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Schlenk Line - Safety Notes

A Schlenk line (also known as a gas/vacuum manifold) is a glass apparatus commonly used when performing air-sensitive and moisture-sensitive syntheses. The gas side of the manifold allows reactions to be run under an inert atmosphere such as argon or nitrogen, while the vacuum side makes removal of oxygen and organic solvents possible. A solvent trap is typically used to protect the vacuum pump, where an extremely cold substance (usually liquid nitrogen) is used to freeze and contain organic solvents once they are extracted from the reaction.

Schlenk line apparatuses are operated under pressure and vacuum, so there are several risks and safety concerns to always be mindful of with their operation. Below is a list of the most common hazards associated with Schlenk line use (not including potential problems with the sensitive nature of the reactions) and suggestions on how to minimize the hazards.


Implosion Hazard: cracks (sometimes invisible) in glassware may cause parts of your reaction set-up to implode.

            Always check your glassware and manifold for cracks prior to use

Dispose of or repair damaged glassware prior to use

 

Explosion Hazard: Schlenk lines are under constant pressure, so extreme caution must be used when manipulating the line and running a reaction.

To minimize the potential of being harmed, use a Schlenk line in a fume hood and/or behind an explosion-proof shield. Lower the hood sash as much as possible when a Schlenk line is operational.

Never heat a closed system, which may explode due to the increase in pressure.

Gas line of the manifold: Always be sure there is pressure relief in the line (e.g. bubbler).

Vacuum line of the manifold: Once liquid nitrogen is removed from the solvent trap and your vacuum is off, open a valve to release the pressure in the line.  Liquid in the trap may convert to gas, which significantly increases the pressure in the line and makes an explosion more likely.


Cooling Media Hazard: The vacuum portion of a Schlenk line requires a solvent trap to be maintained at a temperature cold enough to rapidly freeze organic solvents, protecting the vacuum pump and further reducing the oxygen present in the manifold. The materials used to cool the solvent trap are cold enough to cause a frostbite hazard. The cooling media also poses a splash hazard, especially while the trap is being initially cooled and small amounts of solvent/cryogen boil off.

Dry ice baths (e.g. dry ice in acetone) are commonly used for this purpose, creating a risk of asphyxiation as well as pressure hazards from the evolution of carbon dioxide gas.

Cryogens, such as liquid nitrogen, have the same hazards described above with the added potential of forming liquid oxygen (described below).


Liquid Oxygen: The presence of oxygen in a liquid nitrogen cooled solvent trap allows liquid oxygen to condense. Liquid oxygen is extremely dangerous, reacting with most organic solvents and the grease on the glassware in addition to adding pressure to the manifold.

Maintaining a trap in liquid nitrogen for more than a few minutes in the absence of vacuum will result in the condensation of liquid oxygen in the trap.

Always ensure your reaction set-up is free of leaks prior to adding liquid nitrogen to cool your trap. A working vacuum gauge is extremely important, and should be used to check that the Schlenk line is holding a good vacuum!

Ensure that an inert gas (such as nitrogen or argon) is used as your gas and not air.

If you lower your nitrogen dewar and see a pale blue liquid in the trap, it is likely liquid oxygen and must be handled with extreme care.


If Liquid Oxygen is Inadvertently Collected in the Solvent Trap follow the procedures outlines by the person who trained you and notify your supervisor. We typically follow this method, but do what you have personally been trained to do:

Lower the liquid nitrogen dewar so it is no longer in contact with the trap
Turn off the vacuum pump

Carefully vent the line to air

Close the fume hood sash

If liquid O2 is observed, leave the area undisturbed for 20-30minutes and notify nearby workers of the hazardous situation

Carefully check that liquid oxygen is no longer present, leaving the set-up untouched for an additional length of time if needed

Once liquid oxygen has completely dissipated, proceed with your standard clean-up procedure

 

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