Answer the question on basis of passage given below:

Lyophobic sols like those of metals (Au, Ag, etc) are unstable and are easily precipitated by addition of electrolytes. However, it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatine, etc. to lyophobic colloids (like a metal sol) render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as 'protection' and the lyophilic colloids are termed as Protective colloids. It is believed that the protective action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid. Evidently, smaller the gold number of a protective colloid, the greater is its protective action.

Evidently, which of the following methods cannot be used for purification of colloidal solution?

Peptization

The correct answer is option 3. Peptization.

The method that cannot be used for the purification of a colloidal solution is peptization.

Let us look at each of the given options:

1. Dialysis: Dialysis is a method used to separate colloids from dissolved substances or ions in a solution by allowing them to pass through a semi-permeable membrane. It is an effective method for purifying colloidal solutions by removing impurities or excess electrolytes.

2. Electrodialysis: Electrodialysis involves the use of an electric field to selectively move charged particles, ions, or colloids through a membrane. It can be used to purify colloidal solutions by selectively removing impurities or controlling the concentration of ions.

3. Peptization: Peptization is not a purification method; instead, it is a process used to convert a precipitate into a colloidal solution by adding a suitable electrolyte or dispersing agent. It does not involve the removal of impurities or purification of a colloidal solution.

4. Ultrafiltration: Ultrafiltration is a method that uses a semi-permeable membrane to separate colloids from larger particles or solutes based on size. It is commonly used for purifying colloidal solutions by removing larger particles or aggregates.

Therefore, the method that cannot be used for the purification of a colloidal solution is peptization.