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Acid or Base? Make your own pH-indicator!

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A great play of colors - pH indicators

Did you know that chemical substances such as acids and bases are not only found in the laboratory? For example, lemon contains acid and the baking powder in our cake consists of bases.

It is interesting to find out which substance is an acid and which is a base, but how do you do it? One way is to taste them. Acids often taste sour, bases often taste bitter. However, we cannot taste many substances because they are harmful to us. So what other options do we have to determine acids and bases?

 

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Red cabbage as a pH indicator: how does it work?

There are so-called “pH indicators” that change color when they come into contact with an acid or base. One example is red cabbage. Depending on the color of the red cabbage, you can determine whether it has grown on acidic or alkaline soil. This is because red cabbage juice contains dyes, known as anthocyanins, which change color when they come into contact with acids and bases.

 

Now it's getting colorful!

We produce our own pH indicator to determine acids and bases ourselves.

 

You will need:

  • Pot
  • Glas container (or a second pot)
  • Sieve
  • Bleached coffee filters
  • Scissors
  • Knife
  • Tweezers
  • Pipette, Brush or teaspoon to distribute liquids
  • Baking paper or aluminum foil
  • Plastic gloves
  • Ca. 500 ml water
  • Ca. 200 g red cabbage

Step by Step:

Cut the red cabbage into small pieces. Heat water until it boils. Wear gloves when cutting and during the next steps, as red cabbage stains badly. It is best to have an adult help you, as we are using a knife and boiling water. Experiments are even more fun together!
Add the red cabbage to the boiling water. Leave to cook for 10 minutes, stirring occasionally. The water will turn purple.
Leave the red cabbage juice to cool. Pour the juice through a sieve into a glass container. Tip: If you have too much red cabbage juice, you can freeze the rest.
Now cut a coffee filter into small pieces. Dip the shreds into the red cabbage juice. Place the shreds on baking paper to dry.
When your pH paper is dry (approx. 3 hours or more), it is ready to use. Immerse it in various liquids or drip the liquids onto the pH paper using a pipette, brush or spoon.
Test different liquids in your household, e.g. water or lemon juice. Observe what color your pH paper is afterwards. Can you draw patterns or even a picture with different liquids?

Explore more!

  • How does your pH paper change color when you drip acidic substances onto it? How does your pH paper change color when you drip alkaline substances onto it?
  • Document your results and write down what you observe or take photos to compare your findings.
  • You can also change the experiment slightly by trying out other types of paper that might work even better as coffee filters.
  • Alternatively, the red cabbage juice can also be used directly. To do this, pour a small amount into a glass container, for example, and drip small amounts of the liquid to be tested into it using a brush or a teaspoon.

Did you know?

Many ants, e.g. wood ants, use formic acid to keep their nest clean. It works like a powerful disinfectant and is very effective against bacteria and fungi.

Background knowledge

In the laboratory, pH indicators are an important part of everyday work, as they can be used to quickly determine important properties of liquids, e.g. whether you are dealing with an acid. The Cremer group at ISTA is researching how ants defend themselves against infectious diseases. Various pH indicators are often used to find out, for example, whether ants are cleaning the nest with formic acid.

The picture shows a queen ant that has just sprayed formic acid to thoroughly clean her new home. The blue pH paper turns yellow in the process.