Christmas Chemistry Demonstration

Color-change demonstrations are classic fare for the chemistry classroom. The most common color change reaction may be the Blue Bottle (blue-clear-blue) chemistry demonstration and the Briggs-Rauscher oscillating clock (clear-amber-blue), but if you use different indicators you can get color-change reactions to suit just about any occasion. For example, you can perform a green-red-green color change reaction for a bit of Christmas chemistry. This color change demonstration uses the indigo carmine indicator.

One of best parts of this demonstration is that you don't need very many ingredients:

• water (distilled is best, but you can use tap water if your pH is close to neutral)
• 15 grams glucose
• 7.5 grams sodium hydroxide
• indigo carmine indicator
• beakers or other clear containers

1. Prepare a 750 ml aqueous solution with 15 g glucose (solution A) and a 250 ml aqueous solution with 7.5 g sodium hydroxide (solution B).
2. Warm solution A to around body temperature (98-100°F).
3. Add a 'pinch' of indigo carmine, the disodium salt of indigo-5,5'-disulphonic acid, to solution A. A pinch is enough indicator to make solution A visibly blue.
4. Pour solution B into solution A. This will change the color from blue → green. Over time, this color will change from green → red/golden yellow.
5. Pour this solution into an empty beaker, from a height of ~60 cm. Vigorous pouring from a height is essential in order to dissolve oxygen from the air into the solution. This should return the color to green.
6. Once again, the color will return to red/golden yellow. The demonstration may be repeated several times.

Indigo carmine, also known as 5,5'-indigodisulfonic acid sodium salt, indigotine, FD&C Blue #2), has the chemical formula is C₁₆H₈N₂Na₂O₈S₂. It is used as a food coloring agent and as a pH indicator. For chemistry, the purple salt is typically prepared as a 0.2% aqueous solution. Under these conditions, the solution is blue at pH 11.4 and yellow at pH 13.0. The molecule may also be used as a redox indicator, since it turns yellow when it is reduced. Other colors may be produced, depending on the specific reaction.

Other uses of indigo carmine include dissolved ozone detection, as a dye for foods and medications, to detect amniotic fluid leaks in obstetrics, and as an intravenous dye to map the urinary tract.

Indigo carmine may be harmful if inhaled. Avoid contact with the eyes or skin, which can cause irritation. Sodium hydroxide is a strong base that can cause irritation and burns. So, wear use care and wear gloves, a lab coat, and goggles setting up the demonstration. The solution may be safely disposed of down the drain, with running water.