Recrystallization without Tears In the method of recrystallization usually presented in organic chemistry lab texts, the dissolved solid is made to separate from solution as ita solubility in the cooling solvent decreases. Crystallization of a solid from solution can also he effected by changing the solvent polarity, rather than its temperature. This "cold" recrystallization technique (which is not new hut perhaps only neglected) has proven for us to he faster, neater, safer, more efficient, and more easily mastered by introductory students than the usual "hot-cold" method. Furthermore, if the easy steps of the "cold" recrystallization technique are repeated several times, decolorization with charcoal is not necessary because oily and colored impurities are removed by infinite dilution. The following procedure for the purification of 2-naphthyl acetate is illustrative of the "cold" method of recrystallization. Cover the crude solid with methanol, stir to dissolve, and add about another 10-15% of methanol. Add distilled water in small portions and mix thoroughly by swirling until the entire mixture is just faintly turbid. Then allow the mixture to stand a t room temperature or chill i t in a n ice bath until reerystallization of the solid is complete. Filter the solid by suction. Soluble and colored impurities are removed with the filtrate, but insoluble impurities remain with the solid. Without drying completely, return the solid to the flask or beaker used originally, cover the solid with fresh methanol, and repeat the procedure as often as necessary. For the final cycle of the procedure, the methanol solution is filtered t o remove insoluble impurities prior to treatment with water to produce the turbidity. After three eyeles of this procedure, 67% of 2-naphthyl acetate is recovered. Using an acetone-water pair, 71% of aspirin and 78% naphthalene are recovered. Further information regarding this improved general method of reerystallization may be obtained by cammunication with the authors. David Bierne Steven Smith Bernard E. Hoogenboom Gustavus Adolphus College St. Peter. Minnesota 56082
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Journal of Chemical Education
