First slide

Buffer Capacity

Question

Statement I : The pH range of any indicator is from pK_In – 1 to pK_In + 1
Statement II : The pH of indicator is given as, $pH = {pK}_{\ln} + \log \frac{[\ln^{-}]}{[Hln]}$ . Colour change of indicator takes place when $[\ln^{-}] = [Hln]$

Easy

A

Statement I and II are correct
B

Statement I is correct and Statement II is incorrect
C

Statement - I is incorrect and Statement II is correct
D

Both statement - I and statement - II are False

Solution

Any indicator is characterised by the characteristic color of its ionised or non-ionised form $\underset{Colorless}{HIn} ⇌ H^{+} + \underset{Colored}{{In}^{-}}$
After equivalents point, the system is a buffer containing the weak acid HIn and its conjugate In^-.
P^H of a such a system is given $P^{H} = {(P^{K_{a}})}_{HIn} + \log \frac{[{In}^{-}]}{[HIn]}$
At equivalents point no color is observed and so [HIn] = 10 x [In^-]
At the end point, color is observed properly and so [In^-] = 10[HIn]
Thus when $P^{H} = P^{K_{In}} + \log \frac{1}{10} = P^{K_{In}} - 1$ system is colorless and when $P^{H} = P^{K_{In}} + \log 10 = P^{K_{In}} + 1$
system is colored therefore, range of P^H for a color change is P^K_^In $-$ 1 to P^K_^In $+$ 1
An average human eye can recognize a color when the concentration of the colored form is roughly 10 times that of the colorless form.

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