1. Tardigrade
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  3. Chemistry
  4. An acid type indicator, Hln differes in colour from its conjugate base (.In-.) . The human eye is sensitive to colour differences only when the ratio [.In-]./[.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 (.Ka=1.0× 10- 5.) ?

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Q. An acid type indicator, Hln differes in colour from its conjugate base $\left(\right.In^{-}\left.\right)$ . The human eye is sensitive to colour differences only when the ratio $\left[\right.In^{-}\left]\right./\left[\right.HIn\left]\right.$ 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 $\left(\right.K_{a}=1.0\times 10^{- 5}\left.\right)$ ?

NTA AbhyasNTA Abhyas 2020Equilibrium

Solution:

The two conditions when colour of indicator will be visible are derived by

$pH=pK_{a}+log\frac{\left[\right. I n^{-} \left]\right.}{\left[\right. H I n \left]\right.}$

$\left(\right.i\left.\right)$ pH = 5 + log 10 = 6

$\left(\right.ii\left.\right)$ pH = 5 + log 0.1 = 4

Thus, minimum change in pH = 2.