What is a Buffer Solution?
A buffer solution resists pH change when small amounts of acid or base are added. Buffers contain a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal amounts.
Buffer = weak acid (HA) + conjugate base (A⁻). pH = pKa + log([A⁻]/[HA]). Buffers resist pH change.
- •pH resists change
- •Weak acid + conjugate base
- •Roughly equal amounts
- •Absorbs H⁺ and OH⁻
- •pH changes drastically
- •Complete dissociation
- •No conjugate base
- •Vulnerable to additions
Step-by-step worked examples
An acetate buffer contains 0.1 M CH₃COOH and 0.1 M CH₃COO⁻. Ka = 1.8 × 10⁻⁵. Find pH.
pH = pKa + log([A⁻]/[HA]) pKa = −log(1.8 × 10⁻⁵) = 4.74 log(0.1/0.1) = log(1) = 0 pH = 4.74 + 0 = 4.74
Same buffer. Add 0.01 M HCl. What happens to [A⁻]?
H⁺ + A⁻ → HA (OH⁻ from HCl consumed) [A⁻] decreases slightly; [HA] increases slightly Buffer capacity resists large pH change
Phosphate buffer at pH 7.4 (biologically important). If pKa₂ = 7.21, find [HPO₄²⁻]/[H₂PO₄⁻].
pH = pKa + log([A⁻]/[HA]) 7.4 = 7.21 + log(ratio) log(ratio) = 0.19, ratio = 10^0.19 ≈ 1.55 [HPO₄²⁻]/[H₂PO₄⁻] ≈ 1.55
Flashcards
Quick quiz
Q1.Buffer main purpose?
Q2.Best buffer pH range?
Q3.Add small amount of HCl to buffer ⇒ ?
Q4.Buffer with [HA] = 10 × [A⁻], pKa = 5. pH = ?
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Common mistakes
Buffers prevent all pH change. — Correct: Buffers resist large pH change, but small changes occur with addition of H⁺ or OH⁻.
Any acid + base mixture is a buffer. — Correct: Must be weak acid + its conjugate base (or weak base + its conjugate acid).
Buffer capacity is infinite. — Correct: Finite — exceeding buffer capacity causes large pH shift.
pH = pKa always. — Correct: pH = pKa only when [A⁻] = [HA]; otherwise use Henderson-Hasselbalch.
FAQ
What is a buffer solution?
A mixture of weak acid and its conjugate base (or weak base and its conjugate acid) that resists pH changes.
How do buffers work?
Added H⁺ reacts with the conjugate base (A⁻) → HA. Added OH⁻ reacts with the weak acid (HA) → A⁻. Equilibrium shifts slightly, pH changes little.
What is the Henderson-Hasselbalch equation?
pH = pKa + log([A⁻]/[HA]). It relates pH to the ratio of conjugate base to weak acid.
How to prepare a buffer of specific pH?
Choose a weak acid with pKa close to desired pH (within ±1). Mix acid + salt in appropriate ratio using Henderson-Hasselbalch.