««« By James Palcik
A solution is prepared containing a dye. The dye, methylene blue, is colourless in a reduced state. Upon shaking the solution, the dye is oxidized and displays a blue colour. As the solution stands undisturbed, the dye fades to colourless as it is reduced. The cycle may be repeated over and over.
Grade 10 Chemistry, Academic and Applied; Grade 11 and 12U Chemistry. This activity examines the concept of Oxidation/Reduction reactions.
- Methylene blue solution, 6 mL
- 5.0M Potassium hydroxide solution, 2 x 25 mL
- Glucose powder, 5 bags (5 g each)
- Safety Goggles, Gloves, Lab Apron
- Caution: 5.0 mol/L potassium hydroxide is very corrosove.
- In a stoppered flask or clear bottle with a top, add 10 mL of the potassium hydroxide solution to 190 mL distilled or deionized water.
- Add 1 bag (5 g) of glucose powder and swirl the solution until the glucose powder is completely dissolved.
- Add several drops of methylene blue solution, seal the container, and swirl to mix. The solution in the container is dark blue.
- Allow the container to sit for 5 – 10 minutes (until the solution becomes clear).
- Vigorously shake the container and observe. Allow to sit again and repeat
Background to the Oxidation and Reduction of Methylene Blue
Methylene blue in reduced form is clear. Upon shaking, dissolved oxygen is introduced to the solution and the dye displays a blue, oxidized form. Upon standing, the dye is reduced by glucoside, which is formed in a reaction between potassium hydroxide and glucose:
- Glucose + OH– → Glucoside
- Glucoside + Dye → Reduced Dye (colourless)
- Reduced Dye + O2 → Oxidized Dye (blue)
Once depleted, the remaining solution may be flushed down the drain with copious amounts of water, OR preferably, dispose of according to your school’s policy.
James Palcik is the owner of Palcik Educational in Ancaster, Ontario. This activity is from the Whiz, Bang, Boom Demos for Science conference session, delivered by James at STAO 2012. Activity from Innovating Science™ by Aldon Corporation, copyright 2008. Used with permission of Palcik Educational.