Filtration is a technique commonly used for separating solids from liquids. Buchner funnel filtration is used in vacuum filtration. A Buchner funnel incorporates with a water aspirator for filtration.
The Buchner funnel is flat at the bottom, porous and porcelain bowl, circular at the base and with a small stem. The Buchner funnel stem has a rubber stopper that is inserted into the open end of a filter flask, on the sidearm.
Buchner Funnel Set-up
The Buchner funnel filtration process occurs when a filter paper, circular and equal to the diameter of the bowl, is placed under the bottom of the bowl. The container is then wetted appropriately with a solvent that seals the stem and the filter paper onto the base of the bowl before filtration.
During filtration, a horse pipe is used to attach the filter flask sidearm onto the vacuum aspirator placed in the laboratory or hood. The vacuum aspirator is responsible for the suction of liquids from the solution, through the filter paper.
The horse pipe used during the Buchner funnel filtration process should be heavy enough to sustain the atmospheric pressure.
This pressure is as a result of turning on the water aspirator which leads to suction. Most importantly, a heavy horse pipe prevents the collapse or pinching of the filtration system.
Buchner Funnel Filtration Process
Mix the solids into the water in a bottle or bowl. Let the mixture to settle down. This will allow most of the solids in the mixture to settle at the bottom of the container or bowl. Now slowly pour the liquid (which always on top of the mix) into the filter paper.
This is faster and does not clog the filter paper because most of the solids would have remained on the bottom of the bowl or bottle. A glass rod can be used to prevent the liquid from spluttering (pour the liquid down the rod).
After the supernatant water is done, pour the solid mixture into the filter funnel and scrape it using a spatula. Then rinse the spatula, the bowl or bottle and the glass rod with water and filter the washings as well. This is useful and ensures all the solid residue remains in the filter paper.
Similarly, great care should be taken while handling a wet filter paper. This is because the wetness causes it to tear easily. After the Buchner funnel filtration is done, put the solid residue on a watch glass for drying.
Test The Filtrate for Purity
Test your filtrate to ensure no solid product is present. The test depends on the solids under separation. For instance, Ba2+ separation forms a BaS04 precipitate after removal. Therefore, Na2SO4 is used.
Buchner funnel filtration is successful if no precipitate forms after the Na2SO4 is added to the filtrate. Formation of a precipitate indicates that Ba2+ is present in the filtrate. Add some precipitating reagents and filter the mixture once more till no precipitate is formed.
Buchner funnel filtration forbids one to remove the funnel while the system is under vacuum. This will flash the water back into the filtrate and damage the filter paper which consequently will lead to the loss of the solid.
Carefully turn off the water aspirator, remove the filter paper slowly to avoid tearing. Label the filtrate and the solid residue and save them in the lab.