Setting up the investigation
Blocks of agar containing a pH indicator allow diffusion of an acid or alkali to be followed through a visible colour change.
For an acid-diffusion setup, the agar can contain universal indicator together with very dilute sodium hydroxide, giving purple agar. As dilute hydrochloric acid moves into the block, its effect on pH changes the indicator colour. Agar made with universal indicator alone can also be used.
To compare different SA:V ratios, cut cubes with different dimensions, for example side lengths of 0.5 cm, 1 cm and 2 cm, and determine the SA:V of each before beginning the diffusion stage.
- Prepare indicator agar cubes of several sizes.
- Work out and record the SA:V ratio for each size.
- Immerse each block in dilute hydrochloric acid.
- Use the indicator colour change to quantify diffusion.
When SA:V is the factor being tested, keep both the volume of hydrochloric acid in each tube and its concentration constant.
Recording diffusion
The change in indicator colour can provide a quantitative result in either of two ways:
- measure the interval from immersion until the block has changed colour throughout, or
- allow diffusion to proceed for a fixed period and then measure how far the changed-colour region extends into the agar.