Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or base are added, or when they are diluted.

There are two types of buffer, acidic and basic.

Let’s look at the reactions that can take place in an acidic buffer made from ethanoic acid and sodium ethanoate.

This reaction happens when hydroxide ions from an alkali are added to the buffer.

Let’s look at the reactions that can take place in a basic buffer made from ammonia and ammonium chloride.

The standard entropy change for the decomposition of calcium carbonate is positive.

This reaction happens when hydrogen ions from an acid are added to the buffer.

This reaction happens when hydroxide ions from an alkali are added to the buffer.

This expression shows the relationship between the acid dissociation constant, and the concentrations of the hydrogen ions, the acid and its salt.

The acid dissociation constant gives a rough guide to the pH of the buffer.

The ratio of the concentrations of acid and its salt provides a 'fine tuning' of a buffer's pH. For an effective buffer, this ratio must not be too large or too small.

We can use this expression to calculate the pH of a buffer if the acid dissociation constant and concentrations of acid and salt are known.

The expression can be rearranged to find the hydrogen ion concentration.

Let's use this expression to work out the pH of an acidic buffer containing ethanoic acid and sodium ethanoate.

The K a value for ethanoic acid is 1.7 x 10 –5 mol dm –3 at 298 K.

In our example, the concentration of ethanoic acid is 0.1 mol dm –3.

In our example, the concentration of sodium ethanoate is 0.2 mol dm –3.

This means that the hydrogen ion concentration is 8.5 x 10 –6 mol dm –3.

Using pH = –log 10 [H + (aq)], we find that the pH of this buffer would be 5.07 at 298 K.