Acid/base indicators are chemicals that help you compare or estimate the strength of an acid or base by changing color depending on the pH value of a tested liquid. The lower the pH value, the more acidic the liquid. The higher the pH value, the more basic, or alkaline, the liquid.
In this project, you will make indicators from foods, such as red cabbage and various plants. You will determine the affect of acids and bases on the color of indicators and create color scales for the different indicators. You will also investigate the effect of soil pH on the color of flowers containing an acid-base indicator.
Purpose: To determine the effect of acids and bases on red cabbage indicator.
- masking tape
- 1-tablespoon (15-ml) measuring spoon
- white vinegar, 5%
- distilled water
- 1⁄2 teaspoon baking soda
- three 10-ounce (300-ml) clear plastic cups
- sheet of white copy paper
- red cabbage indicator (see Appendix 6)
- 4 stirring spoons
- Use the pen and tape to label the cups "Acid," "Base," "Neutral."
- Add 1 tablespoon (15 ml) of vinegar to the Acid cup, 1 tablespoon (15 ml) of distilled water and 1⁄2 teaspoon (2.5 ml) baking soda to the Base cup, and 1 tablespoon (15 ml) of distilled water to the Neutral cup. Note: Wash the spoon in distilled water and dry it after each use so that you do not contaminate or dilute the contents of the different cups.
- Set the cups on one of the sheets of white paper so that it will be easier to detect color changes.
- Add 1 tablespoon of red cabbage indicator to each cup. Using different spoons, stir the contents of each cup. Then observe and record their color in a Red Cabbage Indicator Data table like Table 27.1.
The red cabbage indicator turns red in vinegar, remains purple in water, and turns blue in baking soda.
Chemicals are substances made of a combination of elements (basic chemical substance of which all things are made). Each element is made of atoms (building blocks of matter), and only one kind of atom makes up each specific element. Indicators are natural or synthetic chemical substances that change color in response to other chemicals. Acid/base indicators change colors in the presence of acids or bases. Acids are aqueous solutions containing hydronium ions—H3O+1 and bases are aqueous solutions containing hydroxide ions—OH–1. Generally the characteristics of acids and bases are opposite. The special scale for measuring the acidic or basic nature of a substance is called the pH scale. The values on the pH (a number used to indicate the acidic or basic nature of a solution) scale range from 0 to 14, with the pH value of 7 being neutral (having no acidic or basic properties). Acids have a pH of less than 7 and bases have a pH greater than 7.
Red cabbage contains a chemical that is one of a class of compounds called anthocyanins (red pigment found in some foods). The anthocyanin pigment in red cabbage is an acid/base indicator. Vinegar is known to be an acid with a pH of about 2.8 and baking soda is a base with a pH of about 8.5. So from the results of this experiment, cabbage indicator is red at pH of 2.8, purple at pH 7, and blue at pH 8.5. For more information about color changes of anthocyanins in red cabbage due to changes in pH, see pages 10–11 in Dianne N. Epp's The Chemistry of Food Dyes (Terrific Science Press: Middletown, Ohio, 1995).