Acid-base indicators are either weak organic acids or weak organic bases. These indicators show one colour in molecular form and another colour in ionic form. Acidic indicator is represented by Hin.
`Hin hArr H^(+) + In^(-)`
In acidic solution, the equilibrium shifts towards left.
Hence, an acidic indicator has the colour of HIn in acidic solutions. In basic solution, the equilibrium shifts towards right ans shows colour of In - in alkaline solution.
Basic indicator is represented by InOH. `InOH hArr In^(+) + OH^(-)`
In acidic solution, the equilibrium shifts towards right. In basic solution, the equilibrium shifts towards left. Hence, basic indicator is having colour of `In^(+)` in acidic solution, and shows colour of InOH in basic solution.
