Stopcocks vs. Teflon Taps

Schlenk lines come in all 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 stopcocks are considered safer than Teflon taps because it is not possible to simultaneously have a stopcock open to both vacuum and inert gas. They also tend to be quicker and easier to use than Teflon taps but may not hold vacuum as well. Double oblique stopcocks are often manufactured specifically for each port on the Schlenk line meaning that replacement parts are less accessible and also means that greater care is required during cleaning (i.e. avoid using base baths for cleaning Schlenk lines with ground glass joints). Double oblique stopcocks must be evenly greased to ensure a good seal – this may contaminate reactions or even be incompatible with certain reagents or conditions employed in a reaction.

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A Schlenk line equipped with double oblique stopcocks.
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Front and side view of a double oblique stopcock when closed, open to vacuum, and open to inert gas.

Teflon taps typically hold better vacuum than double oblique stopcocks and also avoid the use of excess grease which may be incompatible for some chemistry. They also give finer control which may be useful for drying solids or removing solvent under vacuum. One major disadvantage of Teflon taps 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 oil or mercury used in the bubbler. Teflon taps are also prone to being over-tightened and can result in a broken Schlenk line.

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A Schlenk line equipped with Teflon taps.