Acid-base indicators

Question 1

Define acid-base indicators

Answer 1

An acid-base indicator is a substance that changes colour according to the pH of the solutions in which it is placed

Question 2

Are most indicators strong or weak acids?

Answer 2

Most indicators are weak acids

Question 3

What must an indicator have in order to be classified as an indicator?

Answer 3

The colour of the undissociated molecule is different from that of the dissociated molecule

Question 4

How do indicators work?

Answer 4

Red litmus example:
If a base containing OH- ions is added to red litmus, the OH- ions react with the H+ ions in order to remove them to form water.
In accordance to Le Chatlier’s Principle, more H+ ions must be formed and therefore the equilibrium is shifted to the right.
Thus, the blue colour of the In- ions predominates, i.e. the red litmus turns blue.
HIn <==> H+ + In-
red (acid) = Hln
blue (base) = ln-

Question 5

Why are only a few drops of indicator added to titrations?

Answer 5

Therefore, the indicator makes a negligible contribution to the H= (or OH-) ion concentration.