It is a substance which is used to indicate the end point or completion of a reaction by it’s colour change.

Type of Indicators

(A)   Redox Indicators: These are used for redox titration.


KMnO_4  (Self indicator)

L_3Fe(CN)_6 (external indicator)


(B)   Acid base Indicators: Here indicate. gives different colours in acidic and basic medium.


HPH gives pink colour in basic medium but it is colourless in acidic medium.


(C)   Adsorption Indicators: Here indicator adsorb a particular thing


Starch adsorb iodine in CuSO_4 vs hypo titration case.


D)   Tracer or Radio Active Indicators: Some radioactive isotopes are used as such indicators to trace mechanism of some reactions.

E. g. 0-18 photo synthesis


Theory of Indicators: The phenomenon of acid base indicator is explained by following two theories mainly:

(A)   Ostwald Theory:

It explains indicator as follows:

  •   Indicators are weak acids or bases which give different colours in their un-dissociated and dissociated states.
  •  The colour of indicator changes with a change of concentration or pH value
  •   Each indicator shows a sharp colour change within a pH range.


(B)   Quinoid Theory: According to it (1) Indicator are aromatic compounds having two tautomeric forms i.e., Benzoid (Light colour) and Quinoid (Dark colour)

(2) Within a certain pH—range these forms changes into each other to colour changes.

E.g. HPH:



Choice of Indicators in Various Titrations:

Type of Titration                                                     Indicator can be used

(I) Weak acid vs Strong base                                        HPH.

(2) Strong acid vs Weak base                                       MeOH, methyl red

(3) Strong acid vs strong base                                      HPH, MeOH, methyl red

(4) Weak acid vs Weak base                 No suitable indicator except phenol red

HPH is not a good indicator for weak base titrations while MeOH not for weak acid titrations.

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