Question

describe preparation, properties and purification of collides​

Answer 1

Preparation methods of colloidal solution:

Lyophilic & Lyophobic Colloids:

Lyophilic colloids can be prepared by just heating the solid with the liquid dispersion medium.

Whereas the lyophobic colloids cannot be prepared just by heating, they need to be prepared by some special methods.

The two methods by which the lyophobic colloids can be prepared are as follows:

Dispersion method

Aggregation method

Dispersion Method:

In this type of preparation method, the larger particles are broken down into particles of the range of colloidal particle size.

Some of the dispersion methods are:

Mechanical Dispersion:

In this method, solid and the liquid dispersion medium are poured into the colloidal mill to form a colloidal sol.

Aggregation methods:  

In this method, small particles are aggregated to form colloidal size particles.

Double Decomposition:

Example: Arsenious oxide is mixed with hydrogen sulfide to form arsenic sulfide sol. The excess amount of hydrogen sulfide is removed by passing stream of hydrogen.

As2O3 + 3H2S → As2S3 (sol) + 3H2O

Reduction:

Example: Noble gases are reacted with organic reducing agents like ethanol, tannic acid, formaldehyde to form their respective sol.

AgNO3 + tannic acid → Ag-sol

Oxidation:

When hydrogen sulfide is passed through a solution of sulfur dioxide it forms a sulfur sol.

2 H2S + SO2 → 2H2O + S

Purification of Colloidal Solution

The following methods are commonly used for the purification of colloidal solutions.

(1) Dialysis

(i) The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane is called dialysis.

(ii) It’s principle is based upon the fact that colloidal particles can not pass through a parchment or cellophane membrane while the ions of the electrolyte can pass through it.

(iii) The impurities slowly diffused out of the bag leaving behind pure colloidal solution

(iv) The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag.

(v) Dialysis can be used for removing  from the ferric hydroxide sol.

(2) Electrodialysis

(i) The ordinary process of dialysis is slow.

(ii) To increase the process of purification, the dialysis is carried out by applying electric field. This process is called electrodialysis.

(iii) The important application of electrodialysis process in the artificial kidney machine used for the purification of blood of the patients whose kidneys have failed to work. The artificial kidney machine works on the principle of dialysis.

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Answer 2

Answer:

methods of preparation of colloids:

• Dispersion Method:In this type of preparation method, the larger particles are broken down into particles of the range of colloidal particle size.Some of the dispersion methods are: Mechanical Dispersion:In this method, solid and the liquid dispersion medium are poured into the colloidal mill to form a colloidal sol.
• Peptization:In peptization method, some electrolytes are added to form a colloidal sol from a freshly prepared precipitate.

Example: Ferric chloride is added to freshly prepared precipitate of ferric hydroxide which converts it into a colloidal sol of reddish brown in color.

• Aggregation methods: In this method, small particles are aggregated to form colloidal size particles.

Double Decomposition:

Example: Arsenious oxide is mixed with hydrogen sulfide to form arsenic sulfide sol. The excess amount of hydrogen sulfide is removed by passing stream of hydrogen.

As2O3 + 3H2S → As2S3 (sol) + 3H2O

Reduction:

Example: Noble gases are reacted with organic reducing agents like ethanol, tannic acid, formaldehyde to form their respective sol.

AgNO3 + tannic acid → Ag-sol

Oxidation:

When hydrogen sulfide is passed through a solution of sulfur dioxide it forms a sulfur sol.

2 H2S + SO2 → 2H2O + S

Properties of a colloid

Colloids are big enough to scatter a beam of light passing through it and make its path visible. They do not settle down when left undisturbed, that is, a colloid is quite stable. They cannot be separated from the mixture by the process of filtration.

Some of the methods of purification of a colloidal solution are stated below:

• Dialysis:The process in which the ions are removed from the solution by the phenomenon of diffusion through a permeable membrane is known as dialysis. In this process, a sol consisting of ions or molecules is filled in a permeable membrane bag, dipped in the water. The ion from the solution diffuses through the permeable membrane. Because of the continuous flow of water, the concentration of electrolyte outside the membrane is neutralized. For e.g.: Ferric hydroxide sol is purified by using this method.
• Electrodialysis:In this process dialysis of colloidal solution is carried under the effect of electric field. Some potential is applied between the metal screens that support the membranes. Due to this potential, the speed of the ions moving in the direction of opposite electrodes is increased. Thus the rate of dialysis is increased. This method is not useful for the non-electrolyte impurities like urea etc.
• Ultrafiltration:If filter paper is made with the colloidal or some regenerated cellulose like cellophane, the size of the pores is decreased. This modified form of filter paper is known as ultrafilter. The process in which the particles of the solution are removed out from the liquid medium by electrolysis and by using this ultrafilter is known as ultrafiltration. It is a slow process. To speed up the process some external gas pressure has to be applied.

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