- 2072 Asoj · 2 marks
Henderson's equation for basic buffer
Short answer
A basic buffer contains a weak base and its salt. From Kb = [BH⁺][OH⁻]/[B], with [BH⁺]≈[salt] and [B]≈[base], we get pOH = pKb + log([salt]/[base]), and pH = 14 − pOH.
Derive Henderson's equation for basic buffer solution.
A basic buffer contains a weak base (B) and its salt with a strong acid (BH⁺ A⁻), for example NH₄OH + NH₄Cl.
Derivation
For the weak base: B + H₂O ⇌ BH⁺ + OH⁻
Kb = [BH⁺][OH⁻]/[B]
[OH⁻] = Kb [B]/[BH⁺]
The salt is fully ionised and its common ion BH⁺ suppresses the ionisation of the base, so [B] ≈ [base] and [BH⁺] ≈ [salt]:
[OH⁻] = Kb [base]/[salt]
−log[OH⁻] = −log Kb − log([base]/[salt])
pOH = pKb + log([salt]/[base])
This is Henderson's equation for a basic buffer. Since pH + pOH = 14:
pH = 14 − pKb − log([salt]/[base])
✓ Answer checked by Claude Opus 5.5 (high effort)
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