Filter Media:
The surface upon which solids are retained in the process of filtration is known as 'filter medium'.
A good filter media should possess the following characteristics:
It should not absorb the constituents of the liquid preparation undergoing filtration.
It should be resistant to corrosive action of the liquid.
It should have a smooth surface for easy discharge of cake.
It should allow free flow of liquid.
It should be resistant to chemical reactions with the contents of the liquid to be filtered.
It should have sufficient mechanical strength to resist the damage that occurs during filtration of liquid under pressure.
The selection of filter media depends on:
The quantity of the material to be filtered.
The nature of the product to be filtered i.e. its viscosity etc.
The size of the solid particles to be retained on the filter
The purpose of filtration e.g. weather sterility required or not?
The various filtering media employed for filtration are:
Filter paper :
Filter paper is most commonly used as filter media for general filtration.
The filter papers are classified as coarse, medium and fine according to their pore size.
For the filtration of pharmaceutical preparations, a fine grade of filter paper is used.
The main drawback of a filter paper is that it has an absorbing property and also it sheds fine particles into the filtrate.
Cotton wool :
Cotton wool is generally used for filtering of moderately coarse particles.
The disadvantage of cotton is that it sheds fine cotton hair in the filtrate.
This problem can be solved to a large extent by using long staple cotton, wool of long staple and wetting the cotton wool plug with liquid before its use.
Glass wool :
It consists of fine fibers of glass which are resistant to corrosive liquids.
So it is used for filtering corrosive liquids, such as strong acids, alkalies and oxidizing agents e.g. solutions of chromic acid and potassium permanganate.
Asbestos :
It is used for the same purpose as glass wool.
It is commonly used in gravimetric analysis for collecting fine precipitates which are to be subsequently ignited.