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Im running a soxhlet extraction using hexane as the solvent. Once the hexane has filled up the soxhlet, a considerable amount of hexane will come out on top of the condenser. Is this an issue with pressure or defects in the glasses?

  • 4
    $\begingroup$ That's not supposed to happen. Are you running the bath at the proper temperature and is your water cold enough? Are you using boiling chips? You also might be overloading the filter cup, which would reduce flow back into the reservoir. $\endgroup$ Commented Apr 28, 2021 at 1:08
  • $\begingroup$ no im not using any boiling chips $\endgroup$
    – prpper12
    Commented Apr 28, 2021 at 1:11
  • $\begingroup$ Heat up slowly. $\endgroup$
    – Alchimista
    Commented Apr 28, 2021 at 9:16
  • 8
    $\begingroup$ The glass joint between the top of the Soxhlet and the condenser looks broken. It is probably leaking hexane vapour profusely. You can't get a good seal with broken glass joints. Solution: don't use broken glassware. $\endgroup$
    – matt_black
    Commented Apr 28, 2021 at 10:12

3 Answers 3


Heating without boiling chips or stirring will cause vapor to burst. Your condenser is not the efficient type either. Suggestion: add boiling chips (after it is cooled down); change a more efficient condenser; heat up slowly.


As noted above by matt_black (eagle eye!) the female joint on the top of Soxhlet is broken. That joint most be replaced ASAP by a qualified glassblower.

Otherwise, you can either injure yourself when moving around the joint, or, due to repeated heating / cooling as well as downward pressure from the weight of the condenser above, the crack will propagate deeper into the Soxhlet itself, potentially resulting in a much bigger accident involving a flammable solvent and a heating element.

  • 1
    $\begingroup$ Plausible cause: OP built this skyscraper, put on top the condenser behind the non-moveable (typically less frequently cleaned) part of the sash. Then the less than lucky clamp (looks like the only one from the stand ...) was not able to sustain the leverage when directing the hose outward (instead of using a clear sink in the rear of the hood) + closing the sash once; the noise of the AC in the hood was louder then the chirping sound of the joint broken, the fracture out of sight if standing in front of the hood. Thankfully it did not topple over (yet) with all the splashes possible. $\endgroup$
    – Buttonwood
    Commented Apr 28, 2021 at 11:42

It is not a defect of pressure (you rightfully leave the system open, because the condenser is open), nor a defect of the glasses used. With the details provided by you, I would reconsider the setup and the scale of the setup.

Instead of the single large (and very tall* [you can't access the top quickly]) apparatus shown in your question, I recommend to use multiple smaller ones (there are smaller Soxhlets, too) than the one used. It looks like even if all of the solvent were in the round bottom flask, it never will be more than half filled. See if you can get two round bottom flasks of $\pu{500 mL}$ (or, two of $\pu{1 L}$) instead which then each would fill up to two thirds at max, topped by their Soxhlet and a good Dimroth condenser. At this size, it is easier to find suitable magnetic stirring bars for a slow, but continuous agitation of the liquid phase to lower the risk of a sudden «shooting», as you name it, especially if using a heating bath easier to monitor and manage than the heater you use now.

For convenience, these magnets should be the shape of an elongated rugby, because a straight cylindrical bar bears too much danger of hitting (and eventually, breaking) the glass. Equally, because of their weight, do not drop them into the flask, but slowly glide them into the container; it is possible to slow their move to the bottom if you tilt the empty flask and held a magnet on the outer side of the wall.

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These are available in different sizes (example entries for these eggs) to match the shape of the round bottom flask's bottom. Start with a rotation set at low rpm and gently increase the rate.

Like for a distillation, you may wrap the parts supposed to be hot (upper part of the round bottom flask, ascend of the extractor but not the beaker, nor the siphon) with aluminum foil). This equally helps to run a gentle extraction.

Consider mechanical support with a clamp for the flask / extractor in addition to the one currently only seen for the condenser. A two / three necked round bottom flask (instead of the single necked one) offers you to refill of hexane (if necessary), to monitor the inner temperature, and to sample the extract for TLC / GC to monitor the advancement of the extraction, too. (The necks at the side need not be of the same large diameter as the one in the centre.)

*) It looks like you use a (an already bent) lab stand to balance the condenser on top of the mount. Despite of its metal plate, given the high centre of gravity of the setup, it would be safer to use a grid of rods mounted to the wall of the fume hood instead (see examples here). Ensure the drain of the hood is clean and working well, and then direct the hose from the condenser to this sink. This allows you to close the sash without worries that the extraction will topple over plus increased lab safety (you don't topple over the hose, less solvent vapours in the lab, less energy consumption (air condition), less noise, etc. Similar for hoods where the sash closes automatically.


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