blue bottle experiment

Blue bottle experiment

Experimental procedure, a) experiment with glucose, naoh, and methylene blue, b) experiment with glucose, cuso 4 and naoh, copy short link.

The Blue Bottle Chemistry Demonstration

When you shake it, the blue liquid turns clear and then back to blue

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• Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
• B.A., Physics and Mathematics, Hastings College

In this chemistry experiment , a blue solution gradually becomes clear. When the flask of liquid is swirled around, the solution reverts to blue. The blue bottle reaction is easy to perform and uses readily available materials. Here are instructions for performing the demonstration, explanations of the chemistry involved, and options for performing the experiment with other colors:

Materials Needed

• 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

• Half-fill two one-liter Erlenmeyer flasks with tap water.
• Dissolve 2.5 g of glucose in one of the flasks (flask A) and 5 g of glucose in the other flask (flask B).
• Dissolve 2.5 g of sodium hydroxide (NaOH) in flask A and 5 g of NaOH in flask B.
• Add ~1 ml of 0.1% methylene blue to each flask.
• Stopper the flasks and shake them to dissolve the dye. The resulting solution will be blue.
• 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.
• The blue color of the solutions can be restored by swirling or shaking the contents of the flasks.
• 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:

CH 2 OH–CHOH–CHOH–CHOH–CHOH–CHO + 1/2 O 2 --> CH 2 OH–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

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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

• 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).
• Warm solution A to body temperature (98-100 degrees F). Warming the solution is important.
• 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.
• 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.
• 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.
• Once again, the color will return to red/golden yellow. The demonstration may be repeated several times.
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The Biology Corner

Biology Teaching Resources

The Blue Bottle Demonstration

Most biology teachers must eventually accept the awful truth that they will need to include chemistry in their biology classes.  For years, the only chemistry I had to worry about was the very simple photosynthesis equation which was fairly easy to explain and also presented in an “unbalanced” form.   When I started teaching AP biology, I had to relearn many concepts of chemistry that confounded me as an undergraduate.

 I can’t say that I didn’t feel a bit of terror at the very idea of having to teach redox reactions.   Luckily, I’ve come to a understanding with chemistry that involves mainly a focus on the organic side of things, focusing on the three main organic compounds found in life forms:   nucleic acids, lipids, carbohydrates, and proteins.  

 We do a little bit with reactions on how polymers are built using dehydration synthesis and how they are broken down with hydrolysis.    I’m also a little more comfortable explain how electron orbitals play a role in carbon bonds and how this in turn results in the long chains of carbon that are both structural and functional polymers of life.  Organic chemistry isn’t so bad!

To be fair, I have to admit that inorganic chemistry has the best demonstrations.   Even a biology teacher can do this one:

Blue Bottle Demonstration

300 ml distilled water 8 g KOH  ( potassium hydroxide ), or similar base 10g of glucose ( dextrose ) 5-10 drops of methylene blue 250-1000 ml erlenmeyer flask with stopper or cap

*Use safety goggles , KOH is strongly alkaline

All materials can be purchased from Amazon, and some can even be substituted.

• Pour 300 ml DI water into flask
• Add 8 g KOH, swirl
• Add 10g glucose and allow to dissolve
• Add 5-10 drops of methylene blue* and swirl
• Wait for the contents to become clear
• Place a cap on the flask and shake vigorously, solution will turn blue

The flask contents will go from clear to blue when shaken repeatedly, though this demonstration must be made within 15 minutes of class, eventually the solution stops changing color.    I actually prepare the contents in front of the class and describe each of the reactants, also demonstrating proper lab protocols by wearing safety goggles.  It’s a good introduction or review of chemistry for biology class.

*You can also substitute the indicator carmine red for a “stoplight demo.”

How It Works

At this point, open questions up to the class for their hypothesis for why the color change.  Some will remember from chemistry that adding oxygen (shaking the contained) will cause some kind of reaction.

Basically, the glucose acts as a reducing agent and turns the methylene blue to a colorless form.  Shaking raises the concentration of oxygen in the solution and then oxidizes the methylene blue back to its blue state.

