Drying tube
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A drying tube is a large-bore glass tube, which may contain a bend and/or a bulb, containing a ground glass joint. A desiccant such as calcium chloride is packed into the tube with the aid of cotton wool.
Reactions which are being heated, or which evolve gases, must never be sealed because an overpressure may shatter the vessel. Drying tubes are usually fitted on top of the reflux condenser, allowing the pressure to be relieved while excluding atmospheric moisture.
Drying tubes are often used in less-demanding applications, typically in organic syntheses. One good example is the Grignard reaction. While the reaction is often run in room temperature, the solvent, usually volatile diethyl ether or tetrahydrofuran is already able to displace air directly, making additional measures to exclude atmospheric moisture less important.
An oil bubbler may be a useful substitute. In this case, gases are allowed to escape, but air is not able to enter because the bubbler acts as a one-way valve. Oil bubblers can tolerate an underpressure in the reaction vessel. Oil is sucked into a sump in lieu of air. However, if the pressure in the reaction vessel falls too low, the oil may be sucked into the reaction vessel, contaminating it.
For more demanding applications, a Schlenk line or glovebox may be used to provide an atmosphere of dry, inert gas such as argon or nitrogen.
