Question

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 $(I{n}^{-})$ forms of the indicator given by the expression

• A. $\log \frac{[I{n}^{-}]}{[HIn]}=p{K}_{In}-pH$
• B. $\log \frac{[HIn]}{[I{n}^{-}]}=p{K}_{In}-pH$
• C. $\log \frac{[HIn]}{[I{n}^{-}]}=pH-p{K}_{In}$
• D. $\log \frac{[I{n}^{-}]}{[HIn]}=pH-p{K}_{In}$

Answer 1

Answer: (D)

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