This guy does a great job of explaining the process and can be used as a substitute if class materials aren’t available.

Shannan Muskopf

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The Blue Bottle Experiment Explained

The famous blue bottle experiment a visually dramatic way to teach reduction-oxidation (redox) chemistry. Students from grade school to grad school find this reaction memorable and it is considered a classic staple in chemical demonstration shows. A half-full bottle of colorless liquid turns blue when shaken, and when the bottle is allowed to sit still, the color fades. Shaking the bottle again causes the color to reappear like magic! What’s going on?

On the molecular level, the blue bottle experiment is a complex system composed of ethanol, the simple sugar glucose, the dye methylene blue, the hydroxide ion, and oxygen from the atmosphere. The color change occurs do to a pair of competing reduction-oxidation reactions. Hence, the blue bottle experiment is a wonderful tool for introducing the key concepts of reduction and oxidation.

All redox reactions involve electrons being transferred from one compound, the reducing agent, to another compound, the oxidizing agent. The term “reduction” means “gain of electrons”. This seems like an odd choice of terminology since “gain” and “reduce” are usually considered antonyms. However, because the electron has an electrical charge of negative one, gaining electrons will reduce the charge of a species. The term “oxidation” means “loss of electrons” and often, but not always, involves reaction with oxygen. A common mnemonic is the phrase OIL RIG, which stands for “Oxidation Is Loss, Reduction Is Gain”.

In first stage of the blue bottle experiment, the methylene blue dye acts an oxidizing agent and the glucose acts as a reducing agent. The methylene blue oxidizes the glucose to gluconic acid and the glucose reduces the methylene blue to its colorless form. The result is a bottle of colorless solution.

When the bottle is shaken, the surface are of the liquid temporarily increases, causing more oxygen to dissolve in the ethanol. The additional oxygen acts as an oxidizing agent and changes methylene blue to its blue, oxidized form. The result is a dramatic color change from colorless to blue.

When the shaking is stopped, the oxygen levels in solution begin to drop. With less oxygen present, the methylene blue once again is reduced to its colorless form by the glucose, and observers will see the color fade and disappear. The color change can be repeated many times simply by shaking the bottle to induce the blue color and then allowing it to sit still in order to make it disappear.

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Beyond the ‘blue bottle’.

By Declan Fleming 2014-05-06T00:00:00+01:00

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Using indigo carmine to produce a range of stunning colours

The ‘blue bottle’ demonstration is one of the most well-known and best-loved chemistry demonstrations. A flask containing a colourless liquid (consisting of an alkaline solution of glucose and methylene blue) is shaken. The resulting increase in dissolved oxygen concentration oxidises the colourless form of the dye back to the blue form, until the glucose reduces it once again.

Despite the fact that instructions for this demonstration often suggest using other dyes, it’s uncommon for teachers to do so. This is a great shame, because these dyes can produce some really beautiful colour changes. Presumably this is because methylene blue is commonly used in schools for other experiments. However, indigo carmine is very cheap, safe, easily obtained and produces a range of stunning colours.

• 70 cm 3 0.4M sodium hydroxide solution (irritant)*
• ~0.02 g indigo carmine (harmful)
• ~2.5 g D-glucose (dextrose)
• Three 100 cm 3 beakers
• 250 cm 3 conical flask with stopper or a 250 cm 3 reagent bottle with lid
• 500 cm 3 beaker to use as a warm water bath (to heat to ~50°C)

*0.4M NaOH is likely to be available in school labs ‘off the shelf’. The reaction rate is first order with respect to hydroxide ions, so similar concentrations are well tolerated, although those of 0.5M and above are classified as corrosive.

© Declan Fleming

Preparation

Wear eye protection. Place 20 cm 3 of water in a 100 cm 3 beaker. Measure out 70 cm 3 of 0.4M sodium hydroxide into a second 100 cm 3 beaker; in a third, dissolve 2.5 g of D-glucose in 10 cm 3 of water. Warm the beaker of sodium hydroxide to 40–50°C in a warm water bath. Add approximately 0.02 g of indigo carmine dye to either a 250 cm 3 conical flask with bung or a 250 cm 3 reagent bottle with lid. There should be little enough dye that unless students are looking for it, they may assume the flask is empty.

