Filtration through Celite

Filtrations through Celite (or Kieselguhr) are commonly employed in 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 flame dried under dynamic vacuum. Avoid directly heating the glass frit for too long to prevent cracking.

Assembling a filter frit and Schlenk flask.

Step 2: Once cooled to room temperature, the apparatus is carefully backfilled with inert gas, and the Schlenk cap is replaced with a rubber septum.

Backfilling the filter frit with inert gas.

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

Transferring the reaction mixture to the filter frit via cannula transfer.

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

Pulling a static vacuum on the receiving flask to initiate the filtration. 

Step 5: Once the filtration is complete, additional solvent can be transferred either into the original reaction flask or directly into the filter stick to wash the solids. The receiving flask is then put back under inert gas, the cannula is removed and cleaned, and the filter stick 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 (see below). This method however is not suitable if Celite is being used. Filter frits without a gas inlet side-arm are also common but are generally used in a similar way as described above.

Filtering reaction mixtures by directly attaching the filter frit to the transfer flask and rotating the filter apparatus 180°