Handling Air-Sensitive Chemicals Safely
In one of our earlier articles we discussed moisture-sensitive chemicals. In the following article we will talk about a technique for working with air-sensitive chemicals.
A variety of techniques have been developed to work with air-sensitive chemicals that are reactive to components of air such as oxygen, carbon dioxide and water vapor. The most common methods involve Schlenk Techniques and glove boxes. Since we discussed glove boxes in an earlier article on moisture-sensitive chemicals, we will focus on the Schlenk Techniques here.
The Schlenk Techniques and specialized equipment named after their inventor, German chemist Wilhelm Schlenk (1879–1943), include flasks, tubes and an apparatus for isolating and manipulating air-sensitive chemicals. The Schlenk techniques and glassware allow for the handling of air-sensitive chemicals and chemical reactions outside of a glove box by providing hook-ups to an inert atmosphere in a closed system.
Schlenk flasks come in various shapes and sizes. Schenk flasks have round or pear-shaped bottoms. An elongated, narrow style called Schlenk tubes or Schlenk traps is also available. Both flasks and tubes have a side arm with a stopcock that serves as a connection point to a vacuum or gas line, so you need to make sure the plugs and barrels of the stopcocks fit and seal properly to prevent accidents.
All styles are made of borosilicate glass to withstand extreme temperatures. Before use, ensure that the glass is clean, free of scratches and completely dry. This is necessary to prevent accidents due to chemical reactions, especially if you are using an inert gas.
To wash your Schlenk flask, first disassemble the parts and then soak in a base bath and/or clean with soap and water.
A Schlenk line is a vacuum/inert gas manifold with multiple ports. It is a dual manifold—one is connected to an inert gas and the other is connected to a vacuum pump. They are also made of borosilicate glass with stopcocks to accommodate multiple tubing attachments and Schlenk flasks. Special stopcocks or Teflon taps are used for the inert gas or vacuum without using a separate line for the sample.
The system is complex, and precautions must be taken especially when several chemical reactions take place at the same time through multiple ports. Two main risks associated with Schlenk lines are explosion and implosion. An explosion can occur due to condensed liquid oxygen when using a cold trap to hold the solvent. Liquid oxygen can be condensed if the vacuum pump draws air through a very cold trap (cooled by liquid nitrogen). Be alert to the possibility of other explosive gases released and condensed in the cold trap during chemical reactions. An implosion can occur due to defects in the glass or pump.
It is important to keep the manifolds clean, being aware that they are delicate and expensive. The manifold can be carefully dismounted from its metal support rods and the stopcocks can be removed before soaking the manifolds in a base bath and/or cleaning with soap and water.
- An air-sensitive reaction may be prepared inside a glove box and then connected to a Schlenk manifold to proceed to completion.
- An isolated sample may be reduced to dryness on a Schlenk manifold using a vacuum pump, as the pump hooked to a Schlenk manifold provides a higher vacuum than typical rotary evaporators.
- Distillation experiments can be conducted in a Schlenk manifold.
This introduction to working with air-sensitive chemicals is by no means a comprehensive reference. A basic awareness of Schlenk lines and glassware can lead you and your students to further research regarding these sophisticated techniques.