Page 43 - Chemistry - XI

In the acidic medium, equilibrium lies to the left. In the alkaline medium, the ionisation of phenolphthalein
increases considerably due to the constant removal of H ions released from HPh by the OH ions from the
+
–
alkali. So, the concentration of Ph ion increases in the solution which imparts pink colour to the solution.
–
HPh H Ph

NaOH Na OH

H OH H O or HPH NaOH Na HO Ph

2 2
Pink
For a weak acid vs strong alkali titration, phenolphthalein is the most suitable indicator. This is so because
the last drop of added alkali brings the pH of the solution in the range in which phenolphthalein shows
sharp colour change.
Methyl Orange
Methyl orange is a weak base and is yellow in colour in the unionised form. Sodium salt of methyl orange
is represented as follows: CH
..
..
N N .. 3
N
Na OS
3
CH 3
The anion formed from the indicator is an active species which on accepting a proton (i.e. acting as Bronsted
Lowry base) changes from the benzenoid form to the quinonoid form. The quinonoid form is deeper in
colour and thus is responsible for the colour change at the end point.

.. .. .. CH 3 H + .. CH
+
– O S N N N – O S NH N N 3
3 3
CH 3 CH
Benzenoid form of the anion (Bronsted-Lowry base) Quinonoid form of the anion 3
(Yellow in colour) (Pinkish red in colour)

Choice of Indicator
In the titration of a strong acid and a weak base, methyl orange is chosen as an indicator. When titration
between a strong base and a weak acid is to be performed, then phenolphthalein is a good indicator. In this
case, alkali is dropped from the burette and acid is taken in the tiration fl ask. Colour of the solution is taken
in the titration fl ask.

EXPERIMENT - 15
Objective
To prepare a standard solution of oxalic acid (0.1 M).

Principle/Theory
We know that the molecular weight of hydrated oxalic acid is 126 g/mole. To prepare 0.1 M solution,
12.6 g of oxalic acid needs to be dissolved in one litre of water.

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