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Sublimation Removal

When a compound sublimates, it goes straight from solid to gas without stopping at liquid in between. Sublimation is sometimes useful as a way to remove a compound that does undergo sublimation from a mixture of other compounds that do not. One of the best examples is the purification of caffeine. Once you've extracted caffeine from tea leaves or coffee beans with a suitable solvent, you can remove it from the resulting mixture through sublimation.

Things You'll Need

Lab coat, goggles and gloves (have these on before you start)
Spatula
Mixture of compounds containing caffeine
2 sidearm flasks
Hot plate
Sand bath
Ring stand
3 clamps
2 vacuum hoses
Vacuum adapter
Non-mercury thermometer
Conical rubber adapter
Glass centrifuge tube
Crushed ice
Pasteur pipette
Beaker
Watch glass

Instructions

- 1
  Place the mixture or sample in the first sidearm flask. Place the hot plate on your bench and plug it in but do not turn it on yet.
- 2
  Place the sand bath on the hot plate and seat the first sidearm flask in the sand. Connect both flasks to the ring stand using clamps. You can also use two ring stands -- one for each flask -- if you need to do.
- 3
  Plug the vacuum hose onto the vacuum outlet; using the vacuum adapter, connect the other end to the second flask -- the one that does not contain any sample. Connect the sidearms of the first and second flasks using the second vacuum hose.
- 4
  Attach the thermometer to the ring stand with the third clamp so that the tip of the thermometer penetrates just below the surface of the sand. You want the thermometer tip to be the same depth in the sand as the base of the flask. This will allow you to monitor the temperature in the flask at all times.
- 5
  Push the centrifuge tube through the rubber conical adapter. The fit should be tight -- tight enough that you have to push hard to get it through. If it is loose, the vacuum will pull the centrifuge tube through the adapter.
- 6
  Insert the conical adapter into the mouth of the flask. The tip of the centrifuge tube should be centimeter above the bottom of the flask.
- 7
  Turn on the vacuum gradually. Do not turn it on suddenly -- you might suck out some of your sample. To check to make sure it's working, temporarily unplug the vacuum adapter from the vacuum-trap flask -- you should hear the hissing sound of the vacuum.
- 8
  Add ice to the centrifuge tube with your spatula. Be careful not to let water run down the side of the centrifuge tube in the process. Fill the tube up to the top with ice.
- 9
  Turn on the hot plate and begin heating your flask. Allow it to heat to 200 degrees Celsius.
- 10
  Watch the ice in the tube. As it melts, withdraw the water it forms using your Pasteur pipette (be careful not to break the tip,) and add more ice to replace the ice that melted.
- 11
  Watch the sample inside the flask. Once you get past 178 degrees Celsius, the caffeine should begin to sublime, and if all goes well it will form a solid on the tip of the cold centrifuge tube (and also possibly on the walls of the flask.) The solid it forms will be white in color but may appear to have a faint yellowish tint.
- 12
  Continue to adjust the hot plate as needed while the caffeine sublimes. Once the caffeine ceases to deposit on the centrifuge tube and/or flask walls and you see no further activity for a couple minutes, turn off the hot plate.
- 13
  Allow the apparatus to cool down for a few minutes, then turn off the vacuum. Carefully remove the centrifuge tube and quickly dump the ice it contains into a beaker. Using your spatula, scrape the caffeine from the tip onto a watch glass. Scrape any caffeine on the sides of the flask onto the watch glass as well. Bear in mind that if left too long, your cold centrifuge tube can collect moisture and thereby dissolve the caffeine collected.
- 14
  Allow the flask and sand bath to cool, then disassemble the remainder of your apparatus and clean up.

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