7.3 Method

  Stoichiometry

A given volume of the aqueous solution of the bases is taken and phenophthalein indicator is added to it. This solution is titrated with an acid of known normality to the end point the volume of the acid used is noted. This end point is called first end point. Now in the same solution methyl orange is added and gain titrated with an acid of known strength to the end point. It is called second end point. The volume of acid, used in the second end point is also noted. The normality of the bases present is then calculated.

Principle: In the presence of phenolphthalein indicator

(i) $NaOH$ is completely neutralized by the acid.

(ii) Only half of the milliequivalents of $N{a_2}C{O_3}$ present are titrated as the phenolphthalein will show the colour change when only $NaHC{O_3}$ (weak base) and neutral species are left in the reaction mixture. The following reactions take place,

\[NaOH{\text{ }} + {\text{ }}HCl \to NaCl{\text{ }} + {\text{ }}{H_2}O\]

\[N{a_2}C{O_3} + {\text{ }}HCl \to \] $\mathop {NaHC{O_3}}\limits_{weak{\text{ base}}} $ $ + {\text{ }}NaCl$

Since phenolphthalein is a weak organic acid, and it changes its colour in weakly basic medium $\left( {NaHC{O_3}} \right),$ so as soon as the $N{a_2}C{O_3}$ is converted to $NaHC{O_3}$ phenophthalein shows the colour change indicating the completion of the reaction.

In the presence of methyl orange, all the basic substances left in the mixture will be neutralized by acid and methyl orange will show the colour change when the medium is weakly acidic

$\left( {{H_2}O{\text{ }} + {\text{ }}C{O_2}i.e.{\text{ }}{H_2}C{O_3}} \right).$