Composition and Action of Buffer Solutions
A buffer solution resists changes in pH when small amounts of acid (\(\text{H}^+\)) or base (\(\text{OH}^-\)) are added. It consists of a weak acid and its conjugate base (acidic buffer) or a weak base and its conjugate acid (basic buffer) 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
Response to Added Acid (H⁺)
When \(\text{H}^+\) is added, the large reservoir of conjugate base (\(\text{A}^-\)) consumes the added protons:
\[\text{A}^-(\text{aq}) + \text{H}^+(\text{aq}) \rightarrow \text{HA}(\text{aq})\]Equilibrium shifts left to absorb the excess protons, keeping \([\text{H}^+]\) virtually unchanged.
Response to Added Base (OH⁻)
When \(\text{OH}^-\) is added, the weak acid (\(\text{HA}\)) donates a proton to neutralise the added hydroxide ions:
\[\text{HA}(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{A}^-(\text{aq}) + \text{H}_2\text{O}(\text{l})\]Equilibrium shifts right to replace reacted protons, maintaining a stable pH.
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.
Ethanoate Buffer System in Action
Example: In an ethanoic acid / sodium ethanoate buffer (\(\text{CH}_3\text{COOH} / \text{CH}_3\text{COONa}\)):
- Add \(\text{HCl}\): \(\text{CH}_3\text{COO}^- + \text{H}^+ \rightarrow \text{CH}_3\text{COOH}\)
- Add \(\text{NaOH}\): \(\text{CH}_3\text{COOH} + \text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{H}_2\text{O}\)
Buffer Capacity vs Dilution
Buffer Capacity: A buffer works best when \([\text{HA}] = [\text{A}^-]\) (\(\text{pH} = \text{p}K_{\text{a}}\)). Dilution does not change the pH of a buffer (since the ratio \([\text{A}^-]/[\text{HA}]\) remains constant), but it does reduce buffer capacity.
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