Advantages Filtering Solids at Scale

Crystallization in the laboratory is rarely performed completely under an inert atmosphere. Most commonly crystal filtration is done in the open air on a Buchner filter followed by washing with ice-cold wash liquid and then partial drying by the passage of air drawn through the filter cake by vacuum derived from a water aspirator.

Because it is conducted in this fashion the final crystallization temperature and the temperature of the wash liquid is rarely taken below zero degrees Centigrade because this would cause moisture from the air to contaminate the solvents used and/or to condense on the porcelain or glass filter funnel.  But conversely, if the Buchner filter is not sufficiently cold, it becomes more difficult to draw off the mother liquors and the wash solvent without partially redissolving the filtrand. Thus laboratory filtration in the open air often has contradictory preferences.

These are not problems in the kilolab, pilot plant, or plant. For safety and to avoid contamination all operations are done in a closed system that is easily kept dry and inert. As a result, cooling to a lower temperature, such as -20° C  is simple during all the operations of crystal formation, collection, and washing.

Sometimes efforts to find a suitable solvent system for recrystallization of a compound, which is crystalline within the typical ambient temperature range, can be replaced with a low-temperature recrystallization from hexane, pentane, or other hydrocarbon liquid. The large temperature range between the liquid’s boiling point and -20°C diminishes the need for a dramatic difference in solubility between the refluxing hot solvent and that same solvent at 0°C.

The special need that must be met to explore such an approach is met by providing means for checking out low-temperature recrystallization at a laboratory scale. Roger Giese described such an apparatus and its mode of use in the Journal of Chemical Education, 45, 610 (1968). Step-by-step instructions are provided. The apparatus is sufficiently simple that it can be put together by modifying a chromatography column that has a fritted glass disc as the plug. Because it operates with its own jury-rigged cold bath made from a plastic bottle, it does not need to fit in a Dewar for cooling, unlike the apparatus described by C. Frank Shaw, III and A. L. Alfred in the Journal of Chemical Education, 47, 165 (1970).