Preparation of Colloidal Solutions: Meaning, Difference, Types

Preparation of Colloidal Solutions: As we know, all solutions contain two entities in them, a solvent and a solute, mixed together to form a solution. A colloidal solution or a colloidal state is a heterogeneous system wherein there are two phases, the dispersed phase and the dispersion medium. The reason why colloids are referred to as solutions is because of these two phases that exist in them.

So, is colloid really a solution? While it can be referred to as one, there is an integral difference between the two. In colloids, the particle sizes of the dispersed phase (or a solute) are much bigger than those in a normal solution. So, when in a true solution, the constituent particles are ions or small molecules; in a colloid, the dispersed phase may constitute particles belonging to a single macromolecule or an aggregate of several ions, atoms, or molecules. This increases the size of the dispersed phase in a colloid. So, depending upon the size of the particles in the dispersed phase, the solutions are of three classes. They are namely, true solution, colloidal solution, and suspension.

Colloidal Solution State and Colloidal Solution System

The colloidal state can be defined as: ‘The state of substance wherein the dispersed phase or the solute is distributed in a dispersion medium or a solvent.’

Colloids are classified based on different parameters:

1. Classification based on the physical state of both phases dispersed phase and the dispersion medium.
2. Classifications based on the nature of interaction between both phases: dispersed phase and dispersion medium.
3. Classifications based on the type of particles in the dispersed phase alone.

Such classifications help in identifying the different types of colloidal systems formed and how they are unique in their nature.

Difference between Three Types of Solutions

The three types of solutions can be differentiated as follows:

Lyophilic & Lyophobic Colloids

While all classifications hold importance, classifications based upon interactions between the dispersed phase and the dispersion medium yields two types of colloids:

Preparation of Colloidal Solutions

While colloidal solutions occur in nature, like, for example, blood plasma and milk, they can also be prepared in the laboratory using special methods of preparation. While lyophilic sols can be prepared by general mixing, lyophobic sols require special methods of preparation.

Chemical Methods of Preparation of Colloids

Colloids can be prepared using both physical and chemical methods of preparation.

Physical Methods of Preparation

1. Dispersion methods

There are two types of dispersion methods:

a. Mechanical dispersion: A colloidal mill of rotating discs is used to prepare colloids. The dispersed phase and dispersion medium are put together in the mill, and the phases are ground together to obtain colloidal solutions.
b. Bredig’s arc method or electrical disintegration method: The process involves dispersion and condensation. Here, two rods of suitable metals are immersed into water placed in an ice bath. When an electric discharge is given, the metal vaporises and condenses in the cold water to form particle sizes of a colloidal solution. Stabilising agents are added to stabilise the colloid. Gold sols, silver sols, etc., are prepared by this method.

1. Peptisation

Peptisation involves converting a freshly prepared precipitate into a colloidal sol. The precipitate is shaken with the dispersion medium in the presence of a limited quantity of electrolyte to form a colloidal solution. The electrolyte added is called the peptising agent.

3. Condensation Method

The condensation method is carried out in three ways:

a. Exchange of solvent where a true solution is mixed with an excess of another solvent (in which the solute is insoluble and the solvent is soluble). Example: Sulphur-alcohol solution in water.
b. The excessive cooling method is in which water in the organic solvent is frozen to form a colloidal solution. Example: ice in ether.
c. Condensing vapours in solvent: Vapours of a solution, such as Sulphur and mercury, are passed over cold water to form colloids. A stabilising agent is used in this method.

Chemical Methods of Preparation:

Chemically, colloids are prepared using four methods:

1. Oxidation:

Sulphur colloids or any non-metal colloids can be prepared by oxidation:

\(2{{\rm{H}}_2}{{\rm{S}}\;} + {\rm{S}}{{\rm{O}}_2}\;\; \to 3{\rm{S}}\left( {{\rm{sol}}} \right) + 2{{\rm{H}}_2}{\rm{O}}\)

2. Reduction:

Colloidal sols of metals are prepared in this method. Example: Gold sols.

