By Sunil Bhardwaj

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There are three main methods for preparation of colloidal solutions:

#Dispersion (Size Reduction)Condensation (Aggregation)Chemical
1Mechanical DispersionExcessive coolingDouble decomposition
2Electro-DispersionExchange of SolventHydrolysis
3PeptizationOxidation
4Reduction

Mechanical Dispersion:

Electro-Dispersion:

Peptization: It is the re-dispersion of the coagulated sol. Peptization is defined as the process by which a colloidal solution can be obtained by dispersing the coagulated mass in which the colloidal particles pre-exist.

Similarly a precipitate of silver iodide can be peptised by shaking with a dilute solution of either KI or AgNO3. When very dilute HCI acid is added to the freshly precipitated aluminium hydroxide, Al(OH)3, a colloidal solution of aluminium hydroxide is obtained. The acid should be in small quantity.

Excessive cooling: This method is employed to get colloidal solution of ice in organic solvent like ether and chloroform. This is done by freezing a solution of water in the given solvent. The molecules of water can no longer be held in solution. They collapse together to yield colloidal sized particles.

Exchange of Solvent: There are certain substances which are more soluble in one solvent as compared to another. Colloidal solution of such substances can be obtained by pouring the solution of the substance from more soluble to the less soluble solvent e.g. (a) Sulphur and phosphorus are more soluble in alcohol than in water. If their alcoholic solution is added to small quantity of water, hydrosol is formed. (b) Phenolphthalein is soluble in alcohol but not in water. If its alcoholic solution is added to water, milky solution is obtained which is a sol of phenolphthalein.

Double decomposition: A colloidal solution of arsenious sulphide (As2O3 ) is obtained by adding water saturated with H2S gas, to a solution of arsenius oxide. (one gm. in 500 ml of water), $${ As }_{ 2 }{ O }_{ 3 } + 3{ H }_{ 2 }S \longrightarrow { As }_{ 2 }{ S }_{ 3 } + 3{ H }_{ 2 }O$$

Hydrolysis: A colloidal solution of ferric hydroxide is obtained by adding 2 or 3 ml of 30% solution of ferric chloride to about 500 ml of hot boiling water. $$Fe{ Cl }_{ 3 } + 3{ H }_{ 2 }O \longrightarrow \underset { deep \ red }{ Fe{ \left( OH \right) }_{ 3 } } + 3HCl$$

Oxidation: A colloidal solution of sulphur is obtained by passing H2S gas through a solution containing some oxidising agent, say dilute HNO3 or by passing H2S into a solution of sulphur dioxide in water. $${ H }_{ 3 }S + \left[ O \right] \longrightarrow { H }_{ 2 }O + S$$ $$S{ O }_{ 3 } + 2{ H }_{ 2 }S \longrightarrow 3S + 2{ H }_{ 2 }O$$

Reduction: A colloidal solution of gold (violet coloured) may be obtained by adding stannous chloride solution (reducing agent) to a solution of gold chloride. $$2Au{ Cl }_{ 3 } + 2Sn{ Cl }_{ 2 } \longrightarrow \underset { violet }{ 3Sn{ Cl }_{ 4 } } + 2Au$$