Engineering Chemistry S Datta

COLLOIDS 285
Microscope
–
+
Powerful
beam of light
Path is
invisible
Path is
visible
True solution
Colloidal
solution
Electrophoresis
Fig. 12.5 Tyndall effect exhibited by a colloidal solution.
Fig. 12.6 Electrophoresis.
(v) Electrical properties
(a) Electrophoresis. Lyophilic and lyophobic colloid particles are either positively or
negatively charged. In a U-tube, containing a colloidal solution, when a potential
gradient is applied the colloidal particles move towards oppositely charged
electrode. After reaching the electrode, they lose their charge and coagulate there.
The movement of the colloidal particles in an electric field is called electrophoresis.
When the negatively charged particles like CdS, As 2
S 3
metallic sols, organic sols
move towards cathode, the phenomenon is called cataphoresis. Positively charged
particles are Fe(OH) 3
, Al(OH) 3
, basic dyes etc. The speed of the colloidal particle
under the potential gradient of 1 volt/cm is called electrophoretic mobility.
(b) Electro-osmosis. When electric current is passed
through a colloidal solution such that the dispersed + –
phase is prevented from movement, the dispersion
medium begins to move in the electric field. This
Diaphragm
phenomenon is called electro-osmosis (Fig. 12.7).
(c) Isoelectric Point. The colloidal sols of protein are
either positively or negatively charged in acidic or
alkaline solution and hence when electrolysed in a
medium, pH of the medium will dictate the
Endosmosis
migration of the colloidal particle to any electrode.
Fig. 12.7
Hence the pH of the dispersion medium at which
the dispersed particles are neutral and hence they do not migrate when subjected
to an electric field is called isoelectric point. The isoelectric point of protein
depends on pH and ionic strength in the medium. Proteins are least soluble in
isoelectric point and hence coagulate at isoelectric point.
Coagulation of Colloids
Colloid particles are electrically charged, so they repel each other and become stable.
When the charge is neutralised, the particles approach each other to form aggregates and
settle down. The precipitation of colloidal solution is called ‘coagulation’ or flocculation. This
can be achieved by
(i) Adding oppositely charged: Two oppositely charged colloidal solutions when mixed,
mutual coagulation of the two sols takes place.
(ii) Electrophoresis: In electrophoresis the electrically charged colloidal particles migrate
towards the oppositely charged electrode and finally coagulate.