Schlenk Line safety
1. When working with the Schlenk line ALWAYS wear safety goggles!
2. Try to prevent working alone in the lab, when working with the Schlenk line.
3. When working first with the Schlenk line try to understand the way of the vacuum and the way of the inert gas (Argon) before using it.
4. Be aware of overpressure (check manometer, gas flow (oil bubbler)).
5. Do not panic if the pressure rises or falls rapidly, keep calm and look at the valves which could be accidentally opened or closed.
6. Be aware of Liquid Oxygen: If a valve is left open to the air, liquid oxygen may condense in the cold trap. Liquid oxygen is a powerful oxidant and reacts violently with many organic compounds including grease and organic solvents collected in the solvent trap. It has a blue colour.
What to do:
Check the manometer: If a stopcock or tap is accidentally left open, or if there is a small leak within the Schlenk line, the manometer reading will be significantly higher than usual and the vacuum pump will usually become a lot louder. Ensure that all taps or stopcocks are closed.
If you see blue colour in the cold trap this could indicate that liquid oxygen has condensed. Replace the cold trap (keep cold trap cool with liquid nitrogen). But only for a few minutes to prepare what to do next. If you can locate the leakage close the valve for example.
Switch of the pump. Remove all organic compounds next to the cold trap, and try to close the fume hood windowpane as much as possible. Carefully flush the system with argon (open an overpressure valve that
you do not cause overpressure). Afterwards immediately remove the liquid nitrogen Dewar. Then quickly open the valve to flush the system with air and leave the lab and inform others not to enter the lab until the liquid oxygen has evaporated. If you do not feel comfortable with flushing with argon, only remove the nitrogen Dewar, open the valve to air, close the fume hood and leave the lab and inform others not to enter. After the warming to RT consider the cold trap still dangerous- organic peroxides could have formed. Fill the liquid from the cold trap in a clean beaker and flush the empty cold trap with water (behind the fume hood glass. If available you can check the mixture for peroxides with a potassium iodide test strip -it will turn purple if peroxides are present. In this case reduce the mixture by adding sodium
thiosulfate or sodium sulphite before disposing the mixture in the solvent waste.
In case of a clogged cold trap due to frozen solvent in the flask (the manometer pressure will rise) turn of the pump, vent the cold trap flask (flush with air), remove it and thaw it by putting the flask in warm water (RT, not hot!) and afterwards remove the liquid solvent. The dry flask can be connected again.
Potential Causes of Explosion:
• An explosion may occur if the inert gas pressure builds up within a closed system. To prevent this, ensure that there is a source of pressure relief attached to the Schlenk line when the inert gas is open (e.g. oil bubbler) or check the overpressure manometer.
• Pressure build-up can also occur if a reaction evolves a large volume of gas. (e.g. LiAlH₄ work-ups, in particular, can evolve a large volume of dihydrogen gas and should be performed controllably with an
adequate source of pressure relief).
• Heating a reaction can also lead to pressure build-up within the Schlenk line. Ensure that known exothermic reactions are kept cold (ice bath or acetone/dry ice bath) to prevent this from happening or work with an overpressure valve or balloon.
• Reactions that have been cooled to -78 °C or below should be left open to the inert gas (and pressure relief) whilst warming back up to room temperature – the inert gas has a lower vapour pressure at lower temperatures à the gas pressure will increase on warming back to room temperature.
• Manipulations that are under a dynamic flow of argon should not be cooled with liquid nitrogen since the argon will begin to condense at these temperatures (-186 °C).
• For freeze-pump-thaw ensure that the flask is sealed before freezing.
Potential Causes of Implosion:
• A flaw in glassware such as star cracks can collapse when placed under high vacuum. Ensure that all glassware is safe before using.
Quenching Pyrophoric Substances
Pyrophoric materials are frequently used in Schlenk line chemistry. They must be safely quenched to prevent the risk of fire.
Read the safety constructions for the pyrophoric substances you are working with.