In front of the audience

You will now have three colourless liquids in beakers and a seemingly empty bottle. First, add the water and shake to dissolve the dye, giving a deep blue solution. Next, add the warm sodium hydroxide to give a vibrant green solution. Finally, add the glucose solution, which will give a different shade of green. If the solution appears more yellow than green at this stage, add a few more crystals of dye until the colour is convincingly green. Replace the stopper or lid and wait. Over the next few seconds, the green colour will change first to red and then to yellow. Shaking the flask briefly makes the colour go red, and shaking further makes it go green. Each time, the colour will change back to red and then yellow. With a little practice, it’s possible to get the first colour change with a single vigorous shake and the second change with a second vigorous shake.

Teaching goal

This demonstration can be used to support teaching of redox topics, as well as illustrating aspects of colour chemistry. The more I researched what was happening, the more interesting things emerged. The reaction has been studied in more detail by Laurens Anderson and colleagues at the University of Wisconsin-Madison, US, 2 since it was last discussed in this column. They suggest the reaction proceeds as follows: under alkaline conditions, the glucose is ionised and tautomerises to the straight chain aldehyde and then to the key intermediate, an enediolate anion. This anion can reduce the dye, forming a keto-aldehyde, glycosulose, in the process. Shaking the flask introduces oxygen into the solution, which reoxidises the dye to the blue form, ready to repeat the process. The hydroperoxide anions this produces can cleave the keto-aldehyde in the presence of excess hydroxide to finally give the sodium salt of arabinonic acid.

Glucose forms an enediolate anion, which reduces the indigo carmine dye. Shaking the flask introduces oxygen into the solution

Indigo carmine itself acts as both a pH and redox indicator. Below pH 11.4 we see the initial blue colour. It has a yellow form above pH 13, but at intermediate pH takes on a green hue. As well as the pH-dependent colours, there are three distinct redox-dependent colours that are the subject of the main demonstration.

Repeating the experiment with 1M NaOH (corrosive) eliminates the green colour. Instead, it goes from one yellow, to red, then to the reduced yellow (which is indistinguishable by eye from the first yellow). 

The demonstration can be started from low pH (blue) or high pH (yellow), as well as the traditional green

It is much more interesting to make up a dye solution at a lower pH: add 0.4M NaOH dropwise until it is close to changing colour to green, then add some glucose and wait. Ask students to predict what will happen to the colour. As you might expect, the reaction proceeds through mixtures of the different forms. The blue colour gradually becomes a beautiful purple colour, then red, orange and finally yellow. Shaking regenerates the blue colour.

Presumably the dye catalyses glucose oxidation in a similar way to methylene blue. Following extensive reading, it’s unclear how well characterised the structures of the reduced species are. But the fact that the demonstration can be started from the low pH blue or the high pH yellow, as well as the traditional green, suggests that the green colour consists of a mixture of indigo carmine (blue) and its conjugate base (yellow).

Indigo carmine itself has been extensively studied and it has been shown that the

E -isomer is favoured, with an efficient hydrogen bond between the C=O and the N–H locking the configuration. Presumably these hydrogen bonds also weaken the N–H bond when compared to that in indole, which explains the lower pKa (12.2 vs 16.2) that facilitates the first colour change in the experiment.

The colour of indigo dyes comes, for the most part, from the cross-linked conjugated system 3 dubbed the ‘H-chromophore’ by Dähne and Leupold, 4 after its shape. As such, the molecule can tolerate substitutions to the benzene rings without affecting the colour too much. Indigo carmine is an acidic, sulfonated indigoid, which is much more soluble, but also binds strongly to basic –NH groups, such as in the amide links of natural wool fibres. Adding the sulfonic acid groups does little to affect the colour. 5 However, the redox chemistry of the reaction in this demonstration goes right to the heart of the chromophore, bringing about significant colour changes through small changes in the oxidation states of these atoms.

