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Schlenk: Filtration
For a filtration under inert conditions a glastube with a bottom cap is used. There are different pore sizes of the filter, which are indicated by numbers. The higher the number, the smaller are the pores.
They're also called frits. A frit with Schlenk valves is always preferred to those without.
Yield of the residue
The frit is plugged into a schlenk valve and sealed with a dummy flask on the other end. Again it's important to make sure, all cuts are greased, turned and secured. This set up is connected below and above the filter to the schlenk line and schlenked three times. Its best to evacuate above and below the filter. After the preparations are finished, the filter and the flask containing the mixture to be filtered are connected under counterflow. Now all valves of the frit are closed, to avoid any leakage into the tube to the line. Those of the flasks are left open to provide pressure balance. Now the set up is unscrewed, swung around a bit to ensure a suspension of the solid and turned around to start the filtration. If the filtration is too slow, the pressure in the lower flask may be reduced. To achieve this, the valve of the flask is closed and vacuum is applied to the tube and closed again, leaving only vacuum within the tube. Now the valve of the flask is opened carefully to speed up the filtration process. To wash the residue the tube above the filter bottom can be removed and washing agent applied under counterflow. It's important not to wash the residue with a solvent, which it is soluble in.
Yield of the filtrate
The filtration can be optimized by adding 2-3 mm of Celite to the filter bottom. This is especially useful for very small particles or substances that tend to clog the filter. Because of the Celite the set up cannot be turned around, so angular pieces are used to connect the flask containing the mixture to be filtered. First, the targeted flask is connected to the frit and secured. Now the angle pieces are connected and sealed with a dummy flask. To avoid dispersion of the Celite a tube to the line is connected above and below the filter bottom. Vacuum should always be applied from below and inert gas always added above the Celite. After the set up is schlenked three times, the flask containing the mixture is added under counterflow, whereas the valve beneath the filter should be closed to provide an exclusive flow through the upper valve. Before starting the filtration the lower valve is opened and the upper valve is closed. By tilting the valve the mixture is drawn to the filtering part. It's important to always move the flask In a way, that no mixture gets into the valve. The flask should be swung around to ensure suspension of the solid and only tilted slowly to avoid dispersion of the Celite. Again, if the filtration speed is too slow, the pressure can be reduced in the same manner as described above. Now the residue should be washed with solvent to solve product, that adhered to the Celite. If the residue has further use, Celite shouldn't be used, because it's hard to separate it from the residue.