Chemical Traffic Light

Chemical Traffic Light

Chemistry Involved:

The oxidized form of indigo carmine is green in an alkaline solution. The indicator reduces in a glucose alkaline solution. The intermediate form of indigo carmine is red. The fully reduced form of the indicator is yellow.

When the flask is shaken, oxygen from air dissolves in the solution and oxidizes indigo carmine. Solution becomes red if a small amount of oxygen is dissolved (moderate shaking), and green if all of indigo carmine was oxidized (intensive shaking).

Waste Disposal

Indigo Carmine

Materials

It is recommended to simply neutralize the sodium hydroxide by adding an appropriate amount of acid. In this case, the 5 grams of hydroxide is neutralized by about 17 mL of 20% sulfuric acid after the demonstration. The indigo carmine acts as an effective indicator for this – when the solution turns blue, it is very close to neutral. This blue liquid can then be safely poured down the drain.

• 3 grams dextrose or glucose
• 5 grams sodium hydroxide pellets
• 5-10 mL indigo carmine indicator solution (1% solution- 1 g/100 mL H2O
• 250 mL distilled water
• 500 mL round-bottom flask
• stopper
• stirring rod

Video Demonstration

Works Cited

• http://chemistry.elmhurst.edu/demos/trafficlight2.htm
• https://practicum.melscience.com/experiments/chemical-traffic-light.html
• http://www.sciencebrothers.org/the-chemical-traffic-light/
• http://chemed.chem.purdue.edu/demos/main_pages/19.2.html

*Indigo Carmine undergoes a reversible reaction.

*Formula: C16H8N2Na2O8S2 + HOCH2(CHOH)4COH + H2O =[OH-] → C16H10N2Na2O8S2 + HOCH2(CHOH)4COOH

2C16H10N2Na2O8S2 + O2 → C16H8N2Na2O8S2 + H2O

Experiment Preparation

1.) In a beaker containing 250 ml of distilled water, add 3 g of dextrose and stir until it dissolves.

2.) Add 5 g of sodium hydroxide pellets into the same beaker, and use a stirring rod to break apart the pellets so that they will dissolve quickly

3.) Pour the contents of the beaker into the glass flask, and add between 5-10 ml of the indigo carmine indicator.

*Enough should be added to the solution so that it turns a yellow color.

4.)Put the stopper in the flask, and let it sit for ten minutes. After ten minutes, the solution should be ready for the demonstration.

5.) Shake the flask once or twice, and it should turn a red color. Shake once or twice again, and it should turn green. Then, if it sits, it should turn back to the yellow color.

Safety Precautions

*Sodium hydroxide is a corrosive chemical, use caution when shaking the flask.

*Make sure the stopper is on tight, and put your hand on top of it when shaking.

*Wear your PPE (Personal Protective Equipment at all times.)