The Chemical Chameleon demonstration

Abstract:
It can be hard to visualise the oxidation states of transition metals but this demonstration has you covered. The rich array of colours of manganese in each of its oxidation states never ceases to amaze. Purple, green, blue, orange. You name it, manganese has it. Exploiting this ability of manganese, and a hydroxide and reducing sugar, this simple to set-up and non-toxic reaction self cycles through almost every colour of the rainbow. It really does earn the name of the Chemical Chameleon!

Introduction:
Transition metals are the fashion models of the periodic table. With many beautiful colours at their disposal, it is no surprise that the transition metal manganese, able to achieve stable oxidation states up to +7, is currently one of my favourite elements. The Chemical Chameleon demonstration aims to share multiple oxidation states of manganese in just one reaction, giving a reaction mixture that switches between various colours.

Our source of manganese will be potassium manganate(VII) (also known as potassium permanganate). This already vibrant purple form of manganese is beautiful in itself, but with the collaboration of sodium hydroxide and sugar, green and orange can also be achieved as well as a multitude of shades in between the three.

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On the left is my potassium manganate(VII) that I store in a jar, it is a little old but it gets the job done. Yes, I am guilty of calling it the outdated name potassium permanganate – a habit I have been trying to break thanks to my chemistry teacher. A bag of cane sugar is in the middle. Some online articles say to use sucrose – cane sugar primarily being sucrose – which works in the reaction despite not being a reducing sugar; more on this later. On the right is some sodium hydroxide, bought in the form of a commercial drain cleaner. Despite the complexity of the reaction, the reactants are simple and few.

The general idea of the reaction is that in alkaline conditions, purple potassium manganate(VII) converts to green potassium manganate(VI). Sodium hydroxide is typically used, but both potassium and ammonium hydroxide will work for this purpose too. The sugar then acts as a reducing agent, reducing green potassium manganate(VI) into orange manganese(IV) oxide. It is also apparently possible for some blue potassium manganate(V) to form as an intermediate.

 

The reduction of the manganate(VII) ion to the manganate(VI) ion:
MnO4 (aq) + e → MnO42- (aq)

 

The reduction of the manganate(VI) ion to manganese(IV):
MnO42- (aq) + 2H2O (l) + 2e → MnO2 (s) + 4OH (aq)

 

Although, the overall redox reactions are a little more complicated:
4KMnO4 (aq) + 4KOH (aq) +2C6H12O6 (aq) → 4K2MnO4 (aq) + 2H2O (l) + 2C6H12O7 (aq)
3K2MnO4 (aq) + 2H2O (l) → 2KMnO4 (aq) + MnO2 (s) + 4KOH (aq)

 

The overall equations are partially constructed by me so please excuse them if they turn out to be wrong! I hunted all over the internet for the best part of three hours for the overall equation. I scanned through my bag of scavenged equations from various articles but none could tell me quite what were the final products other than manganese(IV) oxide and a slightly different sugar. In the end I found a couple of equations, funnily enough on Wikipedia, that if modified with the knowledge from a few other sources seemed to make the most sense out of everything I had seen.

The idea is that potassium manganate(VII) is converted to potassium manganate(VI) and water with help from potassium hydroxide and a reducing sugar. Then the potassium manganate(VI) reacts with water and undergoes a disproportionate reaction to form manganese(IV) oxide and reform potassium hydroxide and some potassium manganate(VII). These reformed chemicals keep cycling through these equations at the cost of some sugar to form manganese(IV) oxide – quickly at first, but then slower and slower as less of the potassium manganate(VII) is left.

The potassium hydroxide in the reaction can be supplied by using potassium hydroxide instead of sodium hydroxide (or ammonium hydroxide), but I suspect the sodium hydroxide and potassium hydroxide might undergo a double displacement reaction, forming a limited amount of sodium manganate(VII) and a potassium hydroxide. The reaction generates more and more potassium hydroxide as it goes though so only a little potassium hydroxide would be needed to kick-start the reaction in this way.

Also sucrose, usually used in the form of cane sugar, is interestingly recommended for this reaction by a few. The whole point of the sugar is to act as a reducing agent in the reaction, however sucrose is a nonreducing sugar. Cane sugar does seem to function as a reducing agent though as I completed the Chemical Chameleon with it half a dozen times and had no problems getting it to work. C6H12O6 is sometimes said to be used so I assume that sucrose does not work for this reaction but monosaccharides, such as glucose, do work as they are reducing sugars and cane sugar must therefore contain a small amount of them. The C6H12O6 would then be oxidised to C6H12O7 such as gluconic acid or something similar. Either that or the sodium hydroxide allows the sucrose to act as a reducing sugar.

This is all speculation on my part so if anyone more experienced could lend a helping hand I would appreciate it! I am very surprised how difficult it is to find out detailed information on this popular reaction.

Safety:

 

Substance Hazard information
Sodium hydroxide Corrosive
It causes severe burns; it is particularly dangerous to the eyes.
Sugar Low hazard
Potassium manganate(VII) Harmful/Oxidising
It is harmful if swallowed and stains the hands and clothing.Many hazardous reactions occur with reducing agents or concentrated acids. On heating, it liberates oxygen gas and releases a fine dust of potassium manganate(VI).
Manganese(IV) oxide Harmful
Harmful by inhalation or if swallowed. It is often encountered as a fine powder. Many hazardous reactions occur with reducing agents or concentrated acids.

Gloves and goggles should always be worn when handling chemicals to protect your sensitive eyes and skin.

