Question

What are indicators ? Name four acid-base indicators and their colour change.

Answer 1

Dear user,

Indicator : It is a substance that shows some colour change with the change in pH.

Acid base indicator : Substance that changes colour with the change in pH of the solution and renders the
end point visible.

4 acid base indicator with their colour change are as follows :

1)Methyl Orange : From red to orange

pH range : 3.1-4.4

Used as an indicator in titration of weak bases with strong acids.
In acidic medium, solution would be red and in basic medium
solution would be orange.

2)Phenolphthalein : From colourless to pink

pH range ; 8.3-10

It is a weak acid. On adding extra hydrogen ion, there is simply shifting of indicator that changes colour from pink to colourless.
And on adding extra hydroxide ion, there is removal of hydrogen ion from the equilibrium which shifts the indicator
and there is colour change from colourless to pink.
That is in acidic medium, solution would be colourless and in basic medium solution would be pink.

3)Litmus : From red to blue

pH range : 5-8

It is a weak acid as well. On adding hydrogen ion, litmus paper turns colour to red, whereas on adding
hydroxide ion, litmus paper turns colour to blue.

4)Phenol Red : From yellow to red

pH range : 6.8-8.4

In strong acidic condition, it gives red colour and in basic condition it gives yellow colour.