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The Dean-Stark apparatus, also called Dean-Stark trap, Dean-Stark receiver, distilling trap or Dean-Stark head, is a piece of laboratory glassware used in chemistry to collect water (or occasionally other liquid) from the distillation of an immiscible azeotropic mixture, such as toluene-water.
The Dean-Stark apparatus typically consists of vertical cylindrical glass tube, often with a volumetric graduation along its full length and a precision stopcock at its lower end, very much like a burette. The lower end of a reflux condenser fits into the top of the cylinder. Immediately below the joint between the condenser and the cylinder is a sloping side-arm that joins the cylinder to a reaction flask. The lower end the side-arm turns sharply downward, so that the side-arm is connected to the reaction flask by a vertical tube.
There are two main types of Dean-Stark trap: one type allows the return of the lighter immiscible phase and another that allows the return of the heavier immiscible phase. Usually, the first type is the most common, as it's simpler and cheaper.
Other variations of the Dean-Stark apparatus are the Barret trap and the Clevenger trap
The Dean-Stark apparatus is sold by lab suppliers, though it's not cheap. It can also be bought online, from eBay and Amazon.
DIY Dean-Stark apparatus
Given that Dean-Stark traps are expensive and sometimes hard to come by, you can make a replacement for a Dean-Stark apparatus can be made by assembling a distillation setup, with a Claisen adapter, where a hose adapter is placed on the second opening, and a long glass tube or another condenser (not connected to cooling), between the Claisen adapter and the still head, to increase the distance between the two. To the still head, a water cooled condenser is connected, where the crude distillate condenses and drips in a graduated cylinder below. A chemically resistant plastic tube it inserted to the cylinder, with the end positioned half-way, the siphoning tube continues near the bottom level of the cylinder, then rises up and connects to the hose adapter from the Claisen adapter of the distillation setup. Secure the tube on the top of the cylinder using a clip. Since this setup requires siphoning to work, it must be properly aligned and the graduated cylinder must be filled to work. Likewise, make sure the position of the siphon tube inlet is below the outlet relative to gravity. Fill the graduated cylinder with toluene or the lighter phase liquid, through the hose adapter then place it back on the Claisen adapter. Begin the distillation of the liquid mixture. The vapor front cannot move upwards through the plastic tube into the graduated cylinder, but it will rise through the tube and condense in the condenser and fall in the graduated cylinder. The heavier phase (water in this case), falls to the bottom of the cylinder, while the lighter phase will siphon back in the distillation flask. This setup can be used to return the lighter phase from the system, but if you want the heavier phase of the dual system, all you need to do is replace the lighter phase (toluene in this case) from the siphon tube with the heavier phase (water) and lower the inlet tube all the way to the bottom of the graduated cylinder, while also lowering the middle of the siphon tube to a position lower than the foot of the cylinder tube. The heavier phase will siphon back in the distillation flask, while the lighter phase will accumulate in the cylinder. NurdRage made a video on the Dean-Stark replacement.