Drying Tubes
Introduction to Drying Tubes
Drying tubes can be used where a reaction needs to carried out with water being excluded or removed from the setup. The use of drying tubes was much more common in the past, but increasingly nitrogen balloons are used in their place.
A drying tube consists of a glass tube which can be packed with a material to absorb water. The other end of the tube is usually fitted with a ground glass cone which can be fitted into the reaction glassware, typically a round bottomed flask or condenser.
Drying tubes can be useful to protect reaction setups to minimise the amount of water which can reach the reaction. E.g. to prevent the reaction of an acid chloride with water in an esterification reaction.
Drying tube fitted into a round bottomed flask
The drying tube is filled with calcium chloride beads used to scrub water from the atmosphere.
Drying tube fitted into a condenser
Setup
The drying agent is usually held in place using cotton wool. A piece of cotton wool is placed into the drying tube and lightly pushed down towards the bend in the tube. It needs to be packed enough to prevent material passing the cotton wool, but should not be packed tightly such that the free movement of air through the glassware is impeded.
Once the cotton wool is in place, the drying tube can be filled with the relevant drying agent. Then another piece of cotton wool is placed into the other end of the tube to keep the drying agent in place.
Drying agents
Calcium chloride is the most commonly used drying agent, and is typically sold in bead form for this purpose, which is easy to handle in a drying tube.
Another commonly encountered drying agent is Drierite® which is impregnated with a dye that results in a colour change when the drying agent has absorbed water.