Buffer Action

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

Additional...

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