Question

Write two chemical methods for preparation of colloids.

Answer 1

However, there are two principal ways of preparation of colloids:

Dispersion of large particles or droplets to the colloidal dimensions by milling, spraying, or application of shear (e.g., shaking, mixing, or high shear mixing).

Condensation of small dissolved molecules into larger colloidal particles by precipitation, condensation, or redox reactions. Such processes are used in the preparation of colloidal silica or gold.

Chemical Methods of Preparation of Colloids



Hydrophilic or Lyophobic colloidal solutions can be prepared by various chemical techniques such as:

Double Decomposition Technique: When hydrogen sulphide is passed through a solution of arsenious oxide in distilled water, we get a colloidal solution of arsenous chloride.

As2O3 + 3H2S → As2S3 +  3H2O

Oxidation Technique: A colloidal solution of Sulphur is made to pass through an aqueous solution of sulphur dioxide. It can also be obtained by passing the gas through a solution of an oxidization agent such as bromine water as well as nitric acid.

SO2 + 2H2S →  2H2O + 3S

H2S + [O] →  H2O + S

Reduction Technique: Another technique of preparing colloidal solutions of metals such as silver, gold as well as platinum involves the use of reducing agent for reduction of the salt solutions of these metals. Example of reducing agent include stannous chloride.

Hydrolysis Technique: It involves the use of boiling water to obtain a reduced solution of ferric chloride.

FeCl3 + 3H2O →  Fe(OH)3 + 3 HCl

Physical Methods of Preparation

Various physical methods can also be employed to obtain colloidal dispersions. Some of them include

Exchange of Solvent

It involves the formation of a colloidal solution of an element by addition of its alcoholic solution to excess water. This colloidal formation can take place only when the element is more soluble in alcohol as compared to water. Example: When an alcoholic solution of sulphur is made to pass through excess water it yields a colloidal solution of sulfur. This is because the solubility of sulphur is more in water as compared to alcohol