Blue Bottle

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves
Performers Required: 1
Equipment	Chemicals
500 mL Florence/Erlenmeyer flask (or larger)	Dextrose (D-glucose, C₆H₁₂O₆)
Rubber stopper to fit flask	Potassium hydroxide (KOH)
50 mL falcon tube	Methylene Blue (M.B.), 1% solution

Procedure:

Add 300 mL water, 8 g KOH, and 10 g dextrose to the flask, swirling the solution until everything has dissolved.
Add 5-6 drops of the M.B. indicator solution, swirling again to mix. The solution will initially turn blue, but after a few moments it will become colorless.
To perform the demo, stopper the flask and shake the solution vigorously, making sure to hold the stopper in place with one hand. The solution will turn blue after sufficient shaking. After the solution has turned blue, stop shaking and let the solution rest for ~10 seconds, at which point it will become colorless again.
This reaction can be repeated several times (usually 10 - 15) before the solution decomposes and turns cloudy.

Table Demo Procedure:

Add 30 mL water, 0.8 g KOH, and 1 g dextrose to a 50 mL tube, swirling the solution until everything has dissolved.
Add 2-3 drops of the I.C. indicator solution, swirling again to mix. The solution will initially turn blue, but after a few moments it will become colorless.

Clean-up: The waste solution should be neutralized, and then rinsed down the drain with copious amounts of water.

Hazards: KOH is a strong base and its solutions are highly corrosive, causing immediate chemical burns on contact.

Principle: This demonstration involves a reversible oxidation-reduction reaction between M.B., oxygen (O₂), and a reducing sugar. M.B. is a common redox indicator that is blue in oxidizing environments and colorless in reducing environments. When the flask is shaken, atmospheric O₂ is dissolved in the solution and oxidizes the M.B. to its blue form. Dextrose is a reducing sugar, and in alkaline solution it is converted to an enolate which reduces the M.B. back to its colorless form; the dextrose is ultimately oxidized into arabinonic acid and formate anions (Scheme 1, see reference). Shaking the flask again introduces more oxygen, which repeats the cycle until no dextrose remains.

Notes: The solution will remain blue at the gas-liquid interface, as there is sufficient oxygen to keep the M.B. in its blue form. The transition time from blue to colorless will increase with successive cycles; also, the conversion can be drastically slowed by halving the concentrations of the reagents. Alternately, stirring/swirling the flask instead of shaking it will cause much more rapid color changes, as less oxygen is dissolved and less of the M.B. is oxidized.

References: Andersen et al. J. Chem. Educ. 2012, 89, 1425-1431. DOI: 10.1021/ed200511d.