Red Cabbage Indicator

This is the red form of an anthocyaninn (in acid). Click on picture to display its Chime image

In base the anthocyanin loses a chloride ion, as well as an H⁺ ion. This changes the structure so the molecule absorbs red light, and turns blue. Click on picture to display its Chime image.

The colored pigment in red cabbage is one of a number of naturally occurring acid-base indicator dyes (another is litmus, which is extracted from a lichen). The red color of cabbage comes from a molecule called an anthocyanin. Like other indicator dyes, including phenolphthalein, the structure of the molecule changes in its acid or base form.

Many other foods contain anthocyanins including cranberry juice, black currants, and strawberries. Some flowers such as hydrangea also contain anthocyanins, and this makes their color sensitive to the acidity of the soil in which they grow. Cornflowers are blue since they contain the basic form of the same anthocyanin which in its acidic form makes roses red (in this case the plant's chemistry changes the acidty).

In this procedure you will extract the dye from cabbage, and test its colors in solutions of acid and base.

MATERIALS:

Extraction	Testing
Red cabbage Distilled water 250-mL beaker Ring stand Large ring wire gauze Bunsen Burner	0.1 M NaOH solution 0.1 M HCl solution 10 18 x 150 mm test tubes calibrated dropper

PROCEDURE:

Wear goggles at all times during this experiment. The acids and bases (sodium hydroxide, hydrochloric acid) you will be using are corrosive, and could cause damage to your eyes.

Extraction:

1. Take a few leaves of red cabbage and tear or cut them into small pieces.
2. Fill a 250-mL beaker about three-fourths full of the cabbage leaf pieces, and add distilled water to the 200mL mark.
3. Place the beaker on the wire gauze of a ring stand and heat with a burner to a slow boil for about 20 minutes. Be careful not to boil over the cabbage, or to let so much water evaporate that it burns.
4. Remove the beaker when you have a dark purple extract. This will be used in the next portion of the experiment.
5. Pour the cabbage extract indicator solution into a labelled stock bottle. If you are going to keep the solution for a while, put it in the refrigerator.

Testing:

1. Label the test tubes as: pH = 1, pH = 2, pH = 3, pH = 4, pH = 5, pH = 9, pH = 10, pH = 11, pH = 12 and pH = 13. Note: it would be difficult to make a solution of pH = 6 through 8 using dilutions as described in the next step.
2. Get 10 mL of 0.1 M HCl in test tube pH = 1. Take out 1 mL of this solution using your calibrated dropper, and put it in test tube pH = 2. Add 9 mL of distilled water, and mix throughly.
3. Rinse your dropper, then take 1 mL of the solution from test tube pH = 2, and put it in test tube pH = 3. Add 9 mL of distilled water and stir.
4. Repeat step 3 (taking 1 mL from test tube pH = 3 to make test tube pH 4, then 1 mL from test tube pH = 4 to dilute to become test tube pH = 5).
5. Get 10 mL of 0.1 M NaOH and put it into test tube pH = 13. Take out 1 mL of this solution using your calibrated dropper, and put it in test tube pH = 12. Add 8 mL of distilled water, and mix throughly.
6. Rinse your dropper, then take 1 mL of the solution from test tube pH = 12, and put it in test tube pH = 11. Add 9 mL of distilled water and stir.
7. Repeat step 6 (taking 1 mL from test tube pH = 11 to make test tube pH 10, then 1 mL from test tube pH = 10 to dilute to become test tube pH = 9).
8. Add 1 mL of the cabbage extract to each test tube, and record the color.