Question

Write briefly about the preparation of colloids by chemical methods.

Answer 1

i) Double decomposition
An Arsenic sulphide Sol is prepared by passing a slowstream of hydrogen sulphide gas through a cold solution of arsenious oxide. This is continued till the yellow colour of sol attains maximum intensity.
$A{s}_2{O}_3+3H_{2}S\to \underset{yellow}{A{s}_2{S}_3}+3H_{2}O$
Excess hydrogen sulphide is removed by passing in a stream of hydrogen.
ii) Oxidation : A colloidal solution of sulphur is obtained by passing $H_{2}S$ into solution of sulphur dioxide.
$2H_{2}S+S{O}_2\to 2H_{2}O+3S$
iii) Reduction : Silver.sols and gold sols can be obtained by treating dilute solution of silver or gold chloride with organic reducing agents like tannic acid or formaldehyde.
$AgN{O}_3\to tannicacid\to AgSol$
$AgCu{l}_3\to tannicacid\to AuSol$
iv) Hydrolysis : Colloidal solutions of the hydroxides of FelCr, Al etc, can be prepared by hydrolysis of their salts. A colloida solution of ferric hydroxide, is obtained by boiling a dilute solution of ferric chloride.
$FeC{l}_3+3H_{2}O\to \underset{redsol}{Fe(OH{)}_3}+3HCl$