Testing the pH of a solution is a fundamental skill in chemistry. Students typically test pH using a pH meter, litmus or pH paper strips, or one of a variety of commercial acid-base indicator solutions. Acid-base indicators are weak organic acids that change color depending on the pH of a solution. A wide variety of pH indicators, such as phenolphthalein, methyl orange, phenol red, and bromthymol blue, can be purchased.
Effective pH indicators can also be made at home or in the classroom from items you probably already have in your kitchen or garden. Anthocyanins are a class of organic compounds found in many plants, fruits, and flowers that are red, blue, or purple in color and change color with pH. In this activity—a great introduction to pH for elementary and middle school students—you will extract anthocyanin pigment from plant material for use as a natural pH indicator.
Most of the indicators used in this activity come from plant pigments and will stain skin and clothing.
Materials (per class)
- Plant Material Containing Anthocyanins (red cabbage, blueberries or blackberries, and purple grape juice)
- Lemon Juice, Vinegar, and Baking Soda Solution (known acids and bases for testing)
- Light-Colored Liquids (unknowns for testing)
- Vegetable Peeler
- Strainer or Colander
- Small Cups or Beakers (to hold prepared indicators and solutions for testing)
- Small Pan
- Disposable Pipets or Medicine Droppers
- If performing the activity at home, clean the space to be used for the activity with household surface cleaner.
- Gather materials.
- Prepare the indicator solutions. For red cabbage: Add about ¼ cup of grated red cabbage to 2 cups of water in a small pan. Heat the water to a boil until the water turns purple. Strain the solution to remove any cabbage. For blueberries or blackberries: Place 2 to 3 berries in a small bowl and mash with a fork to make a paste, or use about a teaspoon of blueberry or blackberry jam. Add about 5 mL of water and stir, then strain to remove any solids. For grape juice: Be sure to use purple grape juice, as white grape juice will not work. The juice can be used as is from its container.
- Place a few milliliters of a known acid in a small cup or beaker.
- Add 1 or 2 drops of one of the prepared indicators.
- Record any color changes.
- Repeat the process using each of the indicators and known acids and bases to determine the color changes for the prepared indicators in the presence of acids and bases.
- Place a few milliliters of a liquid with an unknown pH, such as a light-colored soft drink, in a small cup or beaker.
- Add 1 or 2 drops of one of the prepared indicators to the solution.
- Record any color changes.
- Use your observations to determine if the solution is an acid or base.
Students should find that most of the prepared indicators are red to purple in the presence of an acid, violet to blue at a neutral pH, and turquoise to green in a basic solution. Have students explore what other plant materials they can use as an indicator. A wide variety of plant materials containing anthocyanins will work as pH indicators. Additionally, turmeric (a spice) and some types of teas will also change colors in the presence of an acid or a base.