Engineering Chemistry S Datta

288 ENGINEERING CHEMISTRY
Hence, there is a potential difference between the particle and the medium. This potential
is called the zeta potential which is responsible for the stability of the colloid.
Application of Colloids
(i) Purification of water. Impure water is treated with alum. Al 3+ ions of alum precipitate
negatively charged clay particles, bacteria, colloidal particles and the impurities.
(ii) Purification of polluted air. Suspended particles, smoke etc., polluted air. Smoke
is negatively charged colloidal suspension consisting of carbon particles. Air is passed
through Cottrell precipitator where the charge is neutralised and carbon settles down
while the hot air passes out through chimney.
(iii) Removal of dirt from sewage. Sewage contains charged dirt particles, which is
passed through tanks fitted with oppositely charged electrodes. By neutralizing the
charge, dirt is coagulated.
(iv) Leather tanning. The raw material skin is positively charged colloidal system,
which is treated with negatively charged colloidal solution of tannin, whereby mutual
coagulation of oppositely charged colloids takes place.
(v) Laundry. Soaps in water form colloidal solution, which adsorb greasy materials,
dirt by emulsion formation and remove dirt from clothes.
(vi) Medicine. Colloidal system of medicines are better assimilated. Blood coagulation
is effected with alum or FeCl 3
.
(vii) Cosmetics. Different types of colloidal systems and emulsion are used in cosmetics.
(viii) Analysis. Noble metals are detected by formation of their colloidal solutions and
observing their colours.
(ix) Artificial rain are made by throwing charged sand on clouds.
(x) Delta formation. Negatively charged sand particles of the river water and Na + , K + ,
Ca 2+ present in the sea water are responsible for the deposition of colloidal particles
where river meets sea.
Highlights:
Dimension of colloidal particles range from 10 –4 to 10 –7 cm.
Purification of colloidal solutions can be done by dialysis, ultrafiltration.
Brownian motion. The random zigzag motion of the colloidal particles in the
suspending medium is called Brownian motion. The motion is rapid for smaller
particles and also for a less viscous medium.
Tyndall effect. When a beam of light is passed through a colloidal solution, the
path of light becomes illuminated due to the scattering of light by the colloidal
particles. This phenomenon is called Tyndall effect.
Schulze-Hardy Law. The coagulating power of an electrolyte depends
predominantly on the valency of the ion bearing a charge opposite to that of the
colloidal particles, and higher the valency of the active ion, the greater is its
precipitating action. This is called Schulze-Hardy law.
Gold number. It is the minimum weight in mg of a protective colloid, which when
added to 1 ml of standard gold sol (containing 0.0053 to 0.0058% gold) also containing
1 ml of a 10% NaCl solution to just prevent its coagulation.
Bancroft rule. The phase in which the emulsifier is more soluble becomes the
outer phase.