Filtration through Celite

Filtrations through Celite are commonly employed in synthetic inorganic chemistry to remove fine solids such as metal salts from reaction mixtures. They are particularly useful for large scale reactions where a standard cannula filtration would take too long.

Step 1: A Schlenk flask equipped with a stir bar, glass filter frit with Celite (pre-dried in an oven), and Schlenk cap is greased, assembled and cycled onto the Schlenk line. It is recommended to leave the glassware under vacuum for at least 1 hour to ensure that the frit and Celite is thoroughly dried.

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Cycling the frit and receiving flask under vacuum.

Step 2: Once cycled onto the Schlenk line, the apparatus is carefully backfilled with inert gas, and the Schlenk cap is replaced with a rubber septum.

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Filter frit under inert gas.

Step 3: The reaction mixture to be filtered is transferred to the filter frit via cannula transfer.

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Cannula transferring the reaction mixture into the filter frit.

Step 4: Once the reaction mixture has been transferred to the filter frit, the bleed needle is removed but the cannula is kept in place. A partial static vacuum is created in the tubing connecting the receiving flask to establish a pressure differential, allowing the mixture to be filtered. This may have to be repeated several times depending on the quantity of suspension to be filtered.

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Completed filtration.

Step 5: Once the filtration is complete, additional solvent can be transferred either into the original reaction flask or directly into the filter frit to further extract the solids. The receiving flask is then put back under inert gas, the cannula is removed and cleaned, and the filter frit is replaced with a greased glass stopper ready for further manipulations.

Hints and tips

  • For reactions mixtures that are too thick to be transferred by cannula, it may be necessary to attach the filter stick and receiving flask directly to the transfer flask and carefully turn the set-up 180° to pour the reaction mixture through the glass frit. A small plug of oven-dried glass wool can be placed above the Celite in the filter stick to prevent it from moving.
  • The same technique can also be used with SiO₂ or Al₂O₃ as a means of removing polar impurities.