Acid base indicators are either weak organic acids or weak organic bases. Indicator change colour in dilute solution when the hydronium ion concentration reachesa 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 range8.3 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 An indicator is a weak acid and pH range is 4.0 to 6.0. If indicator is 50% ionized in a given solution then what is ionization consatant of the acid?