Disappearing Rainbow (Demonstration)
(Reference: Flinn Scientific--http://flinnsci.com/homepage/chem/disrain.html)
 

Indicators   :   Endpoints   :   Acids & Bases
 

Purpose:
Illustrate the idea of acid/base endpoints using multicolored indicators

Safety:
Safety goggles should be worn by instructor
 

Materials:
6 250 mL beakers
2 g phenolphthalein
1 1000 mL beaker
1 g thymolphthalein
50 mL 6.0 M NaOH
10 g p-nitrophenol
50 mL 6.0 M HCl
250 mL Ethanol (95%)
 

Procedure:
First you will need to make the indicators if you havenít already (30 mL each)

Violet
0.45 g phenolphthalein
0.2 g thymolphthalein
30 mL ethanol

Blue
0.2 g thymolphthalein
30 mL ethanol

Green
0.2 g thymolphthalein
2.0 g p-nitrophenol
30 mL ethanol
5 drops 1M HCl (to acidify)
 
 

Yellow
1.0 g p-nitrophenol
30 mL ethanol
5 drops 1M HCl (to acidify)

Orange
2.0 g p-nitrophenol
0.15 g phenolphthalein
30 mL ethanol
5 drops 1M HCl

Red
1.5 g p-nitrophenol
0.75 g phenolphthalein
30 mL ethanol
5 drops 1M HCl

Add 3 drops of each indicator to each of 6 different beakers, on overhead projector.  Fill each with about 50 mL water and 50 mL of the HCl.  Solutions should be clear
 
Add some NaOH until colors change, to each beaker.  Now add more base.
 

Explanation:
This is a pretty simple acid/base reaction, where the colors appear at the endpoint of each titration.  You could show a titration curve and have students predict if it is acidic or basic depending on whether it has color or not.
 

Clean-up:
Save acids, bases and indicator.  All waste can go down drain.
 
 

Ginger Chateauneuf, 2000.
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