Making a pH indicator from red cabbage.
When it rains in areas where there is a lot of air pollution, the rain will be acidic. This is called acid rain. In this experiment you will be using vegetables to make a liquid which you can use to measure how acidic something is, that is what level of pH it has. The red colour found in certain vegetables changes into other colours according to acidity, so it can be used as a pH indicator.
You will need:
- Red cabbage or beetroot
- Boiling water
- A fine sieve or a coffee filter
- Two large glass bowls
- Small transparent cups
- Things to test, like baking powder, lemon juice, vinegar, grapefruit juice, heart burn medicine and laundry washing powder
- A knife
And last but not least ... a helping hand from an adult.
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Method
Step 1: Preparing pH indicator
Chop the cabbage or beetroot into small pieces until you have about 2 cups of chopped vegetable. Place the pieces in a large glass container and add enough boiling water to cover them. Wait at least ten minutes for the color to seep out into the water, giving them a stir now and then.
You should notice that the water is no longer clear – if you are using cabbage it has turned violet, while if you are using beetroot it has turned red. Now gently tip the contents of the bowl into a sieve, and let it drain into another glass bowl. If you’ve cut the cabbage or beetroot too finely, you’ll need to use a coffee filter to remove the smaller pieces.
Step 2: Testing different items for pH level
First you need to dissolve any powdery ingredients, such as baking powder, washing powder, and heart burn medicine, in water. Put a small amount of baking powder into a cup, some washing powder in another and some heart burn medicine into a third cup. Then add water and stir well until all the powder disappears.
Next pour about two to three centimetres of the red cabbage or beetroot indicator you prepared before into separate cups. Add the different ingredients to the cups and watch closely to see how the colours of your indicator change as you add these test materials.
These colours show different levels of acidity or pH.
The beetroot pH indicator becomes lighter in acids, and darker in alkaline mixtures. On the other hand, the red cabbage pH indicator turns into a variety of colours, from red in strong acids to yellowish green in strong alkaline mixtures.
The following table gives an indication of the pH value according to the colour:
| pH | 1-2 | 3-4 | 5-6 | 7-8 | 9-10 | 11-12 |
|---|---|---|---|---|---|---|
| Color | Pink | Dark Red | Violet | Blue | Blue - Green | Greenish - Yellow |
Solutions with a pH value above 7.0 are alkali, and solutions with a pH value below 7.0 are acidic. Solutions with a pH value of 7.0 are neutral.