Buffer Solutions

What Is a Buffer Solution?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added to it. Buffers do not prevent pH changes entirely — they minimise them.

Buffer solutions are essential in biology (blood pH must stay between 7.35 and 7.45), in industrial processes, and throughout analytical chemistry. At A-Level, you need to understand how they work and how to calculate their pH.

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Types of Buffer

There are two types:

Acidic buffer (pH < 7): Made from a weak acid and its conjugate base (usually as a salt). For example:

• Ethanoic acid (CH₃COOH) + sodium ethanoate (CH₃COONa)
• The solution contains both the undissociated weak acid HA and the conjugate base A⁻

Basic buffer (pH > 7): Made from a weak base and its conjugate acid (usually as a salt). For example:

• Ammonia (NH₃) + ammonium chloride (NH₄Cl)

Most Edexcel exam questions focus on acidic buffers.

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How an Acidic Buffer Works

An acidic buffer contains a large reservoir of:

• Weak acid molecules, HA — ready to donate protons
• Conjugate base ions, A⁻ — ready to accept protons