Some plants, such as red cabbage, are rich in anthocyanins (see pp. 2–6). Anthocyanins are compounds that change colour depending on the pH. We can use this property to good effect, making a natural pH indicator.
You can make your own pH indicator by boiling up some roughly chopped red cabbage in water (do not add anything else to the water). For a concentrated solution, use just enough water to cover the cabbage, but keep checking to make sure that it does not boil dry. Once the leaves have lost all their colour (they will look rather grey), drain off the purple liquid and allow it to cool. If you want to keep your red cabbage indicator to use later, you can store it in a bottle in the fridge, where it will last for several weeks as long as it is not contaminated with anything that might cause it to go mouldy.
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