Cabbage Juice - pH indicator
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Cabbage Juice - pH indicator

Area of Science: Chemistry
Meant for at least Grade 4-6 (age 8-10).
This experiment is edible.
An adult need not be present.

Overview:
Make your own acid/base indicator by boiling red cabbage. Use the juice to pH different fluids.

Equipment:
1. 1/2 head of *red* cabbage or so.
2. Metal grater.
3. Pot filled with enough water to cover the grated cabbage.
4. Strainer
5. Some acid/base solutions: for an acid try vinegar, for a base - mix some detergent in water.

Safety:
Be careful with the fingers when grating the cabbage.

How to do the experiment:
1. Grate the cabbage into small pieces and place them in the pot + water.
2. Boil the mixture for 20-30 minutes, until the liquid turns a dark purplish color.
3. Decant the fluid into a glass or jar, pouring through a strainer to remove the cabbage. Save the cabbage.. mix with a little vinegar and you can eat it on hot dogs, etc.. The collected fluid should be bluish/dark purple in color.
4. Make up some 'test' acid/base solutions. A good acid to use would be white vinegar. You could also try soda water/sprite or diluted juice from a lemon or orange. You can make a basic solution by mixing some washing detergent in water, or by adding some baking soda (sodium bicarbonate) to water. It's useful to have a control solution of (neutral) water - distilled water is best if you have access to it.
5. Add a few drops of the cabbage juice to your solutions, and note any color changes. The juice should turn pink in acidic solutions, and green in basic solutions.
6. You can use the indicator on any other solutions of interest, or try drying it on coffee filters to create a simple form of "pH paper."

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

For most pH indicators, the compound acquires a proton at low pH (lots of H+) but looses it at higher pH. This seemingly minor alteration is sufficient to alter the wavelengths of light reflected by the compound, thus creating the color change with respect to pH. Anthocyanins behave somewhat inversely in that the pigments "gain" an -OH at basic pH, but loose it at acidic pH. The WWW link below describes the chemistry with structures if you want to see the details.

The chemistry behind pH, acids and bases.. An acidic solution contains an excess of protons or H+. pH is a measure of how 'acidic' a solution is. The lower the pH, the more acidic the solution. In chemical terms, pH means "the negative log of the concentration of protons" in solution. Chemistry students should recognize this as pH = -log[H+]. If the concentration of H+ is .01M, the pH will be:

-log[.01] = -log[10^-2] = -(-2) = 2 (very acidic!).

"Neutral" solutions (water, e.g.) have a pH of 7. This number coicides with the amount of H+ naturally formed in water from the equilibrium reaction: H2O <--> H+ + OH- (H+ experimentally known to be ~10^-7M; OH- is also the same concentration). "Basic" solutions have a pH greater than 7 - meaning they have less free H+ than that of neutral water.

Useful References:
Plant pigments


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Further comments:
Other plants that contain anthocyanins include beets, cranberries and blueberries. Try assessing the pH properties of the pigments from these sources.


Experiment submitted on Tue Mar 25 17:28:17 1997 by:
Name: MadSci Admin
Institution: WashU
Position:


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