How Buffers Resist pH Changes

The Mechanism in Detail

In the previous lesson you learned the equations describing buffer action. Now let us examine the mechanism more deeply, including the role of Le Chatelier's principle, buffer capacity, and real-world applications.

An acidic buffer contains a weak acid HA and its conjugate base A⁻ in equilibrium:

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

The key to buffer action is that both HA and A⁻ are present in large, comparable quantities. They act as reservoirs that can absorb added acid or base without significant changes in pH.

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Adding Acid to a Buffer: Le Chatelier's Explanation

When a small amount of strong acid (H⁺) is added to the buffer:

1. The added H⁺ ions increase [H⁺] momentarily
2. The conjugate base A⁻ reacts with the added H⁺: H⁺ + A⁻ → HA
3. This removes the added H⁺ from solution
4. [HA] increases slightly while [A⁻] decreases slightly
5. The ratio [HA]/[A⁻] changes only marginally because both are present in large amounts
6. pH decreases only very slightly