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.

The reactions involved are not generally part of the curriculum, but this experiment has a number of features that make it ideal for investigating reaction kinetics - it is very quick, the chemicals are relatively cheap and safe, and the measurements are straightforward. It also has great visual impact and so is a good way of stimulating interest in chemistry, perhaps via an open day.

The demonstration lasts 3–5 minutes, but 15–20 minutes is needed for the preparation beforehand.

Equipment

Apparatus

• Eye protection: goggles should be worn when preparing the solution
• Conical flask (1 dm³)
• Stopper or bung, to fit flask

Chemicals

• Potassium hydroxide (CORROSIVE, IRRITANT), 8 g
• Glucose (dextrose), 10 g
• Methylene blue (HARMFUL), 0.05 g
• Ethanol (IDA – Industrial Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL), 50 cm³
• Access to a nitrogen cylinder (optional)

Health, safety and technical notes

• Read our standard health and safety guidance
• Eye protection. Wear goggles when preparing the solution.
• Potassium hydroxide, KOH(s), (CORROSIVE, IRRITANT) – see CLEAPSS Hazcard HC091b.
• Glucose (dextrose), C₆H₁₂O₆(s) – see CLEAPSS Hazcard HC040c.
• Methylene blue (HARMFUL) – see CLEAPSS Hazcard HC032.
• Ethanol (IDA – Industrial Denatured Alcohol), C₂H₅OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040a.

Procedure

Before the demonstration

Less than 20 minutes beforehand, preferably.

1. Make a solution of 0.05 g of methylene blue in 50 cm³ of ethanol (0.1%).
2. Weigh 8 g of potassium hydroxide into the 1 dm³ conical flask.
3. Add 300 cm³ of water and 10 g of glucose and swirl until the solids are dissolved.
4. Add 5 cm³ of the methylene blue solution. The exact quantity used is not critical.
5. The resulting blue solution will turn colourless after about one minute. Stopper the flask and label it IRRITANT (due to the potassium hydroxide present).

The demonstration

1. Holding the stopper securely in place, shake the flask vigorously so that air dissolves in the solution.
2. The colour will change to blue and will fade back to colourless over about 30 seconds.
3. The more shaking, the longer the blue colour will take to fade.
4. The process can be repeated for over 20 cycles.
5. After some hours, the solution will turn yellow and the colour changes will fail to occur.

Follow up

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.

Tips

• 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.
• 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.

Teaching notes

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 re-oxidises the methylene blue back to the blue form.

This experiment could be used to determine the kinetics of the reaction and thus the mechanism.  

The reaction is first order with respect to the hydroxide ion, methylene blue and glucose but zero-order with respect to oxygen. The rate law can be found by measuring how long it takes for a solution of known concentration to go colourless.  

The activation energy can be calculated using a normal Arrhenius plot - natural logarithm of the decolouration time (lnt) against the reciprocal of absolute temperature (1/T). Campbell2 explains this can be done because the rate of the slow step is independent of the oxygen concentration, and thus the time, t, which is required for the total oxygen to disappear, is directly related to the rate constant, k. A straight line is obtained from the plot of lnt against 1/T. The rate law for the reaction is:3

Rate = k[Dox][CH][OH-] 

where Dox is the oxidised (blue) form of methylene blue and CH is the carbohydrate, glucose. A simple mechanism for the reaction is: 

CH + OH- ⇌ C- + H2O

O2 + D → Dox (Fast)

Dox + C- → D + X- (Slow) 

where D is the reduced (colourless) form of methylene blue and X- represents the oxidation products from glucose (arabinoic, formic, oxalic and erythronic acids). The enthalpy of the reaction has been reported as 23 kJ mol-1. 

Using other redox indicators

Redox indicators other than methylene blue can be used to present other colours and make the demonstration really striking. 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. Mixtures of the dyes can also be used.

Phenosafranine 

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.

Indigo carmine 

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. Find the full equipment list and procedure for the Traffic light demonstration in the Colour chemistry activities. 

Resazurin 

IRRITANT – see CLEAPSS Hazcard HC032. 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.