6.12: Handling Air- and Water-Sensitive Chemicals Using a Schlenk Line

1. General procedure for carrying out a reaction under an inert atmosphere

1. Evacuating the glassware
   1. Start the vacuum pump and fill the dewar flask in which the schlenk line trap is immersed in with liquid nitrogen.
   2. Use oven dried or flame dried glassware to ensure that there is no residual moisture adsorbed on the walls of the glassware.
   3. Seal the flask using either ground glass adapters, or rubber septa. It is highly recommended that vacuum grease is used to avoid seizing of the ground glass joints.
   4. Connect the sealed glassware to the Schlenk line using an appropriate adapter/connection.
   5. Turn the two-way valve to open the line to vacuum.
   6. Evacuate the system and let the flask cool to room temperature
2. Purging with inert gas
   1. Make sure the inert gas line is open and sufficient flow is seen through the bubbler. A typical flow for operation of the schlenk line is about one bubble per second. The flow should be increased while initially purging the system after evacuation.
   2. Once the flask has cooled to room temperature, turn the two-way valve slowly to open the line to inert gas, taking care that some gas still manages to reach the bubbler. Use caution to not change over to inert gas too quickly, as this can lead to the liquid sealant from the bubbler to flow back into the schlenk line.
   3. Once the inert gas flow has stabilized back to its original state (as observed on the bubbler), switch the two-way valve back to vacuum, and repeat the above procedure two more times.
3. Adding reagents
   1. Once the flask has been evacuated and backfilled with inert gas a total of three times, it is now ready for use in handling air/moisture sensitive reagents. The inert gas flow can be readjusted to one bubble per second to save the consumption of inert gas.
   2. The reagents are typically added from a septum sealed bottle or from a solvent dispensing station.
   3. Use an oven dried cannula or needles to transfer air sensitive reagents into the flask. Make sure the flask is under an inert atmosphere, and not under vacuum as addition of chemicals to a flask under vacuum can damage the vacuum source.
4. Quenching the reaction
   1. Once the reaction is complete, carefully quench the reaction using an appropriate reagent.
   2. The quenching step regularly results in the generation of gases, make sure that the two-way valve is open to the inert gas so that there is no pressure build-up in the glassware.

2. Basic design and set up of Schlenk line.

The design of Schlenk lines varies from lab to lab but the key features are the same. Figure 1 shows a schematic set up of the Schlenk line with key features pointed out.

Figure 1. Schematic set up of a Schlenk line.
The dual manifold consists of two parallel glass tubes, which form the main body of Schlenk line. A gas manifold is connected to the inert gas supply and a vacuum manifold is connected to the vacuum line, which allows switching between inert gas and vacuum by tuning the taps. The gas exits the manifold through a visible oil bubbler in order to monitor the flow of the gas. The vacuum manifold is closed at one end and the other end is connected to a cold trap and a vacuum pump. The cold trap is used to prevent volatile or corrosive solvents from damaging the pump.

3. Safety concerns with Schlenk line.

1. Liquid oxygen.
   Liquid oxygen can accumulate if a constant steam of air goes into the vacuum line with the cold trap. This can be indicated by a light blue liquid in the trap, since liquid oxygen is light blue. Liquid oxygen is dangerous due to its violent reaction with organic solvents, including the vacuum grease and the high pressure generated once vaporization occurs in this confined space. Remember to never open the vacuum line to air when the cold trap is in place.
2. Explosion.
   1. Pressurized gases can become dangerous, so make sure the system is open with the inert gas flowing. Explosions can occur if pressure is built up in the case of a closed system.
   2. Violent reactions: If the reaction gets out of control, a large volume of gas might be generated quickly and may cause an explosion. Be aware of the reaction before operating in the Schlenk line.
   3. Heating a closed system: In a closed system, the increase of the temperature will increase the pressure also. Be aware of any reactions with thermal properties and make sure the line is open and there is a bubbler for pressure relief.
3. Implosion.
   Cracks in the glassware will cause breaks under vacuum. Repair or replace cracked glassware.

A Schlenk line is a piece of laboratory equipment that allows for the safe handling of air- and water-sensitive chemicals.

A Schlenk line requires a vacuum, inert gas, and cryogenic temperatures. It must be operated with care to prevent equipment failure and thus release of hazardous chemicals. This video discusses the potential hazards associated with the Schlenk line, and precautions necessary for its safe use. For more information about the operation and applications of the Schlenk line, please watch JoVE's video on "Schlenk Lines Transfer of Solvent."

