Question

An acid type indicator, Hin differs 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 changes. `(K_(a) = 1.0xx10^(-5))`?

Answer 1

`"HIn" hArr H^(+) + In^(-), K_(a) = ([H^(+)][In^(-)])/(["HIn"]) or [H^(+)] = K_(a) (["HIn"])/([In^(-)])`
When `([In^(-)])/(["HIn"])=10, [H^(+)]=10^(-5)xx(1)/(10) = 10^(-6), pH = 6`
When `([In^(-)])/(["HIn"])=0.1 , [H^(+)]=10^(-5)xx10=10^(-4), pH=4" " :. ` Minimum change in pH = 2