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.) ?

Question

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.) ? (A) 4 (B) 2 (C) 6 (D) 1

Tardigrade Admin · Accepted Answer

The two conditions when colour of indicator will be visible are derived by pH=pKa+log([. I n- ]./[. H I n ].) (.i.) pH = 5 + log 10 = 6 (.ii.) pH = 5 + log 0.1 = 4 Thus, minimum change in pH = 2.

4

2

6

1

The two conditions when colour of indicator will be visible are derived by pH=pKa​+log[HIn][In−]​ (i) pH = 5 + log 10 = 6 (ii) pH = 5 + log 0.1 = 4 Thus, minimum change in pH = 2.

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

$p H = p K_{a} + l o g \frac{[ I n ^{-} ]}{[ H I n ]}$

$(i)$ pH = 5 + log 10 = 6

$(ii)$ pH = 5 + log 0.1 = 4

Thus, minimum change in pH = 2.