Making Natural Acid-Base Indicators in the Science Classroom

Fig.2 - Red Cabbage Natural Indicator

Acids are substances that are sour to the taste and corrosive. Weak acids found around the home include lemon juice, vinegar and soft drink, while car battery acid and rust remover (phosphoric acid) are of a much stronger acidity. All acids are contain hydrogen ions (positively charged hydrogen atoms), and act to donate these ions in reactions.

Bases act in the opposite way to acids and can accept hydrogen ions in reactions. Bases are slippery to the touch, have a bitter taste and can also be corrosive in some cases. Common bases found around the house include soaps, detergents, toothpaste, baking powder and drain cleaner. Most bases are the oxides or hydroxides of metals – examples include sodium hydroxide and magnesium oxide. Metal carbonates such as sodium carbonate may also behave as bases.

Anthocyanin – A Natural Indicator Found in Many Plants

Interestingly, the juices of plants containing anthocyanin pigments have been found to change colour in the presence of acids or bases. Red cabbages, grapes, blackcurrants, beetroot, eggplant and turnips are included in this group. Solutions of these plant materials turn pink when acid is added to them and blue/purple when in the presence of bases.

This is because the anthocyanin molecule has both acidic and basic properties and can itself donate or accept hydrogen ions (see figure 1). When it accepts a hydrogen ion it turns pink and when it loses a hydrogen ion it turns blue .A whole spectrum of colours within this range can be achieved depending upon the strength of the added acid or base (see figure 2).

Making a Natural Indicator – Materials and Teaching Method

This indicator can be prepared in a science laboratory or, alternatively, in a home kitchen using a saucepan, hotplate and several glasses. Many of the substances that are tested are readily available in most homes.

The following materials and equipment are required per group of around 4 students:

•                one small handful of chopped red cabbage or beetroot

•                beaker or small saucepan.

•                hotplate

•                enough water to cover chopped plant material

•                7 test tubes or small glasses

•                1 teaspoon of each of lemon juice, vinegar, lemonade, detergent, shampoo, baking powder and liquid soap

•                eyedropper

Students should be instructed to copy down the following directions:

Fig.1 - Anthocyanin Equilibrium

•                Add about 10 ml of each of the household products listed above to the 7 test tubes.

•                Place the chopped cabbage or beetroot in a saucepan or beaker and cover with water.

•                Boil the mixture for 5 minutes, then strain the resulting purple liquid into a beaker or cup.

•                Add a few drops of the purple solution to each of the household products in the test tubes.

•                Record the colour change that occurs in each of the 7 solutions.

A suitable results table could have the following headings at the top of each column: 'Solution Tested', 'Colour in Red Cabbage/Beetroot Indicator' and 'Is the Solution an Acid or a Base?'. Students should complete this table as they carry out their tests.

The following questions could be written on the board after students write up their experiment;

1.             Name the pigment present in cabbages and beetroots that acts as an acid-base indicator.

2.             Which of the household solutions tested was the strongest acid? Explain your answer.

3.             Which of the household solutions tested was the strongest base? Explain your answer.

4.             Name some other fruits and vegetables that contain anthocyanins.

Natural Indicators Follow -Up Activities

The double arrows in figure 1 indicate that in aqueous solution, anthocyanin exists in an equilibrium between its corresponding (conjugate) acid and base, as indicated below:

HIn = H+ + In-

More capable students could research how this equilibrium shifts from one side to the other, and thus from one colour to another, depending on the hydrogen ion concentration of the solution. Students could also experiment with other natural indicators such as tea, turmeric powder, the petals of petunia, impatiens, primrose, rose, marigold and hydrangea flowers, grapes and blackberries.

References

Carboni, G. "Experiments With Acids and Bases." Funscience,funsci.com, 2004.

Sonawane, L.V. "Natural pH Indicators." Pharmainfo, pharmainfo.net, 2007.

Withers, G. "Natural Indicators: How Do They Work?" cmu.edu, 2001.