Question

Which of the following acts as a self-indicator? (a) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (b) \(\mathrm{KMnO}_{4}\) (c) Oxalic acid (d) Iodine

Short answer

Potassium permanganate (\textrm{KMnO}_{4}) acts as a universal self-indicator, changing from purple to colorless until a pink color persists at the endpoint.

Step-by-step solution

Understand Self-indicating Property

Identify what a self-indicator means. A self-indicator is a substance that acts as its own indicator by undergoing an observable physical change, typically a color change, at the endpoint of a titration or a chemical reaction.

Examine Option (a) - Potassium Dichromate (\textrm{K}_{2}\textrm{Cr}_{2}\textrm{O}_{7})

When used in redox titrations, potassium dichromate acts as an oxidizing agent. It turns green as it gets reduced, indicating the endpoint. However, it is not always used as a self-indicator because it doesn't always provide a strong, clear endpoint.

Examine Option (b) - Potassium Permanganate (\textrm{KMnO}_{4})

Potassium permanganate acts as a self-indicator. In titrations, the solution remains colorless as long as there are other substances to oxidize. Once the oxidation is complete, a persistent pink color appears, indicating the end of the reaction.

Examine Option (c) - Oxalic Acid

Oxalic acid is not a self-indicator. It is often used in titrations but requires an external indicator to show the endpoint of a reaction, as it does not induce a color change by itself.

Examine Option (d) - Iodine

Iodine can act as a self-indicator in specific reactions such as iodometric titrations. When all the reducing agent is consumed, iodine imparts a blue color to the starch indicator, signaling the endpoint. It is not a universal self-indicator but is in certain cases.

Identify the Correct Self-indicator

Based on the analysis, while both potassium permanganate and iodine can act as self-indicators under certain conditions, potassium permanganate is more widely acknowledged as a self-indicator due to its clear color change from purple to colorless, and then to a pink endpoint in titrations.

Key concepts

Potassium Permanganate as a Self-Indicator

Potassium permanganate (\textbf{KMnO}\(_4\)) is a chemical compound known for its strong oxidizing properties and is widely used in analytical chemistry during redox titrations. What makes potassium permanganate unique as a self-indicator is its ability to signal the titration endpoint with a distinct color change. As an oxidizing agent, it reacts with the analyte in the solution until it is completely consumed.

During the titration process, if any reducing agents are present, they react with \textbf{KMnO}\(_4\), keeping the solution clear. Once all the reducing agent has been oxidized, and excess \textbf{KMnO}\(_4\) remains, it imparts a pink or light purple hue to the solution. This change is readily perceptible to the naked eye, eliminating the need for an additional indicator. Through its innate characteristic as a self-indicator, \textbf{KMnO}\(_4\) simplifies the titration process and enhances the accuracy of the results.

Redox Titrations Explained

Redox titrations are a type of titration based on a redox reaction between the analyte and the titrant. They are essential in determining the concentration of a substance by oxidizing it with a known concentration of oxidizing agent. Students need to understand that in redox reactions, one species is reduced as it gains electrons, and the other is oxidized as it loses electrons.

• Redox titration steps include preparing the sample and titrant, monitoring the titration process, and identifying the endpoint where equivalent oxidation and reduction have occurred.
• A self-indicator like potassium permanganate reduces the need for an external indicator, making the process more straightforward.
• The clear end point marked by a permanent color change provides a precise stop signal for the titration.

A grasp of redox mechanics and a knowledge of different self-indicators ensures students can accurately conduct these titrations.

Color Change Indicators

Color change indicators are substances that experience a distinct color change under specific conditions, particularly when the pH of the solution changes. They are intrinsic to titrations as they visually demonstrate the completion of the reaction.

The effectiveness of an indicator depends on its clear and distinct color change at the exact point where the stoichiometric equivalence is reached.

• Types:

  There are universal indicators that show a gradation of color changes over a wide pH range and specific indicators designed for particular reactions.

• Choice of Indicator:

  The choice of an indicator is crucial because it must undergo a clear change as close as possible to the equivalence point to ensure accuracy.

Although many titrations require external indicators, self-indicators like \textbf{KMnO}\(_4\) during redox titrations simplify the process by signaling the endpoint without the addition of another substance.

Understanding the Titration Endpoint

The titration endpoint is a fundamental concept in analytical chemistry, as it marks the completion of the chemical reaction in titration. It's the moment when the titrant has been added in just the right amount to react stoichiometrically with the analyte in the solution.

The precision in spotting the endpoint directly affects the accuracy of the titration's result. Students must practice identifying the endpoint correctly, as errors at this stage can lead to incorrect conclusions about the analyte’s concentration.

• Visual Indications:

  For many titrations, the change in color of the indicator, including self-indicators like \textbf{KMnO}\(_4\), provides a clear visual signal of the endpoint.

• Instruments:

  In some cases, instruments may be used to detect the endpoint, such as pH meters in acid-base titrations.

Grasping the concept of the titration endpoint and recognizing the visual cues from self-indicators contribute to successful and accurate titrations for students.