MICRO SCALE CRYSTALLIZATION PROCEDURE IN PHOTOSThe following photos illustrate steps in a micro scale crystallization, using the pipet method to remove solution from the crystals. When the crystals are too small for the pipet method to be used, the crystals can be separated by suction filtration on a Hirsch funnel. This is described elsewhere.
Adding a solid to a reaction tube with a spatula, using a powder funnel to make a clean transfer. Note that a small beaker can be used to hold reaction tubes in an upright position.
Using a Pasteur pipet to add solvent to the solid in the reaction tube.
Heating the reaction tube in a sand bath. Note that a boiling stick is added to the tube to promote smooth boiling which helps prevent the solution from boiling over. The sand bath voltage control on the left is usually not set higher than 30. The temperature of the reaction tube is controlled by the depth to which it is placed in the sand, not by adjusting the voltage.
Once enough solvent has been added to dissolve the solid at the boiling point (bp) of the solution, the reaction tube is removed from the heat, the boiling stick is removed, and the tube is allowed to cool slowly to room temperature (rt). Slow cooling can be achieved by placing the tube into an insulated container (here, a small beaker into which a paper towel has been stuffed). Once the solution has cooled slowly to rt and the solid has crystallized, the tube is often cooled further in an ice-H2O bath to produce more crystals. In the following steps, the tube can be kept in an ice-H2O bath to keep the solution cold and to prevent crystals from redissolving.
Once crystallization is complete, a pipet is placed into the reaction tube,
the crystals are stirred around a bit, and the pipet is pushed to the bottom of the tube while expelling air from the pipet. It is essential that the pipet be placed squarely on the bottom of the tube, without crystals getting between the tube and the pipet.
The pipet bulb is released, allowing solution to be separated from the crystals. Using the pipet to separate the solution from the crystals is equivalent to doing a filtration.
A few drops of fresh ice-cold solvent is added to rinse impurities off the surface of the crystals, the whole time keeping the tube cold. This rinse solvent is then removed with a pipet in the same way as before.
The crystals must now be dried. This can be done in several ways. To allow solvent to evaporate, the tube can be left open for at least a day, or to speed up things a bit, the solid can be scraped from the tube onto a piece of clean dry filter paper.