Buffer Solutions
- A buffer resists pH changes when small amounts of acid or base are added
- Made from a weak acid + its conjugate base (or weak base + conjugate acid)
- Henderson-Hasselbalch: pH = pKₐ + log([A⁻]/[HA])
- Added H⁺ reacts with A⁻ → HA (base component absorbs acid)
- Added OH⁻ reacts with HA → A⁻ (acid component absorbs base)
Buffer Capacity
- Buffer capacity = amount of acid/base a buffer can absorb before pH changes
significantly
- Higher capacity when: higher concentration of buffer components
- Maximum capacity when [HA] = [A⁻] (ratio = 1:1, pH = pKₐ)
- Buffer breaks when one component is fully consumed
- Effective range: pKₐ ± 1 (pH where buffer still works)
IB HL Exam Tips
- Identifying a buffer: look for weak acid + salt of conjugate base (or vice
versa)
- Calculating pH: use Henderson-Hasselbalch directly — pH = pKₐ + log([A⁻]/[HA])
- After adding acid: [HA] increases, [A⁻] decreases → ratio decreases → pH
decreases (but only slightly)
- Blood buffer: H₂CO₃ / HCO₃⁻ system maintains blood pH at 7.4
- Common mistake: using strong acid/base to make a buffer — only WEAK acid
components work