Schlenk flask

Schlenk flask

A selection of Schlenk flasks and, bottom right, a Schlenk tube
Other names	Schlenk tube
Uses	Vacuum Inert gas
Inventor	Wilhelm Schlenk
Related items	Schlenk line

A Schlenk flask, or Schlenk tube is a reaction vessel typically used in air-sensitive chemistry, invented by Wilhelm Schlenk. It has a side arm fitted with a PTFE or ground glass stopcock, which allows the vessel to be evacuated or filled with gases (usually inert gases like nitrogen or argon). These flasks are often connected to Schlenk lines, which allow both operations to be done easily.

Schlenk flasks and Schlenk tubes, like most laboratory glassware, are made from borosilicate glass such as Pyrex.

Schlenk flasks are round-bottomed, while Schlenk tubes are elongated. They may be purchased off-the-shelf from laboratory suppliers or made from round-bottom flasks or glass tubing by a skilled glassblower.

Typically, before solvent or reagents are introduced into a Schlenk flask, the flask is dried and the atmosphere of the flask is exchanged with an inert gas. A common method of exchanging the atmosphere of the flask is to flush the flask out with an inert gas. The gas can be introduced through the side arm of the flask, or via a wide bore needle (attached to a gas line). The contents of the flask exit the flask through the neck portion of the flask. The needle method has the advantage that the needle can be placed at the bottom of the flask to better flush out the atmosphere of the flask. Flushing a flask out with an inert gas can be inefficient for large flasks and is impractical for complex apparatus.

An alternative way to exchange the atmosphere of a Schlenk flask is to use one or more "vac-refill" cycles, typically using a vacuum-gas manifold, also known as a Schlenk line. This involves pumping the air out of the flask and replacing the resulting vacuum with an inert gas. For example, evacuation of the flask to 1 mm and then replenishing the atmosphere with 760 mm inert gas leaves 0.13% of the original atmosphere (¹/₇₆₀ × 100%). Two such vac-refill cycles leaves 0.000173% (i.e. ¹/₇₆₀² × 100%). Most Schlenk lines easily and quickly achieve a vacuum of 1 mm Hg.