Abstract

Objective

Make your own pH indicator and use it to test the pH of various household solutions.

Introduction

A solution is a mixture of a soluble chemical dissolved in water. Think about the difference between salt water and tap water. The salt in the salt water has dissolved and the solution looks clear, but the salt is still there and will taste salty if you taste it. Because solutions are made with water, which is made of hydrogen and water, the hydrogen in the water can make a solution into an acid or a base.

You might think about an acid as something that an evil villain uses to trap a super hero, but actually some very common household solutions are acids. Acids are solutions that will donate hydrogen ions in a solution, and usually taste sour. Some common acids are citrus fruit juices and household vinegar. Bases are solutions that accept hydrogen ions in solution, and usually feel slippery. Bases have many practical uses. "Antacids" like TUMS or Rolaids are used to reduce the acidity in your stomach. Other bases make useful household cleaning products.

How do you tell if something is an acid or a base? You use a chemical called an indicator, which changes in color when it goes from an acidic to basic condition. There are many different types of indicators, some are liquids and some are concentrated on little strips of "litmus" paper. Indicators can be extracted from many different sources, including the pigment of many plants.

Red cabbage contains an indicator pigment molecule called flavin, which is one type of molecule called an anthocyanin. This water-soluble pigment is also found in apple skin, red onion skin, plums, poppies, blueberries, cornflowers, and grapes. Very acidic solutions will turn anthocyanin a red color. Neutral solutions result in a purplish color. Basic solutions appear in greenish-yellow. Therefore, it is possible to determine the pH of a solution based on the color it turns the anthocyanin pigments in red cabbage juice.

In this experiment, you can extract your own cabbage juice indicator and use it to test the pH of different solutions around your home. You might be surprised to find out what common items around your house are acids or bases.

Terms, Concepts, and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!

• chemical
• acid
• base
• pH
• pigment
• indicator

Bibliography

• This web site at Chemistry 4 Kids has a great tutorial on chemistry, matter properties and mixtures. Go check it out:
  Rader, Andrew, 2005. "Rader's Chem4Kids." Andrew Rader Studios. 12/13/05.
  http://www.chem4kids.com/files/matter_intro.html
• Here is a good website about acids and bases, including information about indicators with very nice pictures:
  Carboni, Giorgio, 2004. "Fun Science Gallery: Experiments with Acids and Bases." Pianoro, Italy. 12/13/05.
  http://www.funsci.com/fun3_en/acids/acids.htm
• Cobb, Vikki, 1972. "Science Experiments You Can Eat." Harper Collins, New York, NY. 12/13/05
  http://www.vickicobb.com/scienceyoueat.html

Materials and Equipment

• cabbage
• boiling pot of water
• strainer
• small white Dixie cups
• medicine dropper
• a series of household items to test the pH of:
  • fruit juice: lemon, lime, orange, apple
  • soda pop (dark sodas might be tricky to see)
  • vinegar
  • baking soda solution
  • cleaning products
  • anything you want!

Experimental Procedure

1. Grate a small red cabbage and place the pieces into a glass bowl.
2. Pour boiling water into the bowl to just cover the cabbage.
3. Leave the cabbage mixture steeping until the liquid is room temperature. The liquid should be red in color.
4. Place a strainer over a collection bowl and pour the mixture through the strainer to remove the pieces of cabbage.
5. Now you should have a clear liquid that will either be purple or blue in color. The color of the liquid will change depending upon the pH. Use this table to figure out the pH of the liquid by observing the color:

   pH	Color
   2	Red
   4	Purple
   6	Violet
   8	Blue
   10	Blue-green
   12	Greenish-yellow

6. Set aside your indicator solution, you will use it as your "stock" solution for your experiments.
7. Next you will test various household solutions with your indicator. It will be important to use separate containers for each household solution—you don't want to mix chemicals that don't go well together or contaminate your results. SO use a separate Dixie cup for each solution you want to test.
8. Fill about half of the Dixie cup with your cabbage indicator solution.
9. Add drops of a liquid you want to test until you see the solution change in color.
10. Record the pH and a description of the item in your data table:

    Item	Color	pH
    Lemon Juice
    7-UP
    Add liquids to the table as you test them

Variations

• There are other plants that can be used to make pH indicators as well: red onion, apple skins, blueberries, grape skins, and plums. Experiment with different sources of pigment, to see which produce the best indicators.
• Can you use your pH indicator to conduct other useful experiments? What about testing the acidity of rain on a smoggy day? Or the usefulness of various antacid brands in reducing acidity of a solution?
• You can use an indicator to write secret messages. Just use full strength lemon juice to write an invisible message on paper and let the message dry. To reveal the message, paint indicator over the paper with a paint brush.

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

Sara Agee, Ph.D., Science Buddies

Last edit date: 2006-01-11 14:44:22