Engineering Chemistry S Datta

280 ENGINEERING CHEMISTRY
Preparation of Colloidal Solutions
The main methods of preparation of colloidal solutions are (a) Condensation method
(b) Dispersion method.
(A) Condensation Method: In this method, a large number of particles of molecular
size are allowed to coalesce or condense to form bigger particle of colloidal dimension. The
various processes used are:
(i) By exchange of solvent: Sulfur or phosphorus is not directly soluble in water. If an
alcohol solution of S or P is poured into water, colloidal solution of S or P is formed.
(ii) By changing physical state: Hg or S sols are produced by allowing their vapours to
pass through cold water. Ammonium citrate is added to stabilize the solution.
(iii) By controlled condensation: Prussian blue sol is obtained by precipitating it in
presence of a protective colloidal system like starch.
(iv) Chemical methods: (a) by double decomposition, colloidal solution of arsenious sulfide
is made by passing H 2
S gas, through a suspension of As 2
O 3
in water, until saturation.
As 2
O 3
+ 3H 2
S ⎯⎯→ As 2
S 3
+ 3H 2
O.
Sols of insoluble inorganic salts are prepared in this way.
(b) By reduction: Colloidal solutions of metals such as Ag, Au, Pt in water can
be prepared by reducing aqueous solution of their salts by reductants like formaldehyde,
carbon monoxide, hydrazine etc.
Thus gold sol is obtained by treating a solution of gold chloride with formaldehyde
(0.3 %).
2AuCl 3
+ 3HCHO + 3H 2
O ⎯⎯→ 2 Au(sol) + 3HCOOH + 6HCl
The liberated acid is neutralized by K 2
CO 3
to stabilize the sol.
(c) By hydrolysis: Colloidal solutions of many oxides and hydroxides of iron, aluminium
can be prepared by this method. On adding a freshly prepared saturated solution
of FeCl 3
dropwise on boiling water, hydrolysis of FeCl 3
takes place with the formation
of deep red colloidal sol of ferric hydroxide.
FeCl 3
+ 3H 2
O ⎯⎯→ Fe(OH) 3
(sol) + 3HCl
(d) By oxidation: Sulfur sol is prepared by passing H 2
S through a solution of SO 2
in
water until the smell of SO 2
is removed. The solution on boiling with saturated
NaCl solution precipitates out S, on filtration, which is washed free of Cl – and
finally dissolves on further addition of water.
(B) Dispersion method: In this method, the bigger or large particles are broken down
to colloidal dimension and dispersing them in a medium to form colloidal solution. The methods
used are:
(i) By mechanical dispersion: Many substances of large size are ground and dispersed in
a medium to get a coarse suspension which is then passed through a colloid mill, which is
essentially two metal discs rotating in opposite directions at high speed with a very narrow
clearance.
The space in between the metal discs breaks down the particles to colloidal size. Black
ink, paints, varnishes, ointments dyestuffs are prepared by this method.