📘 IB Understanding
A buffer solution resists changes in pH when small amounts of acid or base are added. It contains a conjugate acid-base pair in comparable concentrations.
Types of Buffer
| Type | Components | pH Range | Example |
|---|---|---|---|
| Acidic buffer | Weak acid + conjugate base | < 7 | CH3COOH + CH3COONa |
| Basic buffer | Weak base + conjugate acid | > 7 | NH3 + NH4Cl |
How Buffers Work
When H+ is Added (Acid)
The conjugate base absorbs the added H+:
A-(aq) + H+(aq) → HA(aq)
When OH- is Added (Base)
The weak acid neutralises the added OH-:
HA(aq) + OH-(aq) → A-(aq) + H2O(l)
Real-World Application: Blood Buffer
Human blood is buffered at approximately pH 7.4 using the carbonic acid-bicarbonate system:
H2CO3(aq) ⇌ H+(aq) + HCO3-(aq)
This prevents fatal pH changes when CO2 (an acidic gas) enters the bloodstream during respiration.
Worked Example
Q: Explain how a CH3COOH / CH3COONa buffer resists pH change when HCl is added.
A: The buffer contains a high concentration of CH3COO- (conjugate base). When H+ from HCl is added, the ethanoate ions react:
CH3COO-(aq) + H+(aq) → CH3COOH(aq)
The added H+ is consumed, so the pH does not drop significantly.
⚠️ Exam Tip
Buffers have a limited buffer capacity. If too much acid or base is added, the buffer components are consumed and the pH will change dramatically. Diluting a buffer does not change its pH, but reduces its capacity.
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