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Universal Indicator `Rainbow'
A long glass tube is filled with a neutral solution of Universal indicator.
Hydrochloric acid is added to one end and sodium hydroxide solution
to the other. The tube is inverted a few times to mix the solutions and
the `rainbow' of Universal indicator colours appears.
Apparatus and chemicals:
For one demonstration:
Bungs (rubber), 2, to fit the glass tube
Beaker (100 cm3)
Dropper pipettes, 3
Clamp stand, boss and clamp
Distilled or deionised water
Hydrochloric acid, 0.1 mol dm-3
Sodium hydroxide solution, 0.1 mol-3
Universal indicator solution
HEALTH & SAFETY: Wear eye protection throughout
A. Add sufficient Universal indicator to about 60 cm3 of distilled or
deionised water in a beaker, to give a solution with a visible
B. Ensure that one end of the glass tube is firmly sealed with a
C. Fill the tube to about 2 cm from the top with the Universal
indicator solution. Then clamp the tube vertically. It is important
to leave a space above the liquid in the tube so that there is an
air bubble this helps the mixing in step H.
D. Add 3-4 drops of the hydrochloric acid solution. The top few
centimetres of the liquid should turn red.
E. Stopper the upper end of the tube, remove it from the clamp,
carefully invert it and then clamp it vertically again.
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F. Remove what is now the top stopper. Add 3-4 drops of the
sodium hydroxide solution. The top few centimetres of the liquid
should turn purple.
G. Stopper the tube. Both ends of the tube should now be firmly
H. Remove the tube from the clamp and carefully invert it 2 or 3