The Schlenk Line

A Schlenk line allows the manipulation of compounds in an inert atmosphere by using a gas that is both oxygen and water-free. The design of Schlenk lines varies, and the labelled photo below shows an example of a typical Schlenk line.

Double oblique taps

The key design in a Schlenk line is that it consists of two separate glass manifolds which are connected to double-oblique taps. These taps allow glassware which is connected via tubing to one or other of the manifolds, or for the tap to be closed. The two manifolds are configured, such that one is under vacuum and the other contains an inert gas, such as N₂. The photo below shows a double obleque tap and the manifolds.

Most Schlenk lines include multiple of these taps, which allows operations to be carried out with several sets of glassware at once. The photo of the Schlenk line at the top of the page has four of these taps, which is fairly typical.

This setup allows the manipulation of compounds in an inert atmosphere by using a gas that is both oxygen and water-free. The double oblique tap enables reaction flasks to be evacuated of air, then refilled to atmospheric pressure with an inert gas. This process of evacuation and refilling is usually repeated several times in order to ensure that all traces of air have been removed from the flask. Typically three cycles are carried out.

Vacuum pump

The vacuum for a Schlenk line is typically provided by a rotary pump. These pumps provide a high vacuum, getting down to the very low pressures which are required on a Schlenk line. Pumps need care, and are usually ballasted before and after use, in order to keep the pumps in good condition. Pumps need regular servicing, such as oil changes, in order to ensure optimal working.

Vacuum gauge

Most Schlenk lines utilise Pirani gauges for measuring the pressure on the vacuum manifold. These gauges can typically display pressures between 1 and 10⁻² mbar. The gauge should indicate a pressure less than 10⁻¹ mbar when under active vacuum. If your gauge is reading a higher pressure then you may have a leak in your system and should consult a demonstrator.

Solvent trap

A solvent trap is usually fitted before the pump. This consists of a tube through which any vapours from the line are drawn through before reaching the vacuum pump. Most commonly the trap is cryogenically cooled (see below).

Cryogenic trap

To prevent solvent vapours from entering the pump, a liquid-N₂ filled Dewar is placed between the pump and the vacuum manifold. This will condense solvent vapours in the trap and therefore minimise any solvent vapours entering the pump. The solvent vapours can cause damage to the pump, and minimising the volume of vapours entering the pump is desirable.

However, as the boiling point of liquid nitrogen (–196 °C) is below that for liquid oxygen (–183 °C), it is possible to condense liquid oxygen in the cold trap if the manifold is left open to the air. It is therefore crucial that you do not place a liquid-N₂ filled Dewar around a cold trap that is not under vacuum. The condensing of liquid oxygen can lead to explosion, particularly if flammable solvent vapours are also condensed into the oxygen, thus providing fuel and oxidant, and a potentially explosive mixture.

Gas manifold & bubblers

The second manifold provides an inert gas which can be connected to glassware via the double oblique taps. The most commonly used inert gas is nitrogen, although other gases (such as argon) may be encountered. The pressurised gas requires an outlet from the line, which in this design is via an oil bubbler. The gas in the manifold will take the easiest route through the system, which most of the time will be via the bubbler into the atmosphere. When the nitrogen line is connected to flasks, the easiest route may become the connected flask, so the bubbling will be observed to cease.

The use of an oil bubbler allows the gas line to operate only marginally above atmospheric pressure, and this can be problematic with oil sucking back into the line (hence the use of the empty trap to prevent this). Some Schlenk lines use pressure release valves (and safety burst discs), enabling lines to be pressurised higher (eg a couple of bar). It is crucial you are familiar with the design of the Schlenk line you are using, as the operations are quite different.

Video introduction to a Schlenk line