THE CHEMICAL CHAMELEON: USING POTASSIUM PERMANGANATE


A beaker containing sugar solution.

Redox reactions can be spectacular, especially when accompanied by a mélange of colour changes. This reaction depicts the reduction of the manganese ion in potassium permanganate (+7 state) to manganese dioxide (+4 state).

Potassium permanganate is an extremely versatile chemical, and a very strong oxidizing agent. It was once widely used to disinfect water. The colour of the compound stems from the coloured permanganate ion. However, when reduced in an alkaline medium, various colours are observed, owing to the intermediate compounds formed. This array of chromatic changes earned this reaction the name 'Chemical Chameleon'.

REAGENTS:

• Potassium permanganate crystals, finely ground.

• Sucrose, or cane sugar

• Distilled water

• Sodium hydroxide or potassium hydroxide solution. If unavailable, ammonium hydroxide solution may be used as well.

PROCEDURE:

• Prepare a potassium permanganate solution by adding a tiny pinch of the crystalline permanganate to distilled water. The colour of the solution should be moderate pink, and not violet, as it obscures the colour changes.

• In another container, prepare a concentrated sucrose solution with constant stirring. 

• Add some sodium/potassium/ammonium hydroxide solution to the sucrose solution, and stir thoroughly. Transfer it to a transparent container, preferably a glass one.

• Quickly add some of the potassium permanganate solution into the alkaline sucrose solution, and observe the flask carefully. The colour changes would start right away if the concentration and temperature of the solutions were ideal.

The colour transitions are due to the following compounds, as manganese is reduced from the +7state to the +4 state:

POTASSIUM PERMANGANATE (+7 state)— PINK

POTASSIUM MANGANATE (+6 state)— GREEN

POTASSIUM HYPOMANGANATE (+5 state)— BLUE (This colour often does not appear as this compound is highly unstable)

MANGANESE DIOXIDE (+4 state)— REDDISH BROWN or YELLOW



In this reaction, hydrolysed sucrose acts as a reducing agent. However, it requires an alkaline medium to do so. Thus, the addition of an alkali is necessary.

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