Different theories have been put forward to explain the role of indicators in the acid-base titrations's like Ostwald's ionic theory, Quinonoid theory etc. Ostwald's theory considers indicator to be a weak acid or base whose unionised forms differently coloured. In presence of acid or base, ie pH change, there is ionization of indicator and hence the colour change appears.
phenolphthalein is a weak acid (PhH)
PhH <_-_-_-_-_-_-> Ph- + H+ ...........(1)
(colourless (Pink in base)
H+ + OH- <-_-_-_-_-_-_> H2O
In presence of an acid (H+) equilibrium (1) is displaced towards the left hand side (a case of LeChatelier's principle); when strong base like NaOH is added, this equilibrium is displaced towards right hand side and there is colour change from colourless to pink when pH changes. This indicator is not suitable for titrating weak base since weak base can't furnish enough OH- that can react with H+ of the phenolphthalein and can impart pink colour only after excess of weak base is added.
Methyl orange behaves like a weak base (MeOH)
MeOH <-_-_-_-_-_> Me+ + OH- .........(2)
(yellow in base) (red in acid)
OH- + H+ <-_-_-_-_-_> H2O
In presence of a base, equilibrium (2) is displaced towards left hand side and appears yellow in base solution. On the addition of strong acid, OH- of MeOH is removed and hence equilibrium (2) is displaced towards right hand side when solution appears red. Thus there is colour changes from golden yellow to red when medium changes from basic to acidic. This indicator is not used in the titration of weak acid since it will not remove OH- of the indicator and can make colour change only after excess of weak acid has been addded.
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