PREPARATION OF COLLOIDS
1.Preparation of lyophilic sols: The colloidal solutions of lyophilic colloids like starch, glue, gelatin etc., can be readily prepared by dissolving these substances in water either in cold or on warming.
2.Preparation of lyophobic sols : Lyophobic sols are prepared by special methods. These methods fall into two categories.
i. Dispersion methods: By splitting coarse aggregates of a substance into a colloidal size.
ii. Condensation methods: By aggregating very small particles into the colloidal particles.
I. Dispersion method
1. Mechanical dispersion
3. Ultrasonic dispersion
1. Exchange of solvents
2. Change of physical state
3. Chemical methods
3.i. Double decomposition
I. Dispersion Methods
1. Mechanical dispersion using colloidal mill
The solid along with the liquid is fed into a colloidal mill. The colloidal mill consists of two steel plates nearly touching each other and rotating in opposite directions with high speed. The solid particles are ground down to colloidal size and then dispersed in the liquid. Colloidal graphite and printing inks are made by this method.
2. Electro-dispersion method: (Bredig's Arc Method)
This method is suitable for the preparation of colloidal solution of metals like gold, silver, platinum etc. An arc is struck between the metal electrodes under the surface of water containing some stabilising agent such as trace of alkali. The water is cooled by immersing the container in a cold bath. The intense heat of the arc vapourises some of the metal which condenses under cold water.
3. Ultra-sonic dispersion :
The sound waves of high frequency are usually called ultra-sonic waves. Ultrasonic waves are passed through the solution containing larger particles. They break down to form colloidal solution.
4. Peptisation :
The dispersion of a precipitated material into colloidal solution by the action of an electrolyte in solution is termed as peptisation. The electrolyte used is called a peptizing agent.
A few examples are
Silver chloride can be converted into a sol by adding hydrochloric acid
Ferric hydroxide yields a sol by adding ferric chloride
II. Condensation methods
1. By exchange of solvent
If a solution of sulphur or phosphorus in alcohol is poured into water, a colloidal solution of sulphur or phosphorus is obtained due to low solubility in water.
2. By change of physical state
Colloidal solutions of certain elements such as mercury and sulphur are obtained by passing their vapour through cold water containing a stabiliser.
3. Chemical Methods
The chemical methods involve chemical reaction in a medium in which the dispersed phase is sparingly soluble. Some of the methods are
(i) Double decomposition: An Arsenic sulphide sol is prepared by passing a slow stream of hydrogen sulphide gas through a cold solution of arsenious oxide. This is continued till the yellow colour of the sol attains maximum intensity.
As2O3 + 3 H2S --- --- -- > As2S3(yellow) + 3 H2O
Excess hydrogen sulphide is removed by passing in a stream of hydrogen
(ii) Oxidation : A colloidal solution of sulphur is obtained by passing H2S into a solution of sulphur dioxide.
2H2S + SO2 ---- -- - > 2H2O + 3 S
(iii) Reduction: Silver sols and gold sols can be obtained by treating dilute solution of silver nitrate or gold chloride with organic reducing agents like tannic acid or formaldehyde.
AgNO3 + tannic acid --- -- > Ag Sol
AuNO3 + tannic acid --- -- > Au Sol
(iv) Hydrolysis: Colloidal solutions of the hydroxides of Fe, Cr, Al etc can be prepared by hydrolysis of their salts. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.
FeCl3 + 3H2O -- --- - -> Fe(OH)3(red sol) + 3HCl