Question

$0.1$N solution of ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ is being titrated with $0.1$N ${\text{HCl}}$, the best indicator to be used is:
A. potassium ferricyanide
B. phenolphthalein
C. methyl orange
D. litmus

Answer 1

When ${\text{NaHC}}{{\text{O}}_{\text{3}}}\,{\text{ + }}\,{\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$is titrated with${\text{HCl}}$in the presence of phenolphthalein indicator, the phenolphthalein indicator gives endpoint only at neutralization of $50\%$amount of the ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ whereas methyl orange indicator gives endpoint at complete neutralization of both of the compounds.

Complete step-by-step answer:
Potassium ferricyanide indicator is used for the titration where iron compound is used in the titration. Here, we are not dealing with the reaction of iron so, we cannot use potassium ferricyanide
It is given that ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ is being titrated with ${\text{HCl}}$. This titration reaction takes place in two steps.
Step-I: ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}\, + \,{\text{HCl}}\,\, \rightleftharpoons \,{\text{NaHC}}{{\text{O}}_{\text{3}}}\,{\text{ + }}\,{\text{NaCl}}$
Step-II: ${\text{NaHC}}{{\text{O}}_{\text{3}}}\, + \,{\text{HCl}}\,\,\, \rightleftharpoons \,\,{\text{NaCl}}\, + \,{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$
Phenolphthalein is a basic indicator. It remains colourless below pH $8$ and shows pink to dark red colour above pH $8$.
So, if we use phenolphthalein indicator in ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$and ${\text{HCl}}$ it gives pink colour for step-I when ${\text{NaHC}}{{\text{O}}_{\text{3}}}$ forms because ${\text{NaHC}}{{\text{O}}_{\text{3}}}$ basic. So, as the ${\text{NaHC}}{{\text{O}}_{\text{3}}}$ forms the pH of the solution increases and phenolphthalein gives pink colour. But in second step, ${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ forms which is an acid. As the ${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ forms the pH of the solution decreases. So, the phenolphthalein does not show colour for the second step.
So, we cannot use phenolphthalein indicator because we will get an end point of $50\%$titration only if we use phenolphthalein indicator.
Methyl orange is a weak base. Methyl orange indicator is used in the strong acid weak base titration.
The colour change of methyl orange depends upon the pH of the solution. In acidic medium means pH below $3.0$ it shows red colour. In the range $3.0 - 4.5$ pH, Methyl orange is of orange colour and in basic medium shows yellow colour.
So, methyl orange gives yellow colour at the first end point which confirms the completion of step-I and then the yellow colour changes to red colour at the second end point which confirms the completion of step-II.
So, the best indicator to be used for the titration of ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ with ${\text{HCl}}$ is methyl orange.

Therefore, option (C) methyl orange is correct.

Note: Potassium ferricyanide indicator is used for the titration where iron metal undergoes oxidation. This indicator turns blue in presence of iron (II) so it indicates the oxidation of iron. The structure of methyl orange is as follows:

The change in methyl orange is represented as:
$\mathop {{\text{HIn}}}\limits_{{\text{red}}} {\text{(aq)}}\, + \,{{\text{H}}_{\text{2}}}{\text{O}}\, \rightleftharpoons {H_3}{O^ + }{\text{(aq)}}\,{\text{ + }}{\mathop {\,{\text{In}}}\limits_{{\text{yellow}}} ^ - }(aq)$. As the base is titrated with acid, initially in the basic medium methyl orange shows yellow colour. As the acid is added, concentration of acid increases, so the methyl orange remains in unionised form which is of red colour.