We begin by briefly examining the components of a Schlenk line. A Schlenk line consists of a pair of glass tubes together called a "dual manifold." One tube connects to a vacuum pump, and the other to a source of inert gas. A Schlenk flask, or other apparatus, connects to the dual manifold via a two-way valve that permits access to either the vacuum line or the inert gas line, but never both simultaneously.

Let's examine the vacuum line in greater detail. The vacuum line is sealed at one end. The other end connects to a cold trap, which is in turn connected to the vacuum pump. A Schlenk flask exposed to this line will be evacuated, and the gases sent to the cold trap, a glass container submerged in liquid nitrogen. It freezes volatile organics, protecting the pump from damage, and the user from exposure. Potential dangers of the vacuum line include glass imploding under vacuum, and the safety hazards associated with liquid nitrogen.

Next, we turn to the inert gas line. The inert gas, usually nitrogen or argon, is regulated by a valve and often passes through a desiccant before entering the Schlenk line. A Schlenk flask exposed to this line will be filled with the inert gas. The excess gas then leaves the system via a mineral oil bubbler, to prevent air from entering the inert gas line. Potential hazards of this line include glass exploding due to over pressurization, and the exposure of the inert gas line to the vacuum.

It is important to perform regular safety checks while setting up the apparatus.

The Schlenk line is mounted inside a fume hood and operated with the sash lowered, to protect the user from inadvertent exposure. A blast shield may be used additionally, when handling very sensitive reagents. Wear safety goggles or a face shield, a flame-proof lab coat, and cryogenic gloves when working with liquid nitrogen.

Check the tubing for tears or other defects before connecting the dual manifold to the vacuum pump and the inert gas line. Ensure there is sufficient oil in the oil bubbler.

The Schlenk line contains check valves to prevent backflow of air into the system, and pressure relief valves on the dual manifold and the cold trap. The connectors use glass clamps sealed with U-clamps and airtight O-rings. Ensure these valves and connectors are not damaged. Apply vacuum grease to the glassware at the interface between surfaces.

Dry the Schlenk flask and other glassware by oven- or flame-drying. Check the glassware and the dual manifold for cracks. Cracked glass can implode under vacuum, exposing the reagents to the atmosphere and injuring the user. Seal the Schlenk flask carefully with a rubber septum or a ground glass adapter before connecting it to the line.

Now that we've seen the preliminary precautions, let's turn to safety concerns while operating the Schlenk line.

One potential hazard is the formation of liquid oxygen in the cold trap. Liquid oxygen is extremely explosive. To prevent its formation, start the vacuum pump and evacuate the vacuum line before submerging the cold trap in the liquid nitrogen Dewar. Never allow the cold trap to contact liquid nitrogen unless the vacuum pump is running, and never open the vacuum line to air while the Dewar is in place.

Liquid oxygen can also form if air inadvertently enters the cold trap during the experiment. Check the cold trap for a light blue liquid. If detected, quench the reaction and call for assistance, but keep the apparatus under vacuum and liquid nitrogen. It is only safe to stop the vacuum and remove the Dewar once the liquid oxygen vaporizes into the pump.

When working with the inert gas line, regularly check the inert gas supply and the flow rate using the inert gas bubbler. The flow rate should be about one bubble per second when the Schlenk flask is open to the line, and slightly greater otherwise. Bubbling should be visible at the oil bubbler at all times. If no bubbles are seen, close the inert gas source to prevent the line from over-pressurizing.

If the inert gas contacts the vacuum, it will depressurize, causing the check valve to close or mineral oil to backflow into the inert line, followed by air. If this occurs, switch off the pump and close all valves.

When purging the Schlenk line or a Schlenk flask, turn the valve very slowly. This prevents glassware from cracking due to rapid pressure changes. Before the reagents are added, expose the Schlenk flask to the inert line slowly, to prevent depressurization of the inert line.

Before withdrawing an air- or water-sensitive reagent, purge syringe and needle and the septum-sealed reagent bottle in the inert gas line. Withdraw the reagent from the bottle, making sure to overdraw and then plunge to the correct volume. Slowly add the reagent to the Schlenk flask and place the needle tip in a beaker of quenching agent if necessary.

After the reaction is complete and quenched, remove the liquid nitrogen Dewar. Then switch off the pump.

You've just watched JoVE's introduction to safe Schlenk line operation. You should now be familiar with its design, its operating procedure, and safety precautions. Thanks for watching!