Question

During the purging steps in the Schlenk line experiment, why does the chemist close the nitrogen before opening the vacuum? What gas is being evacuated?

Answer 1

Schlenk lines are generally used for reactions performed in solution as they lend themselves well to cannula and counterflow techniques. Manipulations involving the measurement or condensation of gases are usually performed on a high vacuum line.

Schlenk lines differ from high vacuum lines by several small features. High vacuum lines, as the name implies, have better vacuum than Schlenk lines. This arises from use of a diffusion pump instead of or in addition to the mechanical pump generally used with a Schlenk line. Schlenk lines employ flexible rubber or plastic tubing to connect the apparatus to the line, while apparatus is connected to a high vacuum line via joints that provide a better seal and vacuum.

The Inert Gas Line

The gas manifold is attached to an inert gas supply and to the gas outlet by flexible rubber or plastic tubing (Fig. 2, A). The inert gas, usually nitrogen or argon, is fed directly to the manifold, or can be passed through a drying or deoxygenation column first. The supply comes from either cylinders of compressed gas, or in the case of N₂, from the run off from the main in-house liquid nitrogen tank. As argon is more expensive than N₂, nitrogen is usually the preferred gas unless the compound(s) under study react with nitrogen.

The gas exits the manifold through a mercury or oil bubbler (Fig. 2). The bubbler provides a pressure release system for the line and a visible means of monitoring the general flow of gas. The bubbler should vent to the back of the fumehood or close to wherever the fumehood exhaust is, especially if it is a mercury bubbler. Mercury bubblers give a slightly higher pressure of gas in the line due to the relative higher weight of mercury compared to oil. This can be useful when it comes to filling a vessel on the line with inert gas (see part 2) as it reduces the chances of contaminating the line with air. However, due to the toxic effects of mercury, these bubblers are becoming less common.

Figure 2. Complete Schlenk line set-up.