Indicators in acid - base titration

The reaction between an acid and a base is called neutralization. It is very fast and the equilibrium constant for a neutralization reaction is so large that it nearly proceeds to completion. An acid-base titration is a simple and convenient volumetric method for quantitatively estimating the concentration of one, if that of the other is known. A known volume of the solution of an acid or base is transferred to a titration flask with the help of a pipette. we add indicator and start adding known volumes of the other solution in steps with the help of a burette. The point at which the reaction is observed to be complete is called the end point of the titration and is noted by the change in the colour of the indicator. For accurate estimation it is necessary for it to coincide with the equivalence point corresponding to the stoichiometric amounts of the acid and base in the neutralization reaction. A number of weak organic acids and bases which can change its colour with in its limits with variations in the pH value of the solution to which it is added act as indicators. The choice of indicator depends on the abrupt change of the pH during neutralization process near the equivalence point.

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.
For example
phenolphthalein is a weak acid (PhH)

PhH <_-_-_-_-_-_-> Ph- + H+ ...........(1)
(colourless (Pink in base)
in acid)

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
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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