Chemical and Ionic Equilibrium - Result Question 56
59. An acid type indicator, HIn differs in colour from its conjugate base $\left(\mathrm{In}^{-}\right)$. The human eye is sensitive to colour differences only when the ratio $\left[\mathrm{In}^{-}\right] /[\mathrm{HIn}]$ is greater than $10$ or smaller than $0.1$ . What should be the minimum change in the pH of the solution to observe a complete colour change? $\left(K_a=1.0 \times 10^{-5}\right)$
(1997,2 M)
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Solution:
$\begin{aligned} & \mathrm{pH}=\mathrm{p} K_{\mathrm{In}}+\log 10=\mathrm{p} K_{\text {In }}+1 \hspace{10mm} & \text { When } \frac{\left[\mathrm{In}^{-}\right]}{[\mathrm{HIn}]}=10 \\ & =\mathrm{p} K_{\mathrm{ln}}+\log (0.1)=\mathrm{p} K_{\ln -1} \hspace{10mm} & \text { When } \frac{\left[\mathrm{In}^{-}\right]}{[\mathrm{HIn}]}=0.1 \\ \end{aligned}$
pH range is $\mathrm{p} K_{\ln -1}$ to $\mathrm{p} K_{\ln +1}$.
BETA
