Chemistry Lab Demonstrations: LIQUID CO2 Extraction!

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It’s the extraction lab this week in the OChem lab I’m TA’ing. It’s a straightforward aqueous base extraction of an acidic unknown from a neutral impurity. Acidify, filter the precipitate, and you’re done. I was trying to come up with a demonstration for the lab. I thought about extracting caffeine from coffee or tea leaves, but that would take a while, and isn’t all that visually appealing.  I’ve only got a few minutes in my pre-lab lecture time.

So I looked around for a while, and finally found this paper by James Hutchison from the University of Oregon (doi:  10.1039/b405810k).  They suggest a new lab for undergraduates involving the extraction of D-limonene from orange peels using liquid carbon dioxide.  That’s right, I said liquid carbon dioxide.

The premise: create a removable filter using copper wire and filter paper to jam into the bottom of a disposable centrifuge tube.  Add grated orange peel.  Add crushed dry ice.  Cap the centrifuge tube tightly (but not TOO tightly! The tube needs to be able to vent so as not to EXPLODE!) and immerse in warm water (T = 40-60 degC).  The pressure rises (naturally) and the temperature increases and you jump into the liquid portion of carbon dioxide’s phase diagram (click for larger)


The liquid carbon dioxide percolates through the orange peels and extracts the limonene.  the oil-in-solvent mixture drains through the filter paper to the bottom of the centrifuge tube.  If you leave the tube in the water long enough, eventually the liquid all evaporates and the pressure decreases.

The goal is that the evaporation of the carbon dioxide leaves the pure oil at the bottom of the tube.  The authors mention that for approximately 2.5 g of freshly-grated orange peel, 0.1 mL of oil should remain after 3 carbon dioxide extractions.  They note this is a yield comparable to typical organic solvent extraction or cold pressing.  I did one extraction on day-old chopped orange peel and did not isolate any oil whatsoever.  Not a drop.  I’m a little disappointed by that, but not really.  It’s still an ok teaching point for the students.  Not all experiments work all the time. I could examine my starting materials and get better quality reagents and it might work.

Now, inside the tube I don’t think we were past the critical point.  I don’t think the temperature inside the centrifuge tube actually makes it up to the temperature of the surrounding water.  I say this because after the examining the tube after the experiment, the orange was cold and there were ice crystals in the tube.  There are two possible explanations for this.  One, the temperature inside didn’t make it past the critical temperature.  Two, when I opened the tube after the experiment, some non-trivial amount of pressure was released.  PV=nRT tells us that a sudden drop in the pressure simultaneously lowers the temperature, and I could have frozen the water out that way.  In fact, the authors note that while exact temperature and pressure readings are impossible with this simple setup, they speculate that the conditions approach the triple point.

In any case, it was a very cool experiment to watch, even if it didn’t do what it was supposed to.  Pictures below.  These pictures are from Monday night when I was practicing the demonstration.  It looked much cooler in person.  The first shows the system when first submerged in the water.  The second is about 15-30 seconds later.  It’s hard to see, but if you look closely, all three phases are apparent in the system.  The third is after the dry ice has completely liquified.  Click for larger.



  1. That’s still cool. How do you “cap the centrifuge tube tightly but not too tightly”?

    • Good question. The first time I tried it, I just had dry ice in the centrifuge tube and nothing else. It worked perfectly.

      Then next 3 or 4 times I tried it with the orange peels and everything, it didn’t work. By that I mean the dry ice never liquefied. It just remained a solid for over a minute (when it works, it liquefies in less than 30 seconds). When I’d had enough, I removed the tube and carefully opened the tube. Pressure was definitely released, indicating there was a build up of pressure.

      What ended up working was un-tightening the cap a quarter turn or so. It seems I was tightening it too tight such that the pressure was building up too high. With the pressure too high, I was missing the liquid range and shooting back up into the solid range. Crazy, I know, but those were my observations. Tighten the cap all the way, un-tighten a quarter turn and it seemed to be at an optimal position. I guess I’m glad I killed the experiment when I did. Otherwise I imagine the pressure would have just kept building…

  2. It’s important to be really careful with this one, because if you change things around a bit you can make it MUCH more dangerous. Instead of putting those tubes in beakers, you should have them in heavy walled plastic cylinders to contain the explosion (in case one happens). Somebody in my department had an explosion in a different kind of container, and it was pretty painful.

    • I would recommend using a stoppered sink for this experiment. I knew a teen who lost a thumb to a CO2 bomb (dry ice in an empty 2-liter pop bottle). I know this is a smaller scale, but its still dangerous. Would an extremely small hole in the plastic cap work better?

      • Would an extremely small hole in the plastic cap work better?

        Probably. The authors note, and I observed, that the pressure slowly escapes from the centrifuge tube through the threads of the cap – the primary reason for not tightening the cap too tight. If the cap is on too tight, the gas cannot leak through the threads.

    • we used to stick fragments of dry ice pellets into those little plastic 1.5 mL eppendorf tubes biologists love so much – 30-60s later you get a very satisfying bang. another thing worth playing with is blocking the needle end of a disposable plastic syringe (a big gob of parafilm is acceptable, but not ideal) and placing a single dry ice pellet in it. hold the blocked end against the bench, replace the plunger and squeeze for all you are worth. the pellet undergoