Question

A base type indicator `B` differs in colour from its conjugate acid.`(BH^(+))` .Acidic form is red in colour while basic form is blue in colour. Human eye can sense blue colour distincity when ratio of blue form concentration to red form cocnentration is `(a)/(b)`. or more.However ,red colour can be sensed by human eye distinctly when ratio of red form concentration of blue form concentration is `(c)/(d)` or more. Determine the `pH` range of solution in which human eyes will be unable to observe distinct red or distinctly blue colour.Take ionisation of `B` as `K_(eq)`.

Answer 1

`:.pOH=pK_(eq)+log((["Acidic form"])/(["Baic form"]))=pK_(eq)+log((["Red"])/([Blue]))`
`(pOH)_(blue)=pK_(eq)+log((b)/(a))` and `p(OH)_(red)=pK_(eq)+pK_(eq)+log((c)/(d))`
so range `=pH_(blue)` to `pH_(red)=14-pK_(eq)-log((b)/(a))` to `14-pK_(eq)-log((c)/(d))`