Fading Blue Solution

What colour is it?

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Purpose -

This demonstration shows the effect of oxidation on the pH of a liquid.

Nature - 

Demonstration

Materials - 

conical or round bottom flask, stopper, 200ml distilled water, 5g potassium hydroxide, 3 gm glucose, 10 -12 drops of methylene blue

Method - 

· Mix ingredients in flask and stopper

· Give flask a good shake to mix.  Solution should be a blue colour (similar to copper sulphate solution).

· Let stand for a few minutes and you should find the solution goes colourless again.  Give it a shake to return it to blue, and let it sit and fade again.

· Repeat several times.

Safety -

Standard precautions when using chemicals

Explanation -

When you shake the flask it forces more oxygen to dissolve into the mixture, which is involved in an oxidation reaction.  This reduces the pH and causes the methylene blue to go blue.  After a sitting for a while some of the oxygen comes back out of solution, raising the ph again, and the methylene blue goes to colourless. 

Notes -

This solution must be made fresh just before use as it breaks down over a few hours.

Upon each subsequent shaking, the blue will be weaker in colour and will last for less time.