Technique primer

P1.5 Distillation In the realm of chemical practice, the physical separation of mixtures is an extremely important consideration. For example, TLC and HPLC analysis is based on the separation of compounds according to their differential affinity for silica (or some other stationary phase). Similarly, aqueous extraction is a means to separate water-soluble components from organic-soluble compounds.

and product isolation. While distillation is energyintensive, it is far less expensive than comparable chromatographic approaches, and equipment exists for distilling ton-quantities of materials in continuous processes (Figure 2).

In some cases, filtration can be used for separation if one component is sparingly soluble (or insoluble) in a medium that dissolves other components. In like fashion, if one component is more volatile than another, the two can be separated via distillation. Simply put, distillation involves a) converting a liquid to vapor by heating in a boiling flask (or “still pot”), b) allowing the vapor to diffuse into a condenser, where it is converted back to liquid, and c) collecting the condensed liquid in another vessel (e.g., the “receiving flask”), a process usually fed by gravity (Figure 1).

Figure 2. Industrial distillation columns. (image from Coldbox.JPG)

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A few guidelines should be observed when setting up a distillation apparatus. The still pot should be securely clamped at the neck, and the heat source should be supported on an iron ring a few inches above the benchtop, so that it can be removed rapidly if need be. A boiling chip should always be added to the still pot before heating is begun. The thermometer should project just below the outlet to the condenser, as shown in Figure 1. This ensures that the bulb is always bathed with the vapor during the distillation. The water coming into the condenser should be adequate for cooling, but not excessive, as high pressure in the line can dislodge a hose connection. Note that the distillation apparatus is open to the atmosphere (in Figure 1, this opening is the hose barb on the vacuum adapter at the far right of the illustration). Unless using equipment specifically designed for high-pressure work (e.g., microwave vials), a closed system should never be constructed. An explosion could easily result otherwise.

Figure 1. Setup for simple distillation (image from commons.wikimedia.org/wiki/File:Simple_distillation_ apparatus.svg)

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Distillation is a very scalable technique, and it is used for many industrially significant processes, including petroleum refining, solvent recovery,

Finally, the still pot should never be boiled completely dry, as the temperature inside the vessel increases dramatically once all the solvent is gone. In a worst-case scenario, the sudden temperature spike can ignite the residual vapor in the apparatus, resulting in an explosion.

this curriculum was developed with support from the National Science Foundation (TUES 1323024)