Acid-base Indicators : Theories –

Theories of Acid-base Indicators

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  Acid-base Indicators : Theories –

(1)       Ostwald’s Theory-

According to this theory

  1. Acid-base indicators are weak organic acid or base
  2. They possess different colours in ionised & unionised state .


(Colourless)                    (Red Colour)

  1. The colour of the indicator depends upon the relative proportions of the unionised indicator molecules & its ions.
  2. Phenolphthalein is a weak acid whose unionised molecule is colourless while ion is red in colour.

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(Colourless)           (Colourless)                (Red )

  1. Similarly Methyl orange is a weak base whose unionised molecules are yellow, while ions are red in colour


(Yellow)                    (Red)              (Colourless)

  1. The ionisation of indicator is effected in presence of acid  & base because

2- Modern Quinonoid theory-

According to this theory

  1. An acid- base indicator is  dynamic equilibrium  mixture of two tautomeric forms, one form is benzenoid while other is quinonoid.
  2. The two forms have different colours.
  3. Out of these one form exist in acidic soln, while the other in alkaline soln.
  4. The change of pH causes the conversion of benzenoid form into quinonoid form & vice versa & consequently  colour changes.
  5. Benzenoid form is colourless or light in colour while the colour of quinonoid form is dark.

Ex. : Phenolphthalein :

HPh has benzenoid form in acidic medium which is colourless while it has quinonoid form in alkaline medium which has pink colour.


Benzenoid -form                                                                         Quinonoid -form

(colourless in acidic medium)                                                          (pink colour in alkaline medium)

Ex. Methyl Orange :

MeOH has quinonoid form in acidic medium & benzenoid form in alkaline medium. The colour of benzenoid form is yellow & quinonoid form is red.





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