Making your own pH indicator is relatively simple. Many berries and vegetables such as beetroot, onion skins and blackcurrants can be used to make a crude indicator which should work well enough to be able to test the pH of a range of household acids and alkalis.
Equipment typically required (per set):
Students may be required to choose a single fruit or vegetable from the list and make an indicator that can be compared to another from the list. Some work better than others. Red cabbage is particularly good and should be selected if only one type is to be made.
- 250ml beaker
- Bunsen burner
- Heat resistant mat
- Range of household acids and alkalis to test such as vinegar, alka seltzer etc.
- Stirring rod
- One of the following: beetroot, blackcurrants, red cabbage, plum, red grapes, red radish.
Students boil a small amount of deionised (distilled) water in the beaker and add small pieces of the fruit or veg. This is stirred occasionally and after around 5 minutes the water should take on a slight colouration. The liquid is left to cool and then used in the same way as you would use conventional pH indicator.
Certain fruit and vegetables work well as pH indicators because they contain substances called anthocyanins. These are present in the kinds of leaves which go red or purple in the autumn when the acidic sap causes them to act as a natural pH indicator.
The spice Tumeric can also be used as a crude indicator when mixed with alcohol. It will turn red in the presence of acid and yellow when an alkali is added. Some grape and cranberry juices can be used straight from the carton as a simple indicator.
Remember that the colours created by these indicators when mixed with an acid or alkali may not be the same as conventional pH indicators. Students may need to produce their own pH colour chart to show the range of colours produced by a particular solution.
pH indicator paper can be created by adding a small amount of the solution to a strip of blotting or filter paper. This can then be dipped into an acid or alkali solution.