Problem 161 - A Complete Resource Book in Chemistry for JEE Main

Question

Amount of oxalic acid present in a solution can be determined by its titration with \(\mathrm{KMnO}_{4}\) solution in the presence of \(\mathrm{H}_{2} \mathrm{SO}_{4} .\) The titration gives unsatisfactory result when carried out in the presence of \(\mathrm{HCl}\), because of (a) Gets oxidized by oxalic acid to chlorine (b) Furnishes \(\mathrm{H}^{+}\)ions in addition to those from oxalic acid (c) Reduces permanganate to \(\mathrm{Mn}^{2+}\) (d) Oxidises oxalic acid to carbon dioxide and water

Step-by-Step Solution

Answer

Option (a): Gets oxidized by oxalic acid to chlorine.

Step 1: Understanding the Reaction in Acid

Oxalic acid (\(\text{H}_2\text{C}_2\text{O}_4\)) reacts with potassium permanganate (\(\text{KMnO}_4\)) in acidic medium to form carbon dioxide, water, and manganese ions. The acid typically used is sulfuric acid (\(\text{H}_2\text{SO}_4\)), which provides the necessary \(\text{H}^+\) ions for the reaction.

Step 2: Specific Role of Sulfuric Acid

In the presence of \(\text{H}_2\text{SO}_4\), the \(\text{H}^+\) ions are essential to maintain acidic conditions, ensuring the permanganate is in its effective oxidative state. This helps oxidize oxalic acid without any side reactions involving the acid.

Step 3: Problem with Using Hydrochloric Acid

When \(\text{HCl}\) is used instead of \(\text{H}_2\text{SO}_4\), the chloride ions \(\text{Cl}^-\) can interfere because they can be oxidized by \(\text{KMnO}_4\) to form chlorine gas (\(\text{Cl}_2\)). This side reaction consumes permanganate, leading to inaccurate titration results.

Step 4: Clarifying the Incorrect Options

Options (b), (c), and (d) are incorrect because: (b) Even though \(\text{HCl}\) does provide \(\text{H}^+\) ions, the issue arises from chloride ion interference; (c) The permanganate reduction is not a direct consequence of using \(\text{HCl}\) specifically; (d) Permanganate oxidizes oxalic acid to \(\text{CO}_2\) and \(\text{H}_2\text{O}\), but the presence of chloride does not affect this primary reaction directly.

Step 5: Conclusion

The issue with using \(\text{HCl}\) in the titration of oxalic acid with \(\text{KMnO}_4\) is related to the oxidation of chloride ions to chlorine gas, represented by option (a).

Key Concepts

Oxalic Acid Titration

Oxalic acid titration is an analytical procedure used to determine the concentration of oxalic acid (\(\text{H}_2\text{C}_2\text{O}_4\)) in a solution by reacting it with a standardized solution of potassium permanganate (\(\text{KMnO}_4\)). During the titration, sulfuric acid (\(\text{H}_2\text{SO}_4\)) is added to the solution to provide the necessary \(\text{H}^+\) ions and maintain an acidic environment. The reaction that occurs can be expressed as follows:

• Oxalic acid reacts with permanganate ions in an acidic medium.
• Manganese ions (\(\text{Mn}^{2+}\)) and carbon dioxide (\(\text{CO}_2\)) are formed as products.

The amount of permanganate used in the reaction is stoichiometrically related to the amount of oxalic acid in the sample. This makes it possible to calculate the concentration of oxalic acid in the original solution.

Interference of Chloride Ions

The interference of chloride ions in the oxalic acid titration process is a critical aspect to understand. When hydrochloric acid (\(\text{HCl}\)) is used instead of sulfuric acid (\(\text{H}_2\text{SO}_4\)), chloride ions (\(\text{Cl}^-\)) can interact with potassium permanganate. This interaction results in the oxidation of chloride ions to chlorine gas (\(\text{Cl}_2\)).

• This side reaction competes with the desired oxidation of oxalic acid.
• The consumption of permanganate by chloride ions leads to inaccurate measurement of oxalic acid concentration.

Thus, it is crucial to avoid using \(\text{HCl}\) in the titration, as it introduces chloride ions which interfere and compromise the results.

Role of Sulfuric Acid in Redox Reactions

Sulfuric acid plays an indispensable role in redox reactions, particularly in titrations involving oxalic acid. Its primary function is to maintain an acidic environment by providing \(\text{H}^+\) ions. This is crucial for the proper functioning of potassium permanganate as a titrant.

• Permanganate ions work efficiently as an oxidizing agent only in acidic conditions.
• Maintaining a stable acidic environment prevents side reactions.
• Sulfuric acid does not introduce any anions that can interfere with the oxidation process.

Due to its inertness and inability to participate in competing reactions, \(\text{H}_2\text{SO}_4\) ensures a smooth titration process, leading to accurate and reliable results.