Making an acid–alkali indicator

Making an acid–alkali indicator

We are learning how to:

• make an acid–alkali indicator.

You should already be familiar with some acid–alkali indicators from Units 10.2 and 10.3. Some indicators are obtained from organic material.

Litmus is extracted from lichen. This is a rather unusual organism in which algae and fungi co-exist for mutual benefit. There are many organic materials that contain coloured chemicals which might provide acid–alkali indicators. For a chemical to be any use as an indicator it must be different colours in acids, and in alkalis.

Organic materials that contain coloured chemicals that may be useful as indicators include the petals of flowers like hibiscus, the leaves of vegetables like red cabbage and the roots of vegetables like beetroot.

Whatever the source, the coloured chemical must be extracted by physically crushing the organic material to break down the cells and then extracting it with a suitable solvent. Many of the chemicals responsible for colour are soluble in water but are more soluble in a solvent like ethanol. When extracting, it is important not to use too much solvent or the extract will be very dilute and colours will be difficult to see.

Activity 10.6.1

Testing for acids and alkalis

Here is what you need:

• Chopped up coloured plant material

• Mortar and pestle

• Beaker

• Ethanol

• Stirring rod

• Test tubes × 3

• Test tube rack