Engineering Chemistry S Datta

COLLOIDS 283
To accelerate this slow process of purification and also to achieve a higher degree of
purity the dialysis is carried out in a direct current electric field, and the process is called
electrodialysis. Electrodialysis is of no use in separating non-electrolytic impurities.
HO
2
–
D E
+
Sol n
A
B
HO
C
2
Fig. 12.4 Electrodialyser (diagrammatic).
(ii) Ultra-filtration. Colloidal particles can pass through ordinary filter papers. But
when ordinary filter paper is impregnated with a solution of gelatin or collodion and hardened
by formaldehyde the pores become smaller, so that colloidal particles cannot pass through it.
Such a filter paper is called ultrafilter paper. The sol when poured over the ultrafilter, electrolytes
can pass through, but the colloidal particles are retained and form a slime over it. The
slime when brought in contact with water, disperses freely and forms colloidal solution. This
process of purifying colloids from the crystalloids using ultrafilters is called ultrafiltration.
By using impregnating solutions of different concentration graded ultrafilters can be
prepared which is capable of separating impurities of different sizes.
Properties of Colloidal Solutions
Colloidal solutions possess some characteristic properties. Important ones are described
below:
(i) Physical properties
(a) Colloidal systems are heterogeneous, intermediate between true solution and
suspension and contain two phases—dispersed phase and dispersion medium.
(b) Colour. Colour and appearance depend on the size, shape, refractive index and
some other properties of the dispersed phase. Gold sol of very fine particles is red
and of coarser particles is blue in colour; sulfur sols may be colorless to faint and
deep yellow in reflected light and reddish in transmitted light.
(c) Shape. Although the most common shape of the particles are spherical; but highly
asymmetrical shape is also found. Vanadium pentoxide particles are rod-shaped,
RBCs (red blood corpuscles) normally are double concave plates and give red
color, but in sickle-cell anaemia they become new-moon shaped. Colloidal particles
are readily filterable through ordinary filter paper.
(d) Adsorption. Generally colloidal particles are either positively or negatively
charged, so they attach to a variety of molecules on their surfaces.
This property of colloid has been utilized in dyeing fabrics where the fabrics are
first treated with colloidal suspension which adsorbs dye.