The process of separating solids or suspended particles from liquids by passing through a filter medium is called filtration.
The liquid obtained after filtration is called filtrate.
The substance which is left on the filter paper after filtration is called residue.
Types of Filtration:
Many of filter mediums can be used for filtration. The selection of medium to be used is decided by the nature of the precipitate. Most convenient ways of filtration are
i) Filtration through filter paper ii) Filtration through filter crucible
Filtration through Filter Paper:
A paper used for filtration purpose is called filter paper.
Whattman Filter paper:
Whattman Filter paper No.41 and 42 is most commonly used for this purpose.
Filtration by a glass funnel and filter paper is slow process.
Pore sizes of filter paper:
The filter papers are available in a variety of porosities i.e. pore sizes. Pore size to be used depends on the size of the particles in the precipitate.
Folding of filter paper:
The first fold should be along the diameter of the paper, the second fold should be such that edges do not quite match. This provides a cone with threefold thickness halfway around and one thickness the other halfway around, and an apex angle very slightly greater than 60°. The paper may then be inserted into 60 degree funnel, moistened with water and firmly pressed down. The filtering operation could be very time consuming if it were not aided by a gentle suction as liquid passes through down the stem.
Important Points of Filtration Method:
(1)The filter paper should be large enough to accommodate one fourth to one half full of precipitate at the end of filtration.
2) The funnel should, in turn, be large enough for its rim to extend 1 to 2 cm above the top circumference of the paper.
3) If the process of filtration is to run smoothly, the stem of the funnel should remain continuously full of liquid as long as there is liquid in the conical portion.
4) The stem of the funnel should be several inches long as that it can extend a few centimeters down into the receiving beaker. In this way, the filtrate runs down the side of beaker without splashing.
Increase in Rate of Filtration:
To increase the rate of filtration through conical funnel a fluted filter paper should be used. This can be prepared through turning a filter paper into a hand fan like arrangement.
Filtration through Filter Crucibles
A convenient way to filter a precipitate is by suction through a crucible. Two types of crucibles are generally used.
(1) Gooch Crucible
It is made of porcelain, which has a perforated bottom, covered with filter paper cut to its size (2.3 a). The filtration can be done quickly by placing the Gooch crucible in a suction filtering apparatus. It is useful for the filtration of precipitates, which need to be ignited at high temperature. If its perforations are covered with asbestos mat then it may be used to filter solutions that react with paper e.g., concentration HCl and KMnO4 solutions.
(2) Sintered Glass Crucible
It contains a porous glass disc seated into the bottom. It is very convenient to use because no preparation is needed as with the Gooch crucible.