The general impurities present in a colloidal dispersion are mainly the suspended particles and the electrolytes. The Suspended particles could be easily separated by a simple filtration.
The following methods are employed for purification of a colloidal solution from electrolytes.
2. Electro 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.
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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.
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 of dfferent sizes and preparing corresponding ultra filters, it is also possible to separated a mixture of colloidal solutions having particles of different sizes
Colloidal particles and crystalloid, both do not settle under gravitation and exhibit Brownian (zig-zag) motion. The continuous zig zag motion of the colloidal particles prevents the colloidal particles from settling under gravity. But if one considers the velocity of this motion colloids move rather slow in comparison to crystalloids. Hence when a sol is placed in a ultra-centriguge and rotated at a speed of 15000 or more revolutions per minute, the colloids settle at the bottom of centrifuging tube leaving the electrolyte free. This suspension can be reconverted into a colloidal solution, employing usual dispersion methods of heating, mechanical stirring etc.
6.Optical Property-Tyndall Effect
7.Kinetic Property-Brownian Movement