Description
In the demonstration, a bottle of colourless liquid is shaken
for a few seconds until the liquid turns blue. Upon standing for
about a minute, the solution goes clear again. This sequence can
be repeated many times.
How does it happen?
Essentially, it is an oxidation-reduction reaction. The bottle contains potassium hydroxide, glucose, water, and an indicator called "methylene blue". Glucose is a reducing agent and in alkaline solution it will reduce methylene blue to a colourless form. Shaking the bottle allows the oxygen in the air to oxidise the methylene blue to the blue form. Upon standing it is reduced by the glucose back to the colourless form. The equations for the reaction are:
| when shaken: | O2 + MB (colourless) -> MBox (blue) | |
| upon standing: | RH + OH- -> R- + H2O | |
| MBox + R- -> MB + oxidation products of glucose |
where MB is the reduced (colourless) form of methylene blue, MBox
is the oxidised (blue) form of methylene blue, and RH represents
glucose.
Methylene blue is an example of a "redox indicator".
Some other redox indicators you may come across are ferroin, diphenylamine
and indigo carmine.
Further details
This demonstration was taken from Ted Lister's Classic Chemistry
Demonstrations, published by the Royal Society of Chemistry,
pages 48-49. Further details of the reaction mechanism are given
in AG Cook, RM Tolliver, JE Williams J. Chem. Ed. 1994,
71, 160-161.
Another article of interest is that by
WR Vandaveer, M Mosher J. Chem. Ed. 1997, 74,
402.
This page was devised by Dr Brian Yates, a lecturer in the School of Chemistry. Please contact me if you have any questions. E-mail: Brian.Yates@utas.edu.au