Acid base indicators are either weak organic acids or weak organic bases. Indicator change colour in dilute solution when the hydronium ion concentration reaches a particular value. For example, phenopthalein is a colour less substance in any aqueous solution with a pH less than `8.3` in between the pH range `8.3` to `10`, transaction of colour (colourless to pink) takes place and if pH of solution is greater than `10`, then solution colour is dark pin. Considering an acid indicator HIn, base `In^(-)` can be represented as :
`HIn hArr H^(+) + In^(-)`
acidic form basic form pH of solution can be computed as :
`pH = pK_(In) + "log" ([In^(-)])/([HIn])`
In general, transition of colour takes place in between the pH range `pK_(In) +-1`
If an indicator is to be used in an acid base titration having an equivalence point in pH range `8` to `10`, the indicator must

Options

A. be a weak base
B. have `K_(a)` of about `1 xx 10^(-9)`
C. ionise in two steps
D. be added to the solution only after it become alkaline

Answer

B