Appropriate personal protective equipment (PPE) including eye protection, suitable gloves and a lab coat must be worn when operating a Schlenk line.
Schlenk line techniques allow chemists to safely handle dangerous and pyrophoric reagents on a routine basis, but still pose many other risks that can be avoided with the correct technique and appropriate precautions. The main risks associated with Schlenk line chemistry include the condensation of liquid oxygen, implosion or explosion, handling pyrophoric substances, or using sharp cannulae and needles. Risk assessments and appropriate training must be completed before operating a Schlenk line. Readers are directed to The Safety Net for detailed safety information including a library of standard operating procedures.
DISCLAIMER: The guides provided herein are an education resource and are no substitute for direct and hands-on Schlenk line training. Never operate a Schlenk line alone – ensure that a senior student, post-doctoral researcher, technician or PI is in the lab to supervise and to assist in the case of an emergency. Appropriate risk assessments alongside necessary training courses (e.g. fire extinguishing, gas cylinder safety) must be completed before operating a Schlenk line. The specific techniques and guides presented on this website are based on the author’s own opinion which have been established from training and experience in numerous main-group and organometallic chemistry laboratories around the world. Alternative methodologies and techniques may be employed by different users depending on personal preference and available equipment.
Liquid oxygen may condense in the liquid nitrogen cold trap if a stopcock or Teflon tap on the Schlenk line is accidentally opened to air. Liquid nitrogen traps should therefore only be used where necessary, and alternative cryogenic traps (dry ice, -78 °C) should be considered. Liquid oxygen is a powerful oxidant and can react violently with many organic compounds including grease and solvents collected in the trap. The use of a manometer can help prevent liquid oxygen condensation by ensuring that the Schlenk line contains no leaks. It is important to regularly assess the status of the Schlenk line (primarily the stopcocks and taps) as well as the condition of hosing and connectors since these are common points of leaks and failure.
In the event that liquid oxygen is identified or suspected in a solvent trap, the original cryogenic state should be immediately restored (vacuum pump on and liquid nitrogen Dewar raised). Do not remove the trap and allow the liquid oxygen to warm as this can lead to an explosion due to rapid expansion or reaction with organic substances. Provided the source of the leak has been identified and stopped, the liquid oxygen will slowly evaporate under dynamic vacuum, even at liquid nitrogen temperatures. For a complete and detailed standard operating procedure on how to deal with and help prevent liquid oxygen condensation, the reader is directed to The Safety Net.
Potential Causes of Explosion and Implosion:
An explosion may occur if the inert gas pressure builds up within a closed system. This is generally prevented by using a bubbler which acts as a pressure relief system within the inert gas manifold of the Schlenk line.
Pressure build-up can also occur if a reaction evolves a large volume of gas, for example when quenching pyrophoric substances10 or LiAlH₄ work-ups; these should be performed controllably (often with appropriate cooling) and with an adequate source of pressure relief (e.g. a septum and bleed needle).
Heating a reaction without a condenser, or unexpected thermal runaways, can also lead to pressure build-up within a Schlenk flask and inert gas manifold. Known exothermic reactions should be appropriately cooled using an ice bath or dry-ice/acetone bath to prevent thermal runaways.
Reactions that have been cooled to low temperatures (from 0 °C down to -78 °C) should be left open to the inert gas (and thus pressure relief) whilst warming back to ambient temperature. Since inert gas has a lower vapour pressure at lower temperature, the inert gas pressure will increase upon warming to ambient temperature.
Manipulations under a dynamic flow of inert gas (nitrogen or argon) should never be cooled with liquid nitrogen since this will lead to condensation of the inert gas. In the event that this occurs, the flask should be left open to inert gas as it slowly warms to room temperature or turned to dynamic vacuum (to evaporate the inert gas) whilst maintained at liquid nitrogen temperatures.
An implosion may occur if the Schlenk line or flask, which contains a crack or defect, is placed under dynamic vacuum. This may also lead to condensation of liquid oxygen in the solvent trap.
Handling and Quenching Pyrophoric Substances
Pyrophoric substances, and the glassware and equipment used to manipulate and handle them, must be safely quenched prior to disposal and cleaning.10 This is achieved by first suspending or dissolving the pyrophoric substance in toluene (a high boiling aprotic solvent) under an inert atmosphere, cooling the flask in an ice bath or dry-ice/acetone bath, and slowly adding isopropanol. Once all visible material has reacted, or when gas evolution cases, ethanol can be added, followed by methanol, and finally water. This should be stirred for a suitable period of time to ensure all pyrophoric material is fully quenched. After disposal of the solvent in an appropriate waste container, glassware should be thoroughly rinsed with water prior to contact with flammable organic solvents (e.g. washed with acetone or in a base bath). Syringes, needles, and cannulas used to transfer pyrophoric compounds should be quenched with a dilute (10-20%) isopropanol-toluene mixture immediately after use to quench residual material and prevent clogging.
When dispensing large volumes (>20 mL) of pyrophoric reagents such as n-butyl lithium, it is recommended to directly cannula transfer the reagent into a graduated pressure equalising dropping funnel, or to select a suitable scale in which the whole reagent bottle is used for a single reaction. Always select an appropriately size syringe and needle gauge for the amount of liquid to be dispensed, and never use multiple syringe transfers to bypass the volumetric limit of a syringe. The death of Sheharbano Sangji in 2008 was in part due to the inappropriate selection and use of a syringe and needle when dispensing large volumes of tert-butyl lithium.
Cannulas and Needles
For a detailed report regarding the safe handling of cannulas, needles and other sharps.