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Sublimation MP and VP

  1. Apr 7, 2012 #1
    I am wondering if I can separate these two compounds:

    Y: M.P.=170 degrees C - Vapor pressure at its MP = 0.05 torr

    Z: M.P.=170 degrees C - Vapor pressure at its MP = 540 torr

    I think having a huge difference in vapor pressures is good. I am kind of confused about this process.
  2. jcsd
  3. Apr 7, 2012 #2


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    You aren't really subliming it if you do the process at or above the melting point. It is just simple distillation. You will need a really cold "cold finger" to isolate Z. I would finely grind it and put in into an ampoule. Pull a mild vacuum on the ampoule and flame-seal the ampoule. Invert the ampoule so that the sample is entirely in the upper bulb of the ampoule. Attach the ampoule with a clamp to a vertical riser being careful to hold it vertically so that all of the sample is still contained in the lower, bulb side of the ampoule.

    Now the fun part... Place a puck of dry ice on the flat side of the ampoule and heat the bulb with something. You can use an oil bath, handheld bunsen burner or lab heat gun. I prefer the oil bath. Replace the dry ice puck often. You will notice a solid slowly build up on this side of flask. When you have enough, remove the heat and dry ice and allow the ampoule to come to room temperature. Snap off the top and the base should contain very pure crystals of Z. The bulb will be quite enriched with respect to Y but will be contaminated with some Z.

    This is how we purify old ampoule samples of osmium tetroxide in the lab.
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