Chemical and Ionic Equilibrium - Result Question 115
####59. An acid type indicator, HIn differs in colour from its conjugate base $\left(In^{-}\right)$. The human eye is sensitive to colour differences only when the ratio $\left[In^{-}\right] /[HIn]$ 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:
- $pH=p K _{\text {In }}+\log 10=p K _{\text {In }}+1$
$$ \text { When } \frac{\left[\operatorname{In}^{-}\right]}{[HIn]}=10 $$
$$ =p K _{\text {In }}+\log (0.1)=p K _{\text {In }-1} \quad \text { When } \frac{\left[In^{-}\right]}{[HIn]}=0.1 $$
$pH$ range is $p K _{\text {In }-1}$ to $p K _{\text {In }+1}$.
