Purification by Sublimation

Purification by sublimation is an effective physical separation method analogous to distillation, but for solid-to-gas transitions. Its effectiveness hinges on the difference in vapor pressures between the target compound and its impurities. The target compound must have a significantly higher vapor pressure at the operating temperature than the impurities, allowing it to sublime while the impurities remain solid. The process is typically carried out in a specialized piece of glassware called a sublimation apparatus.

The apparatus consists of a vessel to hold the impure solid, a heating source (like a heating mantle or oil bath), and a cooled surface (the cold finger) placed above or within the vessel. A vacuum is often applied to the system. Reducing the pressure lowers the temperature at which the compound will sublime, which is crucial for thermally sensitive compounds that might decompose at their atmospheric-pressure sublimation point. As the impure solid is heated, molecules of the target compound escape into the gas phase. These gaseous molecules travel a short distance until they collide with the cold finger, which is typically cooled by circulating water or a cryogen. Upon contact with the cold surface, the molecules lose thermal energy and undergo deposition, transitioning directly back into a highly pure crystalline solid. The non-volatile impurities, having much lower vapor pressures, are left behind. This technique is valued for its simplicity and for yielding very high-purity crystals, making it a standard procedure in many synthetic and analytical chemistry laboratories.