Degassing solvents

Introduction to Degassing solvents

Some reactions can be affected by the presence of dissolved gases (especially oxygen) in solvents or other reagents used in a reaction. Degassing is a process carried out in order to displace any dissolved gases prior to their use. There are a number of ways to carry out degassing with common routes including:

Bubbling gas through solvent
Freeze-thaw degassing
Inert atmosphere distillation

Degassing often goes alongside using solvent which is water-free, which may be obtained from a solvent purification system or distilled (with appropriate drying agents) prior to use. However, this doesn't always need to be the case, as some reactions may require oxygen free reaction conditions, but tolerate the presence of, or even be performed in, water.

Degassing by gas bubbling

This route involves bubbling an inert gas (typically nitrogen or argon) through the solvent prior to use, in order to displace any dissolved gases. The gas can be supplied either from a balloon or from a nitrogen tap and is usually bubbled through a flask containing the solvent and an exit needle for a sufficient period of time to ensure that any dissolved gases have been displaced.

Freeze-thaw degassing

An alterntive route to degassing solvent is to carry freeze-thaw cycles with a solvent on a Schlenk line.

Step by step process for degassing solvent using a balloon

Photo showing a conical flask clamped up in a fume hood, the flask is fitted with a septum. A long needle with a nitrogen balloon attached is in the flask so the end is submerged below the surface of the liquid forcing the gas to bubble through the liquid.

Take a ground glass joint fitted flask (this may be a round bottomed flask, or other vessel such as a ground-glass jointed conical flask) containing the solvent, and fitted with a septum. It is usually prudent to clamp flasks which are being degassed, as balloons may cause the flasks to become unstable through a high centre of gravity.
Fill a balloon with inert gas (usually nitrogen) and attach to a needle of sufficient length to reach into the solution. Insert the needle through the septum and place below the level of solvent in the flask.
Insert an exit needle into the septum, and the solution will begin to bubble as the balloon pressure causes the gas in the balloon to be expelled through the solvent, and will exit the flask from the exit needle.
Allow the gas to bubble through, replacing the balloon as nescessary. The length of time required to purge solvent will depend on the solvent volumes being degassed and the sensitivity of reaction being undertaken.
Once the solution has been sufficiently degassed, attach a filled balloon to the exit needle to maintain the gas atmosphere in the headspace of the flask, then remove the balloon and long needle from the solution.

Degassing solvent using a continuous flow of gas

Degassing solvents using a continuous nitrogen flow presents more potential safety hazards than with using a balloon. Care must be taken to ensure that it is not possible to over-pressurise any vessels being degassed. Flows of gases need to be carefully controlled to ensure that more gas is not being introduced into the system than can safely be removed via an exit needle, and care must be taken to ensure that pressure cannot build up, for instance through removing an exit needle whilst gases are still being introduced into a flask.

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