pH indicator dye

Make your own pH indicator dye at home or in the classroom (with adult supervision)

Red cabbage contains a coloured pigment which is one of a number of naturally occuring acid-base indicators. This means that the structure of the molecule changes when it is placed in an acid or a basic solution and this changes the colour. Red cabbage can be used to test solutions for their pH:

Ingredients:

Red cabbage leaves

Distilled (Fresh) water

A container (beaker or bottle)

Heating device (either a bunson burner or stove)

Extracting the dye (on a bunson burner – if you are doing this at home you can heat the cabbage leave in a pan on a stove instead):

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. (If you are doing this at home, then use a sauce pan on the cooker to boil the cabbage instead).
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 pH of solutions:

You can test a number of different household solutions to see whether they are acid or base. When the cabbage water is in its red form the solution is more acidic and when it is in blue form it is basic. For example, Lemon juice & vinegar have a pH 2, Orange juice has a pH 3, Banans have a pH 5, Milk has pH 6, pure water has pH 7, Sea water has pH 8, Baking soda has pH 9 and Soapy water has pH 12.

If you have access to a known concentration of acid (e.g. in school use 0.1 M  HCl – Wear goggles at all times during this experiment.  The acids and bases are corrosive, and could cause damage to your eyes) you can use serial dilutions to create a pH colour scale.

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