Red Cabbage - red or blue?

Demonstration Experiment on Video

Red Cabbage - red or blue?

Objective: Anthocyanins as pH Indicators

Peter Keusch

German version

Chemical:
ethanol

Household products:
1 head red cabbage
vinegar
lemon
Sprite lemonade
baking soda
soap
drain cleaner

Apparatus and glass wares:
hotplate
knife and cutting board
sieve
2 beakers 1000 mL
7 gas collecting cylinders h = 20 cm, d = 4 cm
measuring cylinder 500 mL
3 measuring cylinders 150 mL
volumetric pipette 10 mL
2 volumetric pipettes 5 mL
3 pipette bulbs
7 snap-cap vials 20 mL
2 pipette bulbs
7 glass rods

Hazards and safety precautions:

Ethanol is highly flammable.

Safety goggles and protective gloves required.

Preparation of the indicator

Cabbage juice: Half a head of red cabbage is chopped up finely. The cabbage is covered with 800 mL of boiling water. The juice cooled down to room temperature is sieved and 5 mL of ethanol are added. The liquid should be a dark reddish-purple color.

Preparation of the solutions to be tested

Seven gas collecting cylinders are set up as described in the following table:

1	2	3	4	5	6	7
130 mL of vinegar	juice of one lemon, made up to 130 mL with dist. water	130 mL of Sprite	130 mL of dist. water	0.15 g of baking soda, 130 mL of dist. water	0.2 g of soap, 130 mL of dist. water	15 mL of drain cleaner, made up to 130 mL with dist. water

Experimental procedure:

The test solutions in the cylinders are mixed with 12 mL of cabbage juice while stirring.

Test results:

Videoclip (Download RealPlayer .rm file)
Discussion:

Diverse household products contain acids or bases.

vinegar	lemon	Sprite lemonade	baking soda	soap	drain cleaner
acetic acid	citric acid	carbonic acid, citric acid	sodium bicarbonate	potassium salts of fatty acids	sodium hydroxide, soda

Red cabbage juice is a pH indicator. The juice will change to a variety of colors when added to solutions of various pH.

Red cabbage contains pigments called anthocyanins. The pigments give it the red/purplish color. Anthocyanins belong to group of chemical compounds called flavonoids.

Reference:
Microscale Projection Experiments Anthocyanins as pH-Indicators and Complexing Agents
Microscale Projection Experiments Light Absorbance of Cyanidin
Demonstration Experiment on Video Reaction of Leavening Ingredients with the Dye of Hybiscus Tea

Index of Lecture Experiments