Alternate methods

If you wish to prepare a bottle in advance, it is better to make up all the solutions at room temperature. Initially, it will take around two minutes for the full yellow colour to return after shaking. However, between approximately 15 and 45 minutes after making up the solution, the colour change will be much faster. After 45 minutes, the colour becomes less impressive, particularly the green. It is possible to rejuvenate by adding a little extra dye, but there are diminishing returns to be had from this and it may be better to make up a fresh solution.

Guidance on the use of other dyes (phenosafranine and resazurin) can be found online. 6

All liquids can be safely washed down the sink with plenty of water.

Wear eye protection

Indigo Blue

• Exhibition Chemistry , November 2006, p155 ( http://rsc.li/PU1DyD )
• L Anderson et al, J. Chem. Educ ., 2012, 89, 1425 (DOI:  10.1021/ed200511d )
• L Serrano-Andrés and B O Roos, Chem. Eur. J. , 1997, 3, 717 (DOI:  10.1002/chem.19970030511 )
• S Dähne and D Leupold, Angew. Chem., Int. Ed. , 1966, 5, 984 (DOI:  10.1002/anie.196609841 )
• Further information and suggested practicals can be found in Education in Chemistry , May 1986, p71
• http://bit.ly/1gOJ7x6
• Acids and bases
• Reactions and synthesis
• Redox chemistry

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Chemical Traffic Light Experiment

The chemical traffic light experiment is a dramatic redox reaction that changes colors between yellow or amber, green, and red. Shaking the solution then reverses the reaction, so the color change goes from red to green to yellow (like a traffic light). Here is how you perform the chemical traffic light experiment and a look at its chemistry. Also, explore chemical substitutions.

Chemical Traffic Light Materials

The classic color change demonstration uses glucose, indigo carmine, sodium hydroxide, and distilled water:

• 6 grams glucose
• 0.01 grams indigo carmine
• 40 milliliters 1M sodium hydroxide (NaOH) solution
• distilled water

It’s fine using indigo carmine indicator solution. Preparing the solutions several days in advance works well and actually increases the color transition speed.

Perform the Chemical Traffic Light Experiment

• Dissolve about 6 grams of glucose in 200 milliliters of water distilled water .
• Add 40 milliliters of the sodium hydroxide solution (3.75 g NaOH in 125 ml water or 1M NaOH).
• In a separate container, dissolve the indigo carmine in water. The amount you use depends on how deep you want the color. Indigo carmine in water is blue.
• Mix the indigo carmine indicator solution and the glucose with sodium hydroxide solution. The alkalinity turns the blue solution green.

As the green solution stands, it changes color and becomes red and then yellow. Shaking the solution and mixing it with air and changes the color green. Upon resting, the color becomes yellow or red.

You can repeat the transitions several times before the colors fade. Adding a bit more indicator solution extends the display up to around 50 cycles.

How It Works

Basically, the chemical traffic light is a variation of the blue bottle chemistry demonstration , except using indigo carmine instead of methylene blue. Both demonstrations are examples of redox reactions and are useful for studies of chemical kinetics because temperature affects the color change rate. The chemical traffic light is an example of a clock reaction.

Indigo carmine is a redox indicator that changes from blue in water to green in the alkaline glucose solution. Shaking the solution dissolves oxygen from air into the liquid and oxidizes indigo carmine. Dissolving a small amount of oxygen by lightly swirling the flask turns the liquid red. Vigorous shaking dissolves a lot of oxygen into the solution, oxidizes all of the indigo carmine, and turns the liquid green. As the oxygen concentration drops, the color returns to yellow.

• Blue: pH < 11.4
• Green: pH between 11.4 and 13
• Yellow: pH > 13

The red color comes from the sugar, which is glucose or dextrose. The reducing sugar converts to an enolate. It first reduces indigo carmine into a red semiquinone intermediate and then into a yellow reduced form. Introducing oxygen into the solution by shaking the flask repeats the cycle until all of the sugar is gone.

