A Simple Method to Perform a Liquid Diffusion Crystallization Of the many methods available to grow crystals from solution for structure determination,' liquid diffusion is often the m w t successful because several days are required to achieve complete mixing (and crystallization). The preparation of such a svstem with a oerfeet intersalvent meniscus (to allow maximum mixing time) is unusallv the main ~rohlem.This may be ov&ome by using a disposable pipet with a plug of glass wool m the neck.
gloss WOI dirposoble pipewe
r m n d solvent (precipitonl) 25 rnl rcinfillotion viol
A
B
T o provide an example, the figure shows the method using a 25-mL screw-top scintillation vial. A concentrated solution of the compound to be crystallized is prepared (about 5 to 10 mL in an appropriate solvent). A precipitating solvent is selected in which the compound is insoluble, and which is miscible with, and lessdense than, the first solvent. Thevial is half-filled with the second solvent and the pipet is placed as shown in part A of the figure. Thesolution isintroduced dropwise via the plugged pipet which allows for a slow flow therehy neatly displacing the second solvent upward and creating a perfect unmixed interface. When the addition iscomplete, the pipet is smoothly withdrawn while the forefinger is held firmly aver the open top to prevent the solution remaining below the plug from leaking and clouding the meniscus area (result is shown in part B of the figure). The vial is capped, and crystals form a t the interface during the three or four days required for mixing. The common practice of layering the precipitant on top of the solution often results in much mixing a t the interface, thereby cutting down the time for crystallization. Common precipitantlsolvent pairs are acetonelwater; hexanelmethylene chloride; and diethyl etherlmethanol or ethanol. This method has been used in this laboratory to provide single crystals of a wide variety of compounds for X-ray crystallography. Jones, P. G., Chem. Brit., 17,222 (1981).
Chris Orvlg Universily of British Columbia 2036 Main Mall Vancouver, B.C. V6T 1Y6 Canada
84
Journal of Chemical Education