\(2{\rm{AuC}}{{\rm{l}}_3}\; + 3{\rm{HCHO}} + 3{{\rm{H}}_2}{\rm{O}} \to 2{\rm{Au}}\left( {{\rm{sol}}} \right)\; + 3{\rm{HCOOH}} + 6{\rm{HCl}}\)

3. Double Decomposition:

Sols of inorganic salts are prepared using this method.

\({\rm{A}}{{\rm{s}}_2}{{\rm{O}}_3}\; + 3{{\rm{H}}_2}{\rm{S }}\; \to {\rm{A}}{{\rm{s}}_2}{{\rm{S}}_3}\;\left( {{\rm{Yellow Sol}}} \right) + 3{{\rm{H}}_2}{\rm{O}}\)

4. Hydrolysis:

Hydroxide sols are prepared using this method.

\({\rm{FeC}}{{\rm{l}}_3} + 3{{\rm{H}}_2}{\rm{O}} \to {\rm{Fe}}{\left( {{\rm{OH}}} \right)_3}\;\left( {{\rm{Red Sol}}} \right) + 3{\rm{HCl}}\)

Purification of Colloids

The colloidal solutions prepared either chemically or physically contain a lot of electrolytes and other impurities. Even if the presence of electrolytes and impurities helps in stabilising the colloidal solution, larger amounts of the same can coagulate it. Hence, it is mandatory to reduce these impurities to a minimum level and to the required extent.

The purification of colloidal solutions can be defined as:

“The process of reducing the amount of impurities to a minimum required amount.”

Purification of colloids can be carried out by the below mentioned:

a) Dialysis

In the dialysis method, dissolved substances are removed from colloidal solutions by diffusing them through a suitable membrane. The colloidal solution is kept in a parchment paper or cellophane membrane, and the whole bag is suspended in water. The impurities diffuse into the water through the membrane, and the colloids become pure in the process. The apparatus used is called a dialyser.

b) Electro-Dialysis

If dialysis is done in the presence of an applied electrical field, it is known as electro-dialysis. The process of dialysis becomes faster with the applied electrical field. The method is as shown in the figure. The electrodes kept in the container attract ions of opposite charge and can speed up the process.

c) Ultra-Filtration

Ultra-filtration uses special filters to separate colloidal particles from solvent and soluble solutes in the colloidal solution. The ultra-filter paper used in the process has a very small pore size. The filter papers are impregnated with colloidal solution, usually a 4% solution of nitrocellulose present in a mixture of ether and alcohol. This is done to prevent the colloidal particles from escaping. The colloidal solution left in the paper is hardened with formaldehyde and dried. Then, the paper is treated with a fresh dispersion medium to get a pure colloidal solution.

Summary

Colloids are a state of substance wherein the dispersed phase or solute is distributed in the dispersion medium or the solvent. While colloids occur in nature, they can also be prepared using both chemical and physical methods. Physical methods of preparation involve condensation, dispersion, and Peptization methods. Chemical methods of preparation involve oxidation, reduction, hydrolysis, and double decomposition methods. The colloids which are prepared using these methods need purification because they might contain more than the required amount of electrolytes. They are purified using dialysis, electro-dialysis and ultra-filtration methods.

FAQs on Preparation of Colloidal Solutions

Q.1. What is meant by the colloidal solution?
Ans: Colloidal solutions are heterogeneous solutions where the solute is dispersed in the solvent or the dispersion medium.

Q.2. Is colloid a solid, liquid or gas?
Ans: Colloids are solutions whose particle sizes are in-between those of true solution and suspension. They can exist as gas, solid or liquid.

Q.3. What are colloidal solids?
Ans: Colloidal solids are solutions of colloids which exist as solids and contain dispersion medium and dispersed phase. Colloidal solids or solid sol have both dispersion medium and dispersed phases as solids. Example: Gemstone and coloured glasses.

Q.4. Give an example of a colloidal solution.
Ans: Examples of colloidal solutions are fat solids in milk, whipped cream, paints, etc.

Q.5. What are the 3 types of colloidal solutions?
Ans: Three types of colloidal solutions are aerosol, gel, emulsion, and so on.

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