MadSci Network: Chemistry

Re: natural indicator solutions (anthocyanins)

Area: Chemistry
Posted By: Lynn Bry, MadSci Admin
Date: Thu Aug 7 11:39:58 1997
Area of science: Chemistry
ID: 870965929.Ch

Dear Amy,

It sounds like you have discovered some of the properties of anthocyanins - the purplish-red pigments found in many fruits and vegetables such as blueberries, cranberries, beets and red cabbage. These pigments are easy to isolate since they are soluble in water. The Edible/Inedible Experiments Archive in MAD labs has a similar experiment you can try with red cabbage juice. The pigment turns pink in acidic solutions (try white vinegar which contains acetic acid), and green in basic solutions. Try mixing some baking soda (sodium bicarbonate) in water to create a basic solution.

The "colorimetric change" noted with most indicators is due the gain or loss of a proton -- H+, by the pigment. This seemingly minor change is sufficient to alter the wavelengths at which the indicator absorbs and reflects light! Anthocyanins are a little different from most acid/base indicators in that the color changes come about via the gain or loss of an OH- (hydroxyl ion).

Hope this helps..

-L. Bry, MadSci Admin

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