|Acid-base indicators are weak acids or bases themselves which establish
an equilibrium between their molecular and ionic forms. The molecular form
has a different colour than the ionic form. Changes in the pH cause a shift
in the equilibrium which favours one species over the other. For example if
we look at phenolphthalein indicator:
HB + Ph-
colourless base conjugate magenta
|Test your understanding of indicator behaviour: The acid colour of
a certain indictor, HIn, is yellow and the basic colour is blue. When this
indicator is added to a certain acid HB, the solution turns yellow. The equation
for the reaction may be written as:
|Which is the stronger acid, HB or HIn?
|Take a look at the Indicator
Table in the Databook.
|Methyl Orange is red in a pH of 3.2 or less and yellow in a solution
of 4.4 or greater. The transition colour of orange is seen between these two
|At the transition point [HIn]=[In-]. that is, the concentration
of the red-coloured species is equal to that of the yellow-coloured species
so that the solution appears orange.
|Thus methyl orange can be used for a titration reaction in which the pH of the solution at the stoichiometric point is between 3.2 and 4.4. The Ka for methyl orange at 25oC is 4.0 x 10-4. Thus ideally, the solution at the stoichiometric point should have a [H3O+] = 4.0 x 10-4 mol/L and a pH of 3.4.|