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 distinctly when ratio of blue form concentration to red form concentration is 
or more. However, red colour can be sensed by human eye distinctly when ratio of red form concentration to blue form concentration is 
or more. Determine the pH range of solution in which human eyes will be unable to observe distinct red or distinct blue colour. Take ionisation constant of B as K eq .
Text Solution
Verified by ExpertsCHECK THE SOLUTION.
14 – pK eq – log 
to 14 – pK eq – log 
Sol. pOH = pK eq + log 
= pK eq + log 
(pOH) blue = pK eq + log 
and p(OH) red = pK eq + pK eq + log 
So, range = pH blue to pH red = 14 – pK eq – log 
to 14 – pK eq – log 
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