The Blue Bottle Chemistry Demonstration

Materials Needed

• Tap water
• Two 1-liter Erlenmeyer flasks, with stoppers
• 7.5 g glucose (2.5 g for one flask; 5 g for the other)
• 7.5 g sodium hydroxide NaOH (2.5 g for one flask; 5 g for the other)
• 0.1% solution of methylene blue (1 ml for each flask)

Performing the Blue Bottle Demonstration

1. Half-fill two one-liter Erlenmeyer flasks with tap water.
2. Dissolve 2.5 g of glucose in one of the flasks (flask A) and 5 g of glucose in the other flask (flask B).
3. Dissolve 2.5 g of sodium hydroxide (NaOH) in flask A and 5 g of NaOH in flask B.
4. Add ~1 ml of 0.1% methylene blue to each flask.
5. Stopper the flasks and shake them to dissolve the dye. The resulting solution will be blue.
6. Set the flasks aside. (This is a good time to explain the chemistry of the demonstration.) The liquid will gradually become colorless as glucose is oxidized by the dissolved dioxygen. The effect of concentration on reaction rate should be obvious. The flask with twice the concentration uses the dissolved oxygen in about half the time as the other solution. Since oxygen remains available via diffusion, a thin blue boundary can be expected to remain at the solution-air interface.
7. The blue color of the solutions can be restored by swirling or shaking the contents of the flasks.
8. The reaction can be repeated several times.

Safety and Cleanup

Avoid skin contact with the solutions, which contain caustic chemicals. The reaction neutralizes the solution, so it can be disposed of by simply pouring it down the drain.

Chemical Reactions

In this reaction, glucose (an aldehyde) in an alkaline solution is slowly oxidized by dioxygen to form gluconic acid:

CH₂OH–CHOH–CHOH–CHOH–CHOH–CHO + 1/2 O₂ --> CH₂OH–CHOH–CHOH–CHOH–CHOH–COOH

Gluconic acid is converted to sodium gluconate in the presence of sodium hydroxide. Methylene blue speeds up this reaction by acting as an oxygen transfer agent. By oxidizing glucose, methylene blue is itself reduced (forming leucomethylene blue) and becomes colorless.

If there is sufficient available oxygen (from the air), leucomethylene blue is re-oxidized and the blue color of the solution can be restored. Upon standing, glucose reduces the methylene blue dye and the color of the solution disappears. In dilute solutions, the reaction takes place at 40 degrees to 60 degrees Celcius, or at room temperature (described here) for more concentrated solutions.

Other Colors

In addition to the blue/clear/blue of the methylene blue reaction, other indicators can be used for different color-change reactions. For example, resazurin (7-hydroxy-3H-phenoxazin-3-one-10-oxide, sodium salt) produces a red/clear/red reaction when substituted for methylene blue in the demonstration. The indigo carmine reaction is even more eye-catching, with its green/red-yellow/green color change.

Performing the Indigo Carmine Color Change Reaction

1. Prepare a 750 ml aqueous solution with 15 g glucose (solution A) and a 250 ml aqueous solution with 7.5 g sodium hydroxide (solution B).
2. Warm solution A to body temperature (98-100 degrees F). Warming the solution is important.
3. Add a pinch of indigo carmine, the disodium salt of indigo-5,5’-disulphonic acid, to solution A. Use a quantity sufficient to make solution A visibly blue.
4. Pour solution B into solution A. This will change the color from blue to green. Over time, this color will change from green to red/golden yellow.
5. Pour this solution into an empty beaker, from a height of ~60 cm. Vigorous pouring from a height is essential to dissolve dioxygen from the air into the solution. This should return the color to green.
6. Once again, the color will return to red/golden yellow. The demonstration may be repeated several times.