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  4. The rapid change of pH near the stoichiometric point of an acid base titration is the basis of indicator detection. pH of the solution is related to ratio of the concentrations of the conjugate acid (HIn) and base (In-) forms of the indicator given by the expression

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Q. The rapid change of $pH$ near the stoichiometric point of an acid base titration is the basis of indicator detection. $pH$ of the solution is related to ratio of the concentrations of the conjugate acid ($HIn$) and base $ (In^-) $ forms of the indicator given by the expression

AIPMTAIPMT 2004Equilibrium

Solution:

For example, an indicator $\left( HI _{ n }\right)$ can dissociate in the solution following way.
$HIn \rightleftharpoons H ^{+}+\text{In}^{\ominus}$
$\therefore E _{\text {Equilibrium Constant }}, K _{ In }=\frac{\left[ H ^{+}\right]\left[ In ^{-}\right]}{[ HIn ]}$
$\therefore pK _{\text {In }}= pH -\log \frac{\left[\text{In}^{\ominus}\right]}{[ HIn ]}$
$\Rightarrow \log \frac{\left[\text{In}^{\ominus}\right]}{[ HIn ]}= pH - pK _{\text{In}}$