Cannula transfers are routinely used during the preparation and manipulation of air and moisture-sensitive compounds to transfer solvents and solutions between flasks. Cannulae are typically made of stainless steel or Teflon and come in a range of gauges depending on the volume of liquid to be transferred.
Step 1: Cycle a solvent ampoule onto the Schlenk line with a minimum of three vacuum-inert gas cycles.
Step 2: Once cycled onto the line and connected to the inert gas inlet, replace the J. Young’s Teflon tap with a rubber septum. Insert a bleed needle into the septum for 5-10 seconds to purge out any air that is introduced into the ampoule. Ensure that the bleed needle only just pierces the septum so that the air/inert gas is forced out of the needle.
Step 3: Insert a cannula through the septum on the solvent ampoule and allow inert gas to purge through the cannula for 10-15 seconds before inserting the cannula into the receiving flask.
Step 4: Insert a bleed needle into the septum of the receiving flask and close the stopcock to stop the flow of inert gas into the flask. Lower the cannula into the solvent and transfer the desired volume into the receiving flask – this is usually estimated based on the size of the flask or time taken to transfer the solvent. Raise the cannula above the level of the solvent in the ampoule to stop the transfer. If an accurate volume of solvent is required, then it is recommended that a syringe or J. Young’s measuring cylinder (essentially a graduated Schlenk flask or ampoule) is used instead.
Step 5: Open the stopcock on the receiving flask to allow inert gas in, and then remove the bleed needle and cannula. Replace the rubber septa with glass stoppers or J. Young’s Teflon taps.
This same technique can also be used to transfer solutions, slurries or liquid reagents between flasks. In these cases, the cannula should be cleaned immediately after use (typically with acetone and water) and placed in a lower shelf of the oven to allow it to dry before its next use.
Step 1: Replace the glass stoppers on the transfer and receiving flask with rubber septa under a flow of inert gas. Insert a cannula in through the septum of the transfer flask and purge for 10-15 seconds before inserting through the septum of the receiving flask.
Step 2: Lower the cannula into the solution of the transfer flask. Ensure that the bottom of the cannula in the transfer flask is higher than the bottom of the cannula in the receiving flask.
Step 3: Insert a bleed needle through the septum on the receiving flask and close the stopcock to stop the flow of inert gas and initiate the cannula transfer. As soon as the first drop of solution passes through the cannula, open the stopcock on the receiving flask and remove the bleed needle to re-equalise the gas pressure and create a syphon.
Step 4: The rate of addition can be controlled by adjusting the relative height of the transfer and receiving flasks – raising the level of the transfer flask or lowering the cannula into the receiving flask will increase the rate of addition and vice versa.
Step 5: Once all of the solution from the transfer flask has been added, remove the cannula from both flasks and replace the rubber septa with greased glass stoppers.
Cannulae can be easily cleaned by flushing them with suitable solvents or quenching solutions. For typical organic reaction mixtures, acetone and water are used, whilst cannulae used to transfer phosphines or thiols are typically washed with bleach. In any case, cannulae should be cleaned immediately after use to prevent blocking.
Using a water aspirator or diaphragm pump to pull a vacuum provides a simple way of sucking/pulling the cleaning solvents/solutions through the cannulae. Ensure that the final rinse is with a low boiling organic solvent (typically acetone) and allow to dry before placing into the lower shelf (or away from clean and dry cannulae) of an oven.