Traffic Light Reaction

SKILL LEVEL

Middle school, High School

TRY IT!

Electrochemical reactions are those that involve the transfer of electrons from one compound or species to another.

In an electrochemical reaction, the species that loses one or more electrons is said to have been oxidized.  The species that gains one or more electrons is said to have been reduced.

Since both processes must occur  together (if something is oxidized, something else has to be reduced), the species that was oxidized is often referred to as the reducing agent. Likewise, the species that was reduced is often referred to as the oxidizing agent.

In this experiment, you'll use dissolved oxygen from the air as an oxidizing agent in the reversible oxidation of the dye, indigo carmine.  The end result is a stoplight chemical reaction that you get to control!

SUPPLIES, TOOLS, MATERIALS

  • Clear plastic or glass container with a stopper or lid.  We used a volumetric flask, but an empty disposable water bottle works great.
  • 250 mL water
  • 3 grams of sodium hydroxide (NaOH).  NaOH may be purchased online or at a hardware store as 'crystal drain opener'.  Make sure that the drain opener says the words "contains 100% lye" on the label.
  • 3 grams glucose*
  • 3-4 drops indigo carmine*

*homesciencetools.com, carolina.com, amazon.com

INSTRUCTIONS

NaOH will produce a mild chemical burn on contact.  Gloves and safety goggles are recommended. Simply rinse with copious amounts of water on first contact to avoid any discomfort.

  1. Fill your container with 250 mL of water.
  2. Add 3.0 g of glucose and swirl until dissolved.
  3. Add 3.0 g of NaOH and swirl until dissolved.  You may note that the outside of the container becomes warm to touch during the dissolution of the NaOH.
  4. If your indigo carmine dye is in powdered form, dissolve about a gram in 50 mL of water in a separate container.
  5. Add several drops to about 20 mL of the dye solution to your glucose/NaOH solution, swirling gently until the color is evenly mixed.
  6. If the solution is red or green at this point, set it down and wait for the color to return to yellow.
  7. Now, cap or stopper the bottle and give it a quick shake.  The solution should turn red!
  8. Now shake it vigorously for about 5 seconds The solution should turn green!
  9. Wait several seconds for the yellow color to be restored and repeat.  You've made a stoplight!
yellow light
red light
green light

WHAT IS HAPPENING?

The details of this reaction are actually fairly complicated and still an area of active research.

When NaOH is dissolved in water, it produces hydroxide ion (OH-) which strips glucose of a hydrogen, leaving behind a negatively charged enediolate ion.

This enediolate ion acts as a reducing agent, reducing the indigo carmine, giving it a yellow color.

When the solution is given a shake, oxygen in the headspace of the container becomes dissolved and oxidizes the dye, producing the observed red color.  Further shaking yields further oxidation and a new green color.

 

indigo carmine

Indigo Carmine. The center of the molecule, called a chromophore, is responsible for its brilliant color.

QUESTIONS TO CONSIDER

  1. Look up and define the following terms:
    Oxidation
    Reduction
    Oxidizing agent
    Reducing agent
  2. Did your stoplight continue to work even after several uses?  If not, why not?
  3. Did the color change or the rate of color change depend on the vigor with which you shook the container?  If so, why?
  4. Would this reaction work in a container that has been purged with nitrogen for several minutes before sealing?  If not, why not?