03. Colour of Indicators   Previous PageNext Page
       Apparatus, Chemicals and Method


1dm measuring cylinders - as many as the number of indicators to be used.
Expanded polystyrene cool box to store the dry ice. The type of box in which Winchester bottles are often supplied is ideal.


The quantities given are for one demonstration
A few cm of universal indicator and/or other indicators as desired.
A few cm of dilute ammonia solution and/or dilute sodium hydroxide solution
About 100g or dry ice (solid carbon dioxide). This should be bought, since dry ice made from a carbon dioxide cylinder attachment will float and is less effective at saturating the solutions. Dry ice can be obtained from universities, higher education institutions, hospitals and industry. It can be stored and transported in expanded polystyrene boxes.


The Demonstration

Fill a 1 dm measuring cylinder to the 1 dm mark with water and add enough universal indicator to give an easily visible colour.

Add a few drops of either ammonia solution or sodium hydroxide solution, to make the water alkaline, and stir. Add a few lumps of dry ice. These will sink to the bottom and bubble as CO is given off. A spectacular fog will be produced at the top of the cylinder and, after several minutes, the indicator will change colour from blue to orange.

The colour change will be more gradual if ammonia is used as the alkali, as the reaction that occurs is a weak acid - weak base reaction. The final pH reached is about 4.5 and therefore, if colour changes that take place at pH's lower than this are to be shown, it will be necessary to add a little concentrated hydrochloric acid at the end.

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