Buffer solutions are specialised solutions that resist changes in pH even when small amounts of acid or base are added, or when the solution is diluted. In both industrial and biological contexts, the ability to maintain a steady pH is essential for various processes.

Composition of Acidic and Basic Buffers

At their core, buffer solutions contain a balance of acidic and basic species. This ensures that any added acids or bases are neutralised, preventing significant pH changes.

Acidic Buffers

• Definition: Solutions made of a weak acid and its conjugate base.
• Components:
  • Weak Acid: Does not fully ionise in water.
    • Example: Acetic acid (CH₃COOH) is a weak acid that partially dissociates in water.
  • Salt of the Weak Acid with a Strong Base: Provides the conjugate base for the weak acid.
    • Example: Sodium acetate (CH₃COONa) dissociates completely, giving acetate ions (CH₃COO⁻) in solution.

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Basic Buffers

• Definition: Solutions made of a weak base and its conjugate acid.
• Components:
  • Weak Base: Partially accepts protons in water.
    • Example: Ammonia (NH₃) is a weak base that doesn't fully react with water to form ammonium ions.
  • Salt of the Weak Base with a Strong Acid: Supplies the conjugate acid for the weak base.
    • Example: Ammonium chloride (NH₄Cl) provides ammonium ions (NH₄⁺) when dissolved.

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Mechanism of Buffer Action

Buffer solutions owe their resistance to pH change to their unique composition. Their ability to counteract added acids or bases is crucial for their function.

In Acidic Buffers

• 1. On Adding a Base: The weak acid in the buffer donates a proton (H⁺) to the added base. This neutralises the base. Although the concentration of the weak acid decreases, its conjugate base increases due to the donation of the proton. This compensatory mechanism helps maintain the pH.
• 2. On Adding an Acid: The conjugate base from the salt captures the proton from the added acid. This action neutralises the acid. The conjugate base's concentration goes down while the weak acid concentration goes up, stabilising the pH.

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In Basic Buffers

• 1. On Adding an Acid: The weak base present in the buffer captures the proton from the acid, neutralising it. Though the weak base's concentration decreases, its conjugate acid rises, preserving the pH.
• 2. On Adding a Base: The conjugate acid (from the salt) in the buffer donates a proton to the added base. This neutralises the base, and while the conjugate acid's concentration drops, the weak base concentration surges, holding the pH steady.

Image courtesy of Essential Pharma Documents.

Solving Buffer Problems Using the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a vital tool that connects the pH of a buffer to the concentrations of its components and the pKa of the weak acid:

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

Where:

• "pH" stands for the solution's pH.
• "pKa" is the negative logarithm of the acid dissociation constant, Ka.
• [A^-] and [HA] are the concentrations of the conjugate base and weak acid, respectively.

Procedure to Solve Buffer Problems:

• 1. Identify Components: Determine the weak acid/base and its conjugate in the buffer.
• 2. Determine Concentrations: Use given data or stoichiometric calculations.
• 3. Use Henderson-Hasselbalch Equation: Insert values to calculate the pH or rearrange to find other variables.

Effect of Dilution on Buffer Solutions

Diluting a buffer solution impacts both its components, affecting its capacity to maintain pH.

• Concentration Effects: Upon dilution, both the weak acid/base and its conjugate's concentrations reduce. However, their ratio remains nearly the same.
• pH Stability: The pH remains largely stable after dilution since the ratio of the two components doesn't change significantly.
• Buffer Capacity: Although the pH remains steady, the buffer's ability to resist large changes in pH (its capacity) diminishes. Thus, while a diluted buffer still resists pH changes, it does so less effectively than a concentrated one.

Buffers, whether in labs, industries, or our bodies, ensure that processes reliant on stable pH environments function optimally. Their mastery in chemistry thus offers a crucial lens to understand myriad processes that hinge on pH stability.