Vacuum Filtration
Buchner filtration
Buchner filtration is a good technique where the isolation of the residue is desired. Buchner filtration make use of a filter paper which is placed over the bottom surface of the Buchner flask which contains a number of holes. The filter paper is sucked down onto the bottom surface of the funnel, with the filtrate passing through the filter paper and the residue being retained. However, it is common for small amounts of residue to pass into the filtrate, particularly around the edges of the filter paper. This imperfect removal of reside, means it is not a suitable method of filtration where collection of the filtrate is desired.
Setup
Buchner filtrations are carried out by fitting the funnel into a Buchner flask setup.
Fit vacuum tubing to the vacuum outlet of the Buchner flask and seurely clamp the flask in place around the neck. Take care when clamping to ensure that the clamp is not above the top of the flask neck, especially when using a Buchner ring, as this may reduce the effectiveness of the filtration
Attach the vacuum tube to the vacuum connector, but keep the vacuum turned off at this stage.
Place a Buchner ring or cone into the top of the Buchner flask.
Find an appropriately sized filter paper for the Buchner funnel. The paper should cover all of the base of the funnel, without overlapping on the sides of the flask. The paper must lay flat on the base of the funnel.
Insert the Buchner funnel through the cone or ring into the flask.
Add a small amount of solvent onto the filter paper
Slowly open the vacuum. The filter paper will be 'sucked' flat and the Buchner funnel will become secured to the Buchner flask, due to the reduced pressure inside the flask. Turn the vacuum to avoid evaporating the solvent from the filter paper.
Running
These instructions assume a conical flask containing a suspension of material to be filtered. The instructions may need to be adapted for other setups.
Carefully pour in some of your suspension onto the centre of the filter paper in the Buchner funnel and carefully open the vacuum. Check that the filtration is proceeeding. Pouring into the centre of the filter paper is desirable, as it minimises any solid being drawn under the sides of the filter paper.
Continue to pour the remaining material onto the filter paper. It is a good idea to swirl the flask before pouring to try and get any solid material into the suspension so that it transfers into the funnel.
Once all the liquid has been transferred into the funnel, it is likely that some residual solid remains in the flask. Ideally the flask is 'rinsed out' using small volumes of additional solvent. The choice of solvent (and volumes used), often needs careful consideration (see notes below).
The isolated product is also often 'washed' in the solvent at this stage. Details on this procedure are listed separately.
Allow the vacuum to pull through all the solvent in the sinter to be left with a reside on the filter paper.
At this stage, the filtration is usually followed by a brief 'drying over vaccuum' in order to remove residual solvent in the material.
Washing a precipitate over vacuum
Often when collecting a product via vacuum filtration, it is desirable to wash the collected material using a solvent. Washing the residue allows the removal of unwanted material, such as small amounts of the filtrate to be removed from the residue. Careful choice of the solvent is important to ensure that product is not lost through dissolving portions of the residue (see notes below).
In order to effectively wash a residue:
If using a different solvent, it may be sensible to empty the collection flask of the filtrate. This is especially important where the material would go into different waste streams.
Under vacuum, add some of the washing solvent onto the residue on the filter paper in the Buchner funnel, and allow to suck through the solid. Repeat as required.
Drying material over vacuum
In order to effectively dry material over vacuum, it is nescessary to empty the collection flask of any solvent. As many of the solvents used in the lab are volatile, prolonged vacuum filtration would cause the flask contents to evaporate. This reduces the effectiveness of any attempts to dry the residue over vacuum.
Turn off and carefully release the vacuum to the setup. The easiest way to release the vacuum is to carefully disconnect the rubber tubing from the vacuum pump connection, which will allow air to enter the previously evacuated system.
Remove the Buchner funnel and place this somewhere secure, where it isn't going to be knocked over or broken.
Empty the collection flask of the filtrate.
Reclamp the flask back in place and reconnect the vacuum tubing to the pump.
Return the Buchner funnel to the setup.
Open up the vacuum and allow the residue to begin to be sucked dry for a few moments.
When the material looks visibly dry, break up the residue using a spatula whilst still over vacuum.
Once the residue is dry, turn off the vacuum and carefully release the vacuum.
Isolate the material.
Isolating the residue
The filter paper can be carefully lifted from the funnel using a spatula and placed onto a watch glass. Allow the paper to dry, before trying to transfer the solid from the paper to the watch glass, and break up to allow to dry. It is good practice to weigh the empty glassware prior to transfer. The mass of the watch glass and product can be periodically recorded, until a constant mass is obtained, which is usually a good indication that the material is now dry.
Common issues
Flask becoming full
When filtering, it is possible that the receiving flask could become filled, and would need emptying before continuing. With a Buchner flask, you should not let the solvent level go above the stem of the sinter funnel, or let the solvent level approach the side arm of the Buchner flask. If you are approaching either of these situations, stop the filtration, and transfer the filtrate contents before resuming the filtration.