Substitutes in the Chemical Traffic Light Experiment

There are multiple variations of this demonstration:

• The colors of the reaction depend on pH . Lowering the starting pH to 11.4 changes the color shift to blue, purple, orange, and yellow.
• Substitute potassium hydroxide (KOH) in place of the sodium hydroxide.
• Substitute dextrose instead of glucose.
• You can use other redox indicators in place of indigo carmine and get different color changes. For example, methylene blue changes between blue and colorless.
• A variation of the chemical traffic light uses indigo carmine, ascorbic acid or vitamin C, sodium bicarbonate, sodium chloride, copper(II) sulfate, sodium hydroxide, and water. The ascorbic acid replaces the glucose in the original project. The copper ions act as a catalyst .
• Another variation uses potassium sodium tartrate ( Rochelle salt ), hydrogen peroxide, and a cobalt(II) salt as a catalyst. This reaction changes colors between green and pink.
• Indigo carmine stains skin and clothing, so either wear gloves or else stopper the container to avoid splashes.
• Sodium hydroxide is a strong base, so wear goggles and gloves and avoid skin or eye contact.
• Engerer, Steven C.; Cook, A. Gilbert (1999). “The Blue Bottle Reaction as a General Chemistry Experiment on Reaction Mechanisms”.  Journal of Chemical Education . 76 (11): 1519–1520. doi: 10.1021/ed076p1519
• Rajchakit, Urawadee; Limpanuparb, Taweetham (2016). “Greening the Traffic Light: Air Oxidation of Vitamin C Catalyzed by Indicators”. Journal of Chemical Education . 93 (8): 1486–1489. doi: 10.1021/acs.jchemed.5b00630
• Shakhashiri, Bassam Z. (1985). Chemical Demonstrations . Madison, Wis.: Univ. of Wisconsin Press. pp. 142–143. ISBN 978-0-299-10130-5.
• Wellman, Whitney E.; Noble, Mark E.; Healy, Tom (2003). “Greening the Blue Bottle”. Journal of Chemical Education . 80 (5): 537. doi: 10.1021/ed080p537

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• Chemical Demonstrations

Blue Bottle Experiment

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The “blue bottle” reaction is a classic chemistry demonstration. It is often used in general science classes to introduce the roles of observation and hypothesis in the scientific method. The demonstration is used in chemistry classes to illustrate oxidation and reduction reactions, and also to study the rates of chemical reactions.

• Reaction Rates
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Blue bottle

Change a solution’s color with a shake of your hand!

• Calcium hydroxide
• Methylene blue
• Put on protective gloves and eyewear.
• Conduct the experiment on the plastic tray.
• Do not allow chemicals to come into contact with the eyes or mouth.
• Keep young children, animals and those not wearing eye protection away from the experimental area.
• Store this experimental set out of reach of children under 12 years of age.
• Clean all equipment after use.
• Make sure that all containers are fully closed and properly stored after use.
• Ensure that all empty containers are disposed of properly.
• Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
• Do not replace foodstuffs in original container. Dispose of immediately.
• In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
• If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
• In case of inhalation: Remove person to fresh air.
• In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
• In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
• In case of injury always seek medical advice.
• The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
• This experimental set is for use only by children over 12 years.
• Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
• The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
• The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
• Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

Don’t worry! Continue with the experiment. The experiment will work with more methylene blue, but the solution will take longer to become colorless.

Yes! Just put the flask into a bowl of warm water (but don’t forget to close it with the stopper first!). The higher the temperature, the faster the liquid will lose its blue color.

Everything’s fine – the solution is simply too hot. Wait until it cools a little and then shake the flask again. The lower the temperature, the slower the solution will lose its blue color.

Try adding some more of both the lactose and NaOH solutions.

The solution stops turning blue when there is no more oxygen in the flask to oxidize the methylene blue. Just remove the stopper and let some air in, then put the stopper back, hold it in place, and shake the flask. The liquid will turn blue again.

