Oxidation-Reduction of Indigo Carmine

Preparation of Solutions:
Indigo Carmine Solution - Dissolve 200 mG of indigo carmine in 100 mL distilled water. (Variations: substitute methylene blue or thionin for the indigo carmine.)

Dextrose Solution (0.1 M) - Dissolve 9 G of dextrose in about 400 mL of distilled and dilute to 500 mL.

NaOH Solution (3 M) - Dissolve 24 G of sodium hydroxide in about 100 mL of water and dilute to 200 mL. (Be careful as this solution process is highly exothermic.)

Pour about 150 mL of 0.1 M dextrose into a bottle. Add 6 mL of 3 M NaOH solution and 4-5 mL of indigo carmine solution. This should be done about 30 minutes before the demonstration is to be used. The color of the solution will gradually change and become yellow. When the solution is shaken gently it will turn back to green or a light blue. This cycle can be repeated many times. After a while the indicator seems to "wear out". When this happens add more indigo carmine to enhance the color changes.

Description of Chemistry:
The dextrose acts as a reducing agent in a basic solution and reduces the indigo carmine. The progress of this reduction reaction is observed by the color changes that the indigo carmine goes through. When the bottle is shaken the oxygen in the air mixes with the solution and oxidizes the indigo carmine back to its original state. An extensive article appeared in the February 1994 issue of the Journal of Chemical Education (page 160). This article gives some other dyes that can be used in place of Indigo Carmine. In 1995 a group of my advanced chemistry students did a research project using a few of the dyes mentioned in the article. Some of these dyes have interesting properties when used in place of Indigo Carmine. However, I still like Indigo Carmine.

Questions? Comments??