Buffers

Rucete ✏ Chemistry In a Nutshell

1. What Is a Buffer?

• A buffer is a solution that resists changes in pH when small amounts of acid or base are added.
• It consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

2. How Buffers Work

• If an acid (H⁺) is added: the conjugate base neutralizes it.
• If a base (OH⁻) is added: the weak acid neutralizes it.
• This helps maintain a stable pH.

3. Common Examples

• Acetic acid (CH₃COOH) + Sodium acetate (CH₃COONa) → buffer with pKa ≈ 4.75
• Ammonia (NH₃) + Ammonium chloride (NH₄Cl) → basic buffer

4. Henderson–Hasselbalch Equation

Used to calculate the pH of a buffer:

$$\text{pH}=\text{pKa}+\log \left(\frac{[{\text{A}}^{-}]}{[\text{HA}]}\right)$$

• [A⁻]: concentration of conjugate base
• [HA]: concentration of weak acid

For basic buffers:

$$\text{pOH}=\text{pKb}+\log \left(\frac{[{\text{BH}}^{+}]}{[\text{B}]}\right)$$

5. Buffer Capacity

• The effectiveness of a buffer depends on the concentration of the acid and base.
• Best buffering occurs when:

[A⁻] ≈ [HA]

→ so pH ≈ pKa