4.3: Filtration
Filtration is a physical separation process that involves passing a suspension through a porous medium to separate solids from fluids. During filtration, solids collect on the porous medium while liquids, also collectively known as the filtrate, pass through. The filtration medium is selected based on the filtration purpose, quantity, and nature of the precipitate. The general criteria for a suitable filtering medium are that it is inert, mechanically strong, nonabsorbent toward dissolved materials and permissive toward rapid filtration.
The simplest filtration apparatus consists of filter paper fitted in a long-stemmed funnel sitting above a beaker. The solution to be filtered is poured down a glass rod onto the filter paper. The filtrate is collected in the beaker, and the solid is retained on the filter paper. Any solid that adheres to the glass rod or the beaker with the original solution is dislodged using a rubber policeman. Because filter paper is hygroscopic, ashless or low-ash filter paper is preferred when weighing is required. Ashless filter paper is also employed in gravimetric procedures that involve igniting the solids before weighing.
Alternatively, the solid can be collected in glass or silica crucibles containing a porous glass disc. The solid is transferred to the crucible fitted into a Buchner flask, and filtration is performed under suction. After filtration, the crucible is dried and weighed directly. The weight difference of the crucible before and after filtration gives the mass of the collected solid.
