Introduction 

A capped 2-liter plastic bottle containing a clear, colorless liquid is shaken.  The liquid turns a deep blue color.  At rest, the liquid returns to colorless.  The cycle can be repeated many times.

Equations

The students should be able to conclude that the blue color is the result of "something in the air" mixing with the solution (they may guess that it is oxygen), and that there must be something in the solution reversing the visible effect.

 O2(g)  → O2(aq)

O2(aq) + methylene blue → methylene blue

(colorless)                        (blue)

 glucose + OH- → glucoside

glucoside + methylene blue → methylene blue + OH-

(blue)                              (colorless)

 To Conduct Demonstration

  1.  Ask students to make observations about the bottle and its contents before shaking.
  2.  Have students tell you what is going on in the bottle, based on their observations when the bottle is shaken and allowed to sit.
  3. OPTIONAL:  Several bottles can be prepared and distributed to students with the instructions to observe the bottle without removing the cap.  In a very few seconds, someone will shake the bottle and "discover" the reaction.  This works well with small groups.
  4. After students have advanced the hypothesis that air or oxygen has something to do with the reaction, ask them how you could test this.  You can then add nitrogen gas to the bottle to flush out the air and oxygen.

An interesting story to go along with this demonstration:

The "Blue People of Troublesome Creek" were descended from a French immigrant, Martin Fugate, and his American wife, Elizabeth Smith.  Because of a recessive gene, they lacked diaphorase.  The inability of their blood to carry enough oxygen gave a blue cast to their skin.

The normal reaction would be:

hemoglobin [Hb(Fe2+) + O2  Oxyhemoglobin [Hb(Fe2+)O2]

Sometimes, however, the hemoglobin is oxidized in the body to methemoglobin [Hb(Fe3+)], which is unable to carry oxygen.  Diaphorase reduces methemoglobin back to hemoglobin.

Daily pills of methylene blue were found to cure this condition.  Methylene blue (blue color) is reduced in the body by pyridine nucleotides to colorless methylene blue, which in turn reduces methemoglobin to hemoglobin.  In doing this the methylene blue is oxidized to the blue form again and eliminated by the body, turning urine blue.  Since the blue color disappears from the skin and appears in the urine at the same time, the saying went, "I can just see that old blue running out of my body."

Safety and Disposal

This is a very safe demonstration.  A thumb should be placed over the stopper before the flask is shaken very vigorously, and eye protection should be worn.  The solution may be sink-disposed.

References

L.R. Summerlin and J.L. Ealy, Chemical Demonstrations: A Sourcebook for Teachers, 1985.
Tested Demonstrations, J.Chem.Ed. 65, p621, 1989.