  • The demo is quite safe, the only real worry is the corrosive sodium hydroxide. The mass of sodium hydroxide I recommend to use only gives a solution concentration of just under 0.25M so is just under what is considered corrosive at 0.5M so is still quite safe and just considered an irritant. I still recommend to use a lab coat in case of spills though.
  • Manganese(IV) oxide is an inhalation hazard and is formed in this reaction as a very fine insoluble precipitation, although this remains in suspension in the solution and so will not become airborne and is not a worry.
  • For disposal, everything is very safe and can just be poured down the drain. You may want to neutralise the sodium hydroxide though to which I recommend sulfuric acid. Do not use hydrochloric acid as this could also react with the fine manganese (IV) oxide precipitate to quickly form a small amount of toxic chlorine gas. Sulfuric acid reacts with manganese(IV) oxide as well but produces safe oxygen gas instead, although this may intensify any nearby open flames.

Experimental:
The demonstration is simple to get to work, but difficult to get just right. It really just takes some trial and error to find the best ratios for you. The amount of potassium manganate(VII) is the biggest deciding factor on how the reaction will turn out: too little and the colours will be too faint to see, too much and the colours will be too strong to distinguish. Due to the trial and error being involved, I will just show you my third and last attempt at the reaction that I was satisfied with.

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I decided to go decently big with the reaction vessel as only small amounts of the cheap reactants were required, so why not? You could try preparing large volumes of the solutions and measuring them out into smaller containers though, this way you can get multiple reactions instead of just the one which often leaves you hungry for more.

 

To a 500ml beaker 350ml of distilled water was added. Tap water should work too, but I can not verify this. The beaker was placed on my brand new hotplate stirrer (show off) and the stirring was switched on medium to get the reactants dissolving quickly when I added them.

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Most of these pictures will be of what is on top of the hotplate stirrer as the top of the device acts as a nice platform to show what I am doing. I also was just a bit lazy and did not want to move the camera from its spot. Typical chemist. May I add, being new to magnetic stirrers, how the whirlpool created never ceases to mesmerise me.

 

You do not need a magnetic stirrer for this demonstration but I highly recommend it. Without proper stirring the colour change is very uneven and not uniform. This can sort of add an interesting effect if you want it, but strong stirring gives the best demonstration in my opinion. Fast swirling or stirring with a glass stir rod should be about adequate but this motion can disturb the viewing pleasure of the observer; leaving the reaction to do its thing by using a magnetic stirrer adds an extra ‘wow’ effect to the demonstration.

IMG_7536[1]
A lovely snowstorm of sugar bliss. It is handy using sugar as the reducing agent as it is probably the easiest chemical to get your grubby chemist mitts on.

Despite the measurements being extremely rough, I nevertheless busted out my 3 decimal place scales to measure out exactly 10g of sugar. This was then added to the solution to dissolve. Again, cane sugar should work fine for some reason, but glucose if on hand should work much better I think. Glucose can usually be bought under the name of dextrose (dextrose is slightly nicer to say than D-glucose).

IMG_7542[1]
The chunkier pellets of sodium hydroxide compared to the grains of sugar are not as pretty but the heat of sodium hydroxide dissolving amazes me every time.

I then weighed out 5g of sodium hydroxide and also added this to the solution to dissolve. The more sugar and sodium hydroxide you use, the faster the colour changes should occur.

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The solution is nice and clear now after allowing everything to dissolve. This gives the maximum effect in the demonstration as what looks like just regular old water causes all these colours to follow the addition of the potassium manganate(VII).

Dissolving the sodium hydroxide is exothermic and slightly raises the temperature of the solution. I left the solution to allow everything to dissolve and to let it cool down a bit. If the temperature is too high the reaction will happen too fast in my opinion to appreciate so I prefer room temperature and the concentration of reactants I use here. This allows the reaction to last just under a minute, giving the most bang for your buck without taking too long. Solution A is now complete! 10g of sugar and 5g of sodium hydroxide dissolved in 350ml of distilled water.

IMG_7550[1]
Just a solution of potassium manganate(VII) is beautiful enough on its own. Such a lovely shade of purple.

Next I prepared solution B. The amount of potassium manganate(VII), contrary to the sodium hydroxide and sugar, needs to be quite precise. I found the tiny amount of 0.01g worked the best for a consistent colour change. This is dissolved in 50ml of distilled water. It you increased the amount of potassium manganate(VII) you use by a little bit it is possible to achieve a final red colour after the normal orange due to more manganese(IV) oxide forming. I was very happy with the colour change by the method I show here but I invite you to experiment to find better proportions of reactants yourself!

Results:
I will leave you with the pouring of solution B into solution A to start the reaction with only the odd remark. I hope you enjoy the magnificent colours manganese achieves in this reaction, the demonstration truly earns the title of the Chemical Chameleon!

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The potassium manganate(VII) solution is added.
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The rich violet of the potassium manganate(VII) fills the solution as it diffuses.
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The solution deepens to a dark purple.
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A dark blue is then just about witnessed.

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A beautiful emerald-green bursts forth. The highlight of the reaction.
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The green becomes paler and heads towards yellow.

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A dull yellow follows.
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With the final colour change the solution deepens into a vibrant orange.
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The orange is more vivid with a white background. Excuse my scramble for a white background and using a sheet of paper full of calculations.
 

 

 

 

 

Further reading or watching:

YouTube channel DanTheAppleMan – Colour change Chameleon Chemical Reaction:

Science Brothers – The Chemical Chameleon:
http://www.sciencebrothers.org/the-chemical-chameleon/

ThoughtCo. – How To Do the Color Change Chameleon Chemistry Demonstration:
https://www.thoughtco.com/how-to-do-color-change-chameleon-4057571

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