The 'blue bottle' experiment
An alkaline solution of glucose acts as a reducing agent and reduces added methylene blue from
a blue to a colourless form. Shaking the solution raises the concentration of oxygen in the
mixture and this oxidises the methylene blue back to its blue form. When the dissolved oxygen
has been consumed, the methylene blue is slowly reduced back to its colourless form by the
remaining glucose, and the cycle can be repeated many times by further shaking.
Read our standard health & safety guidance
Lesson organisation
The reactions involved are not part of a normal chemical curriculum, but this experiment has a
good visual impact and would be one way of stimulating interest in chemistry, perhaps via an
Open Day.
The demonstration lasts about 3 - 5 minutes, but about 15 - 20 minutes is needed for the
preparation beforehand.
Apparatus and chemicals
Eye protection: goggles should be worn when preparing the solution
For each demonstration:
Conical flask (1 dm3)
Stopper or bung, to fit flask
Potassium hydroxide (Corrosive), 8 g
Glucose (dextrose) (Low hazard), 10 g
Methylene blue (Harmful), 0.05 g
Ethanol (IDA – Industrial Denatured Alcohol) (Highly flammable, Harmful), 50 cm3
Access to a nitrogen cylinder (optional)
Technical notes
Potassium hydroxide (Corrosive) Refer to CLEAPSS Hazcard 91
Glucose (dextrose) (Low hazard) Refer to CLEAPSS Hazcard 40C
Methylene blue (Harmful) Refer to CLEAPSS Hazcard 32
Ethanol (IDA – Industrial Denatured Alcohol) (Highly flammable, Harmful) Refer to
CLEAPSS Hazcard 40A
Procedure
Before the demonstration (less than 20 minutes beforehand, preferably)
HEALTH & SAFETY: Wear goggles
a Make a solution of 0.05 g of methylene blue in 50 cm3 of ethanol (0.1%).
b Weigh 8 g of potassium hydroxide into the 1 dm3 conical flask.
c Add 300 cm3 of water and 10 g of glucose and swirl until the solids are dissolved.
d Add 5 cm3 of the methylene blue solution. The exact quantity used is not critical.
e The resulting blue solution will turn colourless after about one minute. Stopper the flask.
The demonstration
a Holding the stopper securely in place, shake the flask vigorously so that air dissolves in the
solution.
b The colour will change to blue and will fade back to colourless over about 30 seconds.
c The more shaking, the longer the blue colour will take to fade.
d The process can be repeated for over 20 cycles.
e After some hours, the solution will turn yellow and the colour changes will fail to occur.
Teaching notes
A white background helps to make the colour changes more vivid. A white laboratory coat is
ideal.
On a cold day it may be necessary to warm the solution to at least 20 °C, otherwise the changes
are very slow.
Methylene blue is a redox indicator and is colourless under reducing conditions but regains its
blue colour when oxidised.
The removal of the blue colour is caused by the glucose which, under alkaline conditions, is
reducing the methylene blue to a colourless form. Shaking the solution admits oxygen, which reoxidises the methylene blue back to the blue form.
To confirm that oxygen is responsible for the colour change, nitrogen can be bubbled through the
solution for a couple of minutes to displace 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.
Natural gas can be used (in a fume cupboard) if nitrogen is not available. Some teachers may
wish to present this experiment as a magic trick. The colour change can be brought about by
simply pouring the solution from a sufficient height into a large beaker.
This experiment can be a popular Open Day activity. If visitors are to be allowed to shake the
bottle themselves it might be wise to use a plastic screw-top pop bottle to eliminate the risk of
the stopper coming off or the bottle being dropped and broken. The solution does not appear to
interact with the plastic over a period of a day but it would be sensible to try out the bottle you
intend to use beforehand.
Redox indicators other than methylene blue can be used. In each case add the stated amount of
indicator to the basic recipe of 10 g of glucose and 8 g of potassium hydroxide in 300 cm3 of
water.
1 Phenosafranine (Low Hazard) This is red when oxidised and colourless when reduced. Use
about 6 drops of a 0.2% solution in water for a bottle that goes pink on shaking and colourless on
standing. The initial pink colour takes some time to turn colourless at first. A mixture of
phenosafranine (6 drops) and methylene blue (about 20 drops of the 0.1% solution in ethanol)
gives a bottle which will turn pink on gentle shaking through purple with more shaking and
eventually blue. It will reverse the sequence on standing.
2 Indigo carmine (Low Hazard, Refer to CLEAPSS Hazcard 32) Use 4 cm3 of a 1% solution in
water. The mixture will turn from yellow to red-brown with gentle shaking and to pale green
with more vigorous shaking. The changes reverse on standing. These colours are those of traffic
lights.
3 Resazurin (Irritant, Refer to CLEAPSS Hazcard 32) Use about 4 drops of a 1% solution in
water. This goes from pale blue to a purple-pink colour on shaking and reverses on standing. On
first adding the dye, the solution is dark blue. This fades after about one minute.
Mixtures of the above dyes can also be used.
Health & Safety checked, August 2008
Vanishing Valentine Materials
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100 ml of a 0.133 M dextrose solution (C6H12O6)
100 ml of a 1.0 M sodium hydroxide solution (NaOH)
1 ml of a 0.1% resazurin solution
a 250-ml or 500 ml Erlenmeyer flask or separatory funnel (resembles a heart)
stopper for the flask
dropper or pipette
Prepare the Solutions
Dextrose Solution: Dissolve 2.4 g of dextrose in distilled or deionized water to make 100 ml of
solution.
Sodium Hydroxide Solution: Prepare the 1.0 M sodium hydroxide solution by dissolving 4.0 g
of sodium hydroxide in enough distilled or deionized water to make 100 ml of solution. Add the
sodium hydroxide a little at time, stirring constantly. Heat is evolved from this reaction.
Resazurin Solution: Dissolve 0.1 g of resazurin in distilled or deionized water to make 100 ml
of solution. The shelf life of resazurin solution is 6-12 months. This solution should be a deep
blue color.
Perform the Vanishing Valentine Demonstration
1. Pour 100 ml of dextrose solution and 100 ml of sodium hydroxide solution into the
Erlenmeyer flask or separatory funnel.
2. Add 8 drops of resazurin indicator solution to the flask or funnel.
3. Stopper the solution and swirl the flask to mix the contents. Initially the solution will be
blue.
4. Allow the solution to sit undisturbed. Once the resazurin is fully reduced the solution will
become clear or colorless. This may take up to 10 minutes.
5. Swirl or shake the solution to turn it a pink Valentine color.
6. The clear-pink cycle may be repeated by allowing the solution to sit and then shaking it
again. Once prepared, the solution lasts approximately an hour (depending on
temperature and available oxygen in the flask). The pink color will become less vivid
over time.
Vanishing Valentine Chemical Reactions
Dextrose irreversibly reduces resazurin to resorufin. The red resorufin molecule is further
reduced (reversibly) to colorless dihydroresorufin. Dihydroresorufin (clear) may be oxidized
back to resorufin (pink) by swirling or shaking the flask to introduce oxygen from the air into the
solution.
Vanishing Valentine Demo Safety
Wear appropriate chemistry lab safety gear when performing this demonstration, such as a lab
apron, gloves and safety goggles. While the resazurin and dextrose solutions are not hazardous,
sodium hydroxide solutions are caustic and could produce a chemical burn if spilled on the skin
or splashed into the eyes.