Red Cabbage Acid Base Indicator

Equipment

Blender, knife, wire mesh, enough 250 mL beakers for every proposed sample + 3, 10-mL graduated cylinder, glass stir rods. A pH meter may be used to confirm the color observations.

Reagents

Head of red cabbage, DI water, 0.5 M solutions of HCl and NaOH, assorted household products such as milk, soap, aspirin, vinegar, etc. Avoid bleach unless your are prepared to talk about oxidation.

Presentation

Procedure:

1. Cut half a head of red cabbage into 1-inch cubes, place in blender, cover cabbage with DI water, and blend thoroughly.
2. Pour the juice into a 250-mL beaker, using a wire mesh to remove the majority of any remaining solids.

Presentation:

1. Add 5 mL of cabbage juice to beakers of 0.5 M HCl and NaOH to provide color reference points.
2. Add 5 mL of the cabbage juice to each sample.
3. Liquid samples: Pour 125 mL into a 250-mL beaker, add cabbage juice and mix.
4. Solid samples: Add ~1 teaspoon of solid to a 250 mL beaker, add 125 mL of DI water, cabbage juice, and mix.
5. You may use a pH meter to determine the pH values of all of the samples.

Hazards

Hydrochloric acid can irritate the skin. Hydrochloric acid vapors are extremely irritating to the eyes and respiratory system. Therefore, it should be handled only in well-ventilated area. Solid sodium hydroxide and concentrated solutions can cause severe burns to eyes, skin, and mucous membranes.

Discussion

Red cabbage and other plants (beets, blueberries, radishes, and many more) contain a class of compounds called anthocyanins. Anthocyanins have different colors that depend upon the numbers of removable protons that remain attached to the molecule. Anthocyanins are weak acids. When weak acids have different colors depending upon the number of protons that remain with the molecule, we call them acid-base indicators. Some of the more common acid-base indicators are phenolphthalein and litmus (neither of which are anthocyanins). If we were to represent an anthocyanin molecule and two removable protons as H₂Antho, the equilibria showing the loss of its protons would be: H₂Antho H⁺ + HAntho^-
HAntho^- H⁺ + Antho^2-
If H₂Antho were red and HAntho^- were blue and Antho^2- were yellow, we might see a red color at low pH, blue at medium pH, and yellow at high pH. As we change from low to high pH we go from a large [H⁺] to moderate [H⁺] and finally to low [H⁺]. As the [H⁺] changes, we gradually convert among the three forms of the anthocyamin. The colors that would be seen would be:

[H+]	color
very high	red
high	mixture of red and blue
moderate	blue
low	mixture of blue and yellow
very low	yellow

References

1. B. Z. Shakhashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry, Wisconsin, 1989, Vol.3, p.50-57 (procedure E).