Question

Identify the method of preparation of a colloidal sol from the following:

• A. Ultrafiltration
• B. Peptisation
• C. Dialysis
• D. Electro-dialysis

Hint:

Peptisation is an important method to convert a precipitate into a colloidal dispersion by adding an electrolyte.

Answer 1

The Correct Option is B

Step 1: Understanding Peptisation Peptisation is the process of converting a precipitate into a colloidal solution by adding a suitable electrolyte. Step 2: Explanation - In peptisation, the electrolyte adsorbs onto the surface of the precipitate, causing it to break into smaller particles that remain suspended in the solution. 
- It is widely used in the preparation of colloidal sols. 
Thus, the correct answer is Peptisation.

Answer 2

To identify the method of preparation of a colloidal sol, we must understand the various techniques used to produce colloidal systems. One of the most important and commonly used methods is called peptisation.

Definition of Peptisation:
Peptisation is the process of converting a freshly prepared precipitate into a colloidal sol by adding a suitable electrolyte, known as the peptising agent. The electrolyte helps to break down the precipitate into smaller particles, which then disperse uniformly in the dispersion medium to form a stable colloidal solution.

How Peptisation Works:
When the electrolyte is added to the precipitate, the ions from the electrolyte get adsorbed on the surface of the particles of the precipitate. This creates a charge on the surface of the particles, which causes repulsion between them. As a result, the particles cannot come together and remain dispersed in the medium. This dispersion leads to the formation of a colloidal sol.

Key Characteristics:
- The process requires a freshly prepared precipitate.
- A small amount of electrolyte (peptising agent) is used.
- The electrolyte should be the same ion as that present in the precipitate to avoid coagulation.

Example of Peptisation:
A classic example of peptisation is the formation of a colloidal solution of ferric hydroxide (Fe(OH)₃).
When freshly prepared Fe(OH)₃ precipitate is treated with a small amount of FeCl₃ solution, a brownish-red colloidal sol of Fe(OH)₃ is obtained.

Chemical representation:
Fe(OH)₃ (s) + Fe³⁺ (from FeCl₃) → Fe(OH)₃ (colloidal sol)

In this reaction, the Fe³⁺ ions from FeCl₃ adsorb onto the surface of the Fe(OH)₃ particles and help to keep them dispersed, forming a stable colloid.

Applications of Peptisation:
- It is used in the laboratory to prepare colloidal solutions from precipitates.
- It helps in the purification of colloidal sols by maintaining their stability.

Conclusion:
Among the various methods of preparing colloidal sols, peptisation is a unique process that involves the conversion of a precipitate into a colloid by the addition of a suitable electrolyte. It is particularly useful when dealing with freshly prepared precipitates.

Therefore, the correct method of preparation of a colloidal sol in this case is:
Peptisation