Buffer Solution Formula
Solutions that resist pH changes when small amounts of acid or base are added
Henderson-Hasselbalch Equation
pH = pKₐ + log([A⁻]/[HA])
For acidic buffers
pOH = pKբ + log([BH⁺]/[B])
For basic buffers
Buffer Components
Acidic Buffer
Weak acid + conjugate base (salt)
Example: CH₃COOH + CH₃COONa
Basic Buffer
Weak base + conjugate acid (salt)
Example: NH₃ + NH₄Cl
Buffer Capacity Formula
Higher capacity = better resistance to pH change
Examples
Example 1: Acetate Buffer pH
Given: 0.10 M CH₃COOH and 0.15 M CH₃COONa (pKₐ = 4.76)
Solution:
pH = pKₐ + log([A⁻]/[HA])
pH = 4.76 + log(0.15/0.10)
pH = 4.76 + log(1.5)
pH = 4.76 + 0.18
pH = 4.94
Example 2: Ammonia Buffer
Given: 0.20 M NH₃ and 0.20 M NH₄Cl (pKբ = 4.75)
Solution:
pOH = pKբ + log([BH⁺]/[B])
pOH = 4.75 + log(0.20/0.20)
pOH = 4.75 + 0 = 4.75
pH = 14 - 4.75
pH = 9.25
Buffer Effectiveness
✓ Best When [A⁻]/[HA] = 1
pH = pKₐ (maximum buffer capacity)
✓ Effective Range: pH = pKₐ ± 1
Ratio of [A⁻]/[HA] between 10:1 and 1:10
⚠️ Higher Concentrations = Higher Capacity
More concentrated buffers resist pH change better