Purification

Introduction to purification of colloids

The following methods are employed for purification of a colloidal solution from electrolytes.

1.Dialysis

2. Electro dialysis

3.Ultra-filtration

4.Ultra-centrifuging

Purification of Colloids: Dialysis

The process of separating a colloidal solution from crystalloid (true solution) is known as dialysis. The property of passing through an animal membrane by the solute and solvent molecules of a crystalloid and inability of colloidal particles was exploited by Graham in designing the apparatus used for purification of a colloid. This apparatus is known as dialyzer. The colloidal solution to be dialyzed is taken in a parchment bag and suspended in a running water trough. The soluble impurities pass through the membrane into the running water leaving pure colloidal suspension in the parchment bag. Completion of dialysis could be ensured by monitoring the conductivity of water in the trough. Presence of ions increases the conductivity of pure water.

Purification of Colloids: Electro Dialysis

The principle behind this technique is same as simple dialysis. Normal dialysis is a slow process and usually takes several hours or sometimes days for completion. Colloidal particles normally have a charge due to adsorption or resting of ions over the surface. Hence diffusing of electrolyte through the membrane is hastend by carrying out the dialysis in the presence of electric field and is known as electro dialysis.

Purification of Colloids: Ultra Filtration

The pore size of ordinary filter paper is >1000 nm allowing the sol particles, which have a diameter of 10 nm - 1000 nm, to pass through them. In the ultra filtration, the filter paper is soaked in a colloidal solution such as gelation or collodion and dried. This reduced the pore size considerably and the impregnated filter paper, known as ultra filter, is used in the purification of colloid, similar to a dialysis membrane. By selectively choosing impregnation suspensions...