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Buffer Action



Acids & Bases

AQA Content

Explain qualitatively the action of acidic and basic buffers
Calculate the pH of acidic buffer solutions.

Specification Notes

A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.

Acidic buffer solutions contain a weak acid and the salt of that weak acid.
Basic buffer solutions contain a weak base and the salt of that weak base.

Applications of buffer solutions.


Buffer solutions:
• Maintain a stable pH when small amounts of acid or base are added
• Consist of a weak acid and its conjugate base or weak base and its conjugate acid
• Can be prepared by mixing a weak acid/base with its salt

Example: Ethanoic acid and sodium ethanoate
Ethanoic acid is a weak acid and sodium ethanoate is a salt that fully ionizes in solution
The buffer solution contains relatively high concentrations of CH3COOH (from ethanoic acid) and CH3COO- (from sodium ethanoate)

Reserve supplies of acid and conjugate base neutralize added H+ and OH- ions, maintaining a stable pH.
Buffers maintain the pH of a solution by neutralizing added H+ and OH- ions.
The pH of a buffer solution is determined by the pKa of the weak acid or weak base used to prepare the buffer. Therefore, it is important to choose an acid or base with a pKa value close to the desired pH when preparing a buffer solution.

Real-life examples of buffers include vinegar, which is a solution of acetic acid (CH3COOH) and its conjugate base (acetate ion, CH3COO-), and citric acid, which is found in lemons and other citrus fruits.

Buffers have a limited capacity and will not be able to neutralize large amounts of added acid or base. Once the buffer is exhausted, the pH of the solution will change rapidly.

Useful Formulae

Calculating the pH of a Buffer

We will need the expression for the acid dissociation


The 0.01 mol of NaOH will produce 0.01 mol of propionate while removing 0.01 mol of propanoic acid. Working with molarity, we can see that the concentration of NaOH is 0.02


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