13. The 'Blue Bottle'        Experiment   Previous PageNext Page
       Theory, Extensions and Further Details


Theory

Glucose is a reducing agent and in alkaline solution will reduce methylene blue to a colourless form. Shaking the solution admits oxygen which will re-oxidise the methylene blue back to the blue form.



Further details


A.G. Cook, R.M. tolliver and J.E. Williams, J. Chem. Ed., 1994, 71, 160. The article The blue bottle experiment revisited gives some details of the reaction mechanism and alternate dyes.


Safety

Wear eye protection.

 


Further Tips

A white laboratory coat provides an ideal background. On a cold day, it may be necessary to warm the solution to 25-30 or the colour changes will be very slow. The demonstration can be used to start a discussion on what is causing the colour changes. Students' suggestions can be tried out as far as practicable.

Extensions

To confirm that oxygen is responsible for the colour change, nitrogen can be bubbled through the solution for a couple of minutes to displace the air from the solution and the flask. If the stopper is now replaced and the bottle shaken, no colour change will occur. Reintroducing the air by pouring the solution into another flask and shaking will restore the system.

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