The blue bottle will work as long as the flask contains oxygen and lactose. You can remove the stopper to let some oxygen in and add some more lactose. You can also use any household source of sugars instead of the lactose solution from the set, such as maple syrup, honey, or berry or fruit syrup. Please note that table sugar (sucrose) isn't suitable for this experiment. You can use this reaction to experiment with different sweet syrups and jams and figure out which of them contain carbohydrates that methylene blue can oxidize!

Step-by-step instructions

First, make a solution containing a reductant (lactose) and methylene~blue.

Now add some Ca(OH) 2 to make the solution basic.

Methylene blue takes some electrons from the reductant, lactose, and turns colorless. You didn't add it intentionally, but the solution contains a powerful oxidant—oxygen. Oxygen can take electrons from methylene blue, making it blue again. But once all the oxygen in the solution is used up, the methylene blue stays colorless.

Even though the solution is now devoid of oxygen, the air in the flask still contains some. Just shake the flask to dissolve it and see what happens.

Expected result

Blue solution in the flask becomes colorless. Shaking the flask turns the solution blue again!

Dispose of solid waste along with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

Why does the solution become colorless.

Initially, the solution contains the components for a potential chemical reaction. Lactose itself is more than happy to surrender its electrons. The oxygen dissolved in the water would be delighted to accept these electrons. Interestingly enough, though, oxygen isn't that willing to interact with lactose. And methylene blue can help: this colored compound acts as a carrier in our experiment, taking electrons from lactose and passing them to oxygen. However, at a certain point, the oxygen in the solution runs out, leaving methylene blue in an awkward position: it’s taken electrons from lactose, but has nowhere to pass them on to. When this happens, methylene blue cannot turn blue anymore and has no choice but to stay colorless.

Methylene blue:

Why does the solution turn blue again?

We can saturate the solution again with oxygen from the air above the solution. When the flask is shaken, oxygen from the air dissolves in the solution. The reaction can then proceed until all the oxygen available in the solution is spent again. However, this trick cannot be repeated endlessly. Since the flask is tightly sealed, sooner or later all the oxygen from the air will be depleted, and the solution will then remain colorless even when shaken. Nevertheless, the process can be reactivated by opening the flask to let some more air in.

Why did we add an alkali to the lactose aqueous solution?

By adding calcium hydroxide Ca(OH) 2 aqueous solution, we created an alkaline environment. Methylene blue needs an alkaline environment in order to accept electrons from lactose; otherwise, the reaction will not proceed, and the solution will remain blue. You can check this condition by conducting the experiment without Ca(OH) 2 .

Why is it so important to seal the flask tightly?

First and foremost, you’ll be able to shake the flask without sending any liquid flying.

Moreover, in sealing the flask we are preventing ambient air from entering, and ensuring that the oxygen in the ambient air will not have access to our solution either. This is why the color can only be restored by shaking the flask (see Why does the solution turn blue again? ). The most diligent observers may notice that the blue tint doesn’t disappear completely after the first shake, but remains at the border between the solution and air in the flask (along the so-called meniscus ) and forms a nice blue fringe. The same would happen if the flask were left open. This is caused by a high concentration of oxygen present in the air above the solution. The oxygen permeates the liquid-gas interface and converts methylene blue to its colored form. However, as the oxygen supply in the flask is gradually depleted, this border gets increasingly thinner and finally disappears.

Milk contains lactose, so try repeating the experiment using fresh milk instead of lactose solution! Share your experience online in your scientific blog.

Using other carbohydrates

This experiment may be conducted with many other carbohydrates. However, only those of them containing an aldehyde –CHO group can be oxidized in this experiment.

For instance, purchase a glucose solution at a pharmacy (else, you can purchase glucose tablets at a pharmacy and prepare an aqueous solution): the reaction would proceed in the same way as with lactose. On the opposite, fructose and sucrose would not participate in the oxidation reaction because they carry no aldehyde –CHO groups.

Another compound you can easily obtain for this chemical reaction is vanillin. It is often used as a flavoring agent in food industry and can be found in a grocery store. Vanillin also contains an aldehyde –CHO group, and the reaction would proceed in the way described above.

Dozens of experiments you can do at home

One of the most exciting and ambitious home-chemistry educational projects The Royal Society of Chemistry