up NH3, while adding alkali uses up NH4+, both keeping concentrations relatively constant.

Step 4: Identify Naturally Occurring Buffers

• Buffers play a vital role in biological systems, particularly in maintaining blood pH (7.35 - 7.45).
• The carbonic acid-hydrocarbonate ion buffer is crucial for stabilizing blood pH.
• Mechanism:
  • Carbonic acid (H2CO3) can dissociate to release H+ ions or react with them, thus regulating pH.
  • Excess CO2 can be exhaled to manage acidosis.

Step 5: Calculate the pH of a Buffer

• To calculate the pH, determine the concentration of H+ ions.
• Use the dissociation constant (Ka) of the weak acid:
  • Assume complete dissociation of the salt.
  • Apply the formula:

    pH = pKa + log([A-]/[HA])
    

• This formula helps find the pH based on the concentrations of the acid and its conjugate base.

Conclusion

Buffer solutions are essential in maintaining pH levels in various contexts, particularly in biological systems. Acidic and basic buffers work by reacting with added acids or bases to stabilize pH. Understanding how to calculate the pH of buffer solutions is also a vital skill in chemistry. To further your knowledge, consider exploring practical applications of buffers in laboratory settings or biological systems.