Question

Solution A3 is considered to be the acid form of the indicator. Solution B3 is considered to be the basic form of the indicator. Consider Le Chatelier’s Principle and explain why this is so.

Answer 1

Acid–base titration indicators are quite often weak acids in which the unionised acid (lets call it HIn) and its 'de–protonated' form, or conjugate base, the anion (In–), have different colours.

• One form can be colourless e.g. phenolphthalein in acid–neutral solutions.

• The equilibrium can be simply expressed as ....

• HIn–(aq, colour 1) H+(aq) + In–(aq, colour 2)

o Applying Le Chatelier's equilibrium principle:

o Addition of acid (A3) favours the formation of more HIn (colour 1)

 HIn(aq)= H+(aq) + In–(aq)

 because an increase on the right of [H+] causes a shift to left increasing [HIn] to minimise 'enforced' rise in [H+].

o Addition of alkali (B3) favours the formation of more I– (colour 2):

 HIn(aq)= H+(aq) + In–(aq)

 The increase in [OH–] causes a shift to right because the reaction

 H+(aq) + OH–(aq) ==> H2O(l)

 reduces the [H+] on the right so more HIn ionises to try to increase the [H+] i.e. minimising the change in [H+].