How does a buffer resist pH change
A buffer resists pH change because it contains a weak acid and its conjugate base (or a weak base and its conjugate acid) that can neutralize added H⁺ or OH⁻, keeping the hydrogen‑ion concentration nearly constant. When strong acid is added, the base component consumes H⁺ forming more weak acid; when strong base is added, the acid component donates H⁺ forming more conjugate base.
Chemistry · Acids and bases
A buffer solution contains a weak acid (HA) and its conjugate base (A⁻) in comparable amounts. The pair can interconvert: HA ⇌ H⁺ + A⁻. Because both species are present, the solution can absorb added H⁺ or OH⁻ without large shifts in [H⁺].
Buffer Capacity
Key factors that determine how much acid or base a buffer can neutralize:
- Concentration of the acid and base components
- Ratio of [HA] to [A⁻]
- The pKa of the weak acid
How a buffer neutralizes an added strong acid:
- 1Strong acid supplies H⁺
- 2A⁻ (the conjugate base) accepts H⁺ forming HA
- 3[H⁺] rises only slightly
How a buffer neutralizes an added strong base:
- 1Strong base supplies OH⁻
- 2HA donates H⁺ to OH⁻ forming water and A⁻
- 3[H⁺] falls only slightly
Common buffer pairs and their pKa values (effective within ±1 pH unit):
| Buffer pair | pKa |
|---|---|
| Acetic acid / acetate | 4.76 |
| Phosphate (H₂PO₄⁻/HPO₄²⁻) | 7.20 |
| Ammonium / ammonia | 9.25 |
Check yourself
What species reacts with added H⁺ in a buffer consisting of acetic acid and sodium acetate?
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