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APPARATUS FOR SEMIMICRO LOW TEMPERATURE CRYSTALLIZATIONS AARON L. BLUHM Quartermagter Research and Development Center, Natick, Massachusetts
AN APPARATUS suitable for the crystallization of small quantities of materials at low temperatures is described. It can be used either to recrystallize solid materials from solvents in which the materials are easily soluble at room temperature, or to crystallize low melting solids, which are liquid a t room temperature. Many liquids can he purified more easily by a crystallization technique than by conventional distillation, especiauy when only small amounts of material are available. Repeated recrystallization of compounds without removal from the assembly is also possible. Materials weighing from 10 t o 200 mg. have been conveniently crystallized in the apparatus a t temperatures ranging from - 10' to -60°C. The apparatus is easily fabricated 'from a Biichner type funnel with a fritted disc (Corning Glass Works, Inc., Cat. No. 36060, 10-mm. disc, medium porosity) as shown in the drawing. In use, the assembled apparatus is joined to a drying tube or tower a t B and to a nitrogen gas tank at A. This tank supplies a positive gas pressure to the bottom of the fritted disc and prevents the solution from passin? through during the crystallization. In order to prevent a huild-up of excessive pressure, an open end u tube containing mercury is also joined to the nitrogen source. The pressure required is about 2-5 mm. of mercury. Before a solution of the material to he recrystallized is introduced to the assembly, the disc is wetted with a small amount of solvent. This step has been found to eliminate clogging of the disc by crystallized materials during the subsequent cooling. The unit is placed in an appropriate coolant in a Dewar flask, up to point C. Temperatures from 0 to -85'C. can he maintained by adding pieces of dry ice as required to acetone. External tapping or rubbing of the assembly with a file aids the formation of crystals. The filtration and drying of the solids deposited are effected at the same temperature by closing off the nitrogen source and applying suction from an aspirator at A. When the compound has a higher melting point than room temperature, the assembly is removed from the cooling bath, and the sample is removed. When the
compound is low melting, the aspirator is closed off after completion of the drying and a positive nitrogen pressure is reapplied. The assembly is removed from the coolant, and the receiving flask is replaced by a fresh flask in which the pure sample is collected by washing through the fritted disc. The sample is then isolated by evaporation of the solvent. The assembly has proved useful for the recrystallization of fatty acids, e.g., C14 labeled oleic and elaidic acids (melting points 14' and 51°C., respectively). It has also been utilized to purify a group of azomethines, e.g., benzalaniline, in order to determine molecular extinction coefficients in the ultraviolet region. Since these compounds were found to be susceptible to hydrolysis, the usual procedure of recrystallizing from water could not be employed. I t was desirable, therefore, to recrystallize the compounds from isooctane, a solvent in which the compounds were easily soluble at room temperature. The recrystallizations were carried out at -10' to -30°C. without difficulty in the described apparatus.
JOURNAL OF CHEMICAL EDUCATION
