Whilst Schlenk lines allow chemists to safely handle dangerous and pyrophoric reagents, they too pose several risks that can be easily avoided with correct technique and proper precautions.
Note: The guides provided are no substitute for direct and hands-on Schlenk line training and supervision.
If a stopcock or tap is accidentally left open to air whilst the liquid nitrogen cold trap is in place, liquid oxygen may condense in the trap. Liquid oxygen is a powerful oxidant and reacts violently with many organic compounds including grease and organic solvents collected in the solvent trap. 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 before disconnecting apparatus from the line to avoid this issue.
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 – this is usually in the form of a bubbler (oil or mercury).
- Pressure build up can also occur if a reaction evolves a large volume of gas. 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.
- 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 meaning that 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 (-185.8 °C). For freeze-pump-thawing ensure that the flask is sealed before freezing.
Potential Causes of Implosion:
- Weaknesses in glassware such as star cracks can fail when placed under high vacuum. Ensure that all glassware is safe for use prior to use.
Quenching Pyrophoric Substances
Pyrophoric materials are frequently used in Schlenk line chemistry. They must be safely quenched or ‘killed’ to prevent the risk of fire. This is typically done by suspending or dissolving the residual pyrophoric material in toluene and adding isopropanol drop-wise under a flow of inert gas. Once all visible material has reacted, or when hydrogen evolution ceases, begin adding ethanol slowly, followed by methanol, and finally water until you are certain that it has been fully quenched. Dispose of the solvent waste and rinse the glassware with water thoroughly before placing into the base bath for cleaning.
For additional safety information, check out The Safety Net.