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A drying tube, sometimes called guard tube, is a tube-like piece of apparatus which holds a small amount of disposable solid desiccant, with one end the tube-like structure terminates in a ground glass joint for use in connecting the drying tube to a reaction vessel or installation, for the purpose of keeping the vessel free of moisture. They are commonly used when the reaction performed is sensitive to moisture but doesn't need inert atmosphere.
A drying tube consists of an open glass tube, with a small bulb at one end, from where the diameter of the tube narrows and a glass elbow begins and ends to a male ground glass joint. Drying tubes are usually fitted on top of the reflux condenser, allowing the pressure to be relieved while excluding atmospheric moisture. Drying tubes are unsuitable to dry reagents, as the volume of desiccant is very small, and their form makes it difficult to properly integrate them in any drying installation.
Another type of drying tube consists of a U-shaped glass tube. It works similar to the one above, only that both ends have to be closed with stoppers and most common models lacks ground glass joints. Some have two additional lateral tubes.
Drying tubes can be bought from lab suppliers and online.
DIY drying tube
A makeshift drying tube can be made by taking a broken piece of glassware with a male joint and stuffing it with cotton and a drying agent.