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25.1.5—Buffer solution

Syllabus
9701–2028–2029
Objective
25.1.5
Level
A2

Buffers consume small additions of acid or base

A buffer solution resists a change in pH when a small amount of acid or base is added. It contains appreciable amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Buffer type How to make it
weak acid/conjugate base mix the weak acid with a soluble salt of its conjugate base, or partially neutralise the acid with strong alkali
weak base/conjugate acid mix the weak base with a soluble salt of its conjugate acid, or partially neutralise the base with strong acid

CHX3COOX−(aq)+HX+(aq)→CHX3COOH(aq)\ce{CH3COO-(aq) + H+(aq) -> CH3COOH(aq)}

CHX3COOH(aq)+OHX−(aq)→CHX3COOX−(aq)+HX2O(l)\ce{CH3COOH(aq) + OH-(aq) -> CH3COO-(aq) + H2O(l)}

In blood, the H₂CO₃/HCO₃⁻ pair moderates pH: HCO₃⁻ consumes added H⁺ to form H₂CO₃, while H₂CO₃ consumes added OH⁻ to form HCO₃⁻ and water. Maintaining a narrow pH range supports pH-sensitive biological processes.

A buffer limits rather than prevents pH change. Its capacity is finite and requires both conjugate components; a neutral salt alone is not necessarily a buffer.

ConceptA-Level CAIE Chemistry A2