Yellow paper dipped in an acid solution (such as vinegar) does not change colour appreciably. The same paper dipped in an alkaline (basic) solution (such as sodium carbonate, soda ash or washing soda) turns a deep red colour.
- Water or alcohol (such as surgical spirit)
- Absorbent paper (blotting paper is ideal, but kitchen towel is also good)
- Acids and alkalis to test
First extract the colour from turmeric powder by adding it to some water or better some strong alcohol which is also clear (e.g. surgical spirits). The alcoholic extract can be mixed with water if required. This should then be applied to absorbent paper. Blotting paper was used here but any strong absorbent paper (e.g. kitchen towel but not toilet paper) should be appropriate. Allow the yellow paper to dry. It will now turn a bright red on application of an alkali but only a small colour change will be observed on the addition of acid.
The same colour change may be carried out in solution. In this case it is best to make the solution alkaline when making up the indicator.
Any coloured compound whose colour depends on the acidity of the solution it is in may be used as an indicator. Many organic compounds change their structures slightly as the acidity of the solution changes. If they are coloured this structure change can cause the colour to change. The molecule which is primarily responsible for the colour change here is circumin. You can see what it looks like and manipulate the molecule here.
This can make a “do it yourself” water into wine experiment where red “wine” can be turned into white “wine”.
These solutions stain everything they touch a bright yellow. It is best to work in disposable containers or to use glass or metal if possible. Cover worktops with disposable sheets of plastic film to avoid stains from splashes.
Prolonged contact with alcohol (and other solvents) can irritate the skin. It is a good idea to use gloves anyway because of the potential of turmeric to stain.