INDICATORS 

1. Topfer's reagent

2. Methyl orange

3. Methyl red 

4. Bromothymol blue

5. Phenol red

6. Phenolphthalein 

INDICATORS 

Certain substances dissociate in solution to give two or more different coloured forms. These substances are known as indicators. The nature of coloured forms is dependent on the pH of the solution. For example, phenol red changes from yellow at pH 6.8 to a deep red at pH 8.4. Indicators are used to determine the pH of liquids and the end points of acid-base titrations. Each indicator has a pH range over which a visible colour change occurs. This is referred to as the "colour change interval".

The commonly used indicators and their preparations are as follows:

1. Topfer's reagent

pH range : 2.9-4.2

Colour change: red to yellow

Preparation: 0.5% dimethylaminoazobenzene in 95% ethanol

2. Methyl orange

pH range: 3.0-4.4

Colour change: red to yellow

Preparation: 0.1 g methyl orange in 100 ml distilled water

3. Methyl red

pH range: 4.4 - 6.2

Color change: red to yellow

Preparation: 0.05 g in 100 ml of 50% ethanol

4. Bromothymol blue

pH range: 6.0 - 7.6

Colour change: yellow to blue

Preparation: Dissolve 0.1g bromothymol blue in 3.2 ml of 0.05 N NaOH. Make the volume to

250 ml with distilled water.

5. Phenol red

pH range: 6.8-8.4

Colour change: yellow to red

Preparation: Dissolve 0.1g phenol red in 5.7ml of 0.05 N NaOH. Make up to 250 ml with distilled water.

6. Phenolphthalein

pH range: 8.3-10.0

Colour change: colourless to pink

Preparation: Dissolve 0.1 g indicator in 100 ml of 50% ehtanol.