Schlenk lines come in many shapes and sizes, but one major difference commonly encountered in the lab is the use of either ground glass double oblique stopcocks or Teflon taps. Both have their advantages and disadvantages which will be discussed in further detail below:
Double Oblique Stopcocks
Double oblique ground glass stopcocks are considered safer to use than Teflon taps because it is not possible to simultaneously have a stopcock open to both vacuum and inert gas. They tend to be easier to use than Teflon taps and are therefore well suited for introducing and teaching Schlenk line techniques – often one or both sides of the stopcock head are marked with coloured ink or tape to indicate which orientation corresponds to vacuum or inert gas. Double oblique stopcocks (and even regular single-holed ground glass stopcocks) are often manufactured specifically for each port on the Schlenk line to ensure the best seal – these may be called “mated” or “matched” ground glass stopcocks. For this reason, greater care is required when cleaning “mated” ground glass stopcocks and the use of base baths should be avoided as this will slowly etch the glassware and compromise the seal. Double oblique stopcocks must be evenly greased to ensure a good seal; this generally means that the vacuum pressure may not consistently reach as low values when compared to Teflon taps. The use of greased stopcocks may also not be compatible with certain reagents or conditions employed in a reaction, and furthermore they are not recommended for the long-term storage of sensitive chemicals or anhydrous solvents since they are prone to seizing (i.e. freezing or locking).
Front and side view of a double oblique stopcock in different positions.
Teflon taps typically hold better vacuum than ground glass stopcocks and avoid the use of grease to ensure a good seal. The main disadvantage of using Teflon taps on a Schlenk line is that it is possible to accidentally have both the vacuum and inert gas taps open simultaneously which can result in suck back of the bubbler oil or the consumption (and potential condensation) of inert gas by the vacuum pump. Teflon taps are also prone to being over-tightened which can damage the Teflon tap or the glassware. Nevertheless, Teflon taps are still recommended for more sensitive chemistry applications and for the long-term storage of solvents and reagents. It should be noted that Teflon taps will contract slightly when cooled, which may compromise the seal of flasks if placed in a fridge or freezer.
A Schlenk line equipped with Teflon taps.