Red Cabbage Juice: A Homemade pH Indicator!
In the laboratory, pH paper and chemicals are commonly used to indicate pH. In the homeschool, students can make their own pH indicator using red cabbage juice, which changes color in the presence of an acid or base. The plant pigment anthocyanin is the active ingredient responsible for the color change. In this activity, students make a pH indicator from red cabbage juice and then use it to test various substances.
- Red cabbage leaves
- 200 mL water
- 250-mL beaker or clear container
- Medicine droppers (1 per test substance)
- Sheet of white paper
- 50-mL beakers or clear containers, (1 per test substance)
- 1 mL vinegar
- 1 mL household ammonia
- Test substances, 1 mL of each (e.g., lemon juice, fruit juice, milk, detergent, soda, antacid tablets, baking soda)
- Put on apron, gloves, and eye protection.
- Chop up several red cabbage leaves.
- Place the leaves in a blender and add 200 mL water.
- Blend the mixture and then pour it through the strainer into the large beaker or container. The red cabbage juice indicator is now ready for use. Note: If you use distilled water the indicator will have a reddish purple color and if you use tap water the indicator will have a violet blue color.
- Fill each of the small beakers or containers with 10 mL of indicator. Place them on the white sheet of paper.
- Add a few drops of vinegar to one container and a few drops of ammonia to another. Vinegar is an acid and ammonia, a base. Vinegar turns the indicator red and ammonia turns it green. Use these 2 samples as references for the other test substances.
- Test each substance by adding a few drops of it to a container of indicator. To test an antacid tablet or other solid, crush it, dissolve it in water, and add a few drops of the resulting solution to a container of indicator. After testing all the substances, students’ results should display a beautiful array of colors ranging from green to blue green, blue, violet, and red. See how the colors correspond to approximate pH ranges in the table below.
|Approximate pH Range||1 to 5||6 to 7||8 to 9||10 to 11||12 to 14|
Students can also make paper pH indicator strips using red cabbage juice. Here’s how to do it. First, cut strips from an unused paper coffee filter. Next, soak them in red cabbage juice for several hours. Finally, take the strips out of the juice and allow them to dry completely. Once the indicator strips are dry, they are ready for use. The dried strips will be light violet or pale blue in color. Simply dip an indicator strip in the substance you’re testing, remove it, and look for a color change. Acids will turn the strips red and bases will turn them green.
Red cabbage juice indicator changes color because its hydrogen ion concentration changes when a test substance is added to it. Acids produce hydrogen ions in aqueous solution and have a pH less than 7. Bases contain hydroxide ions and have a pH greater than 7. At a pH of 7, a substance is neutral (neither acid nor base) due to equal numbers of hydrogen and hydroxide ions. Carolina offers a full line of kits, chemicals, and pH indicators to further your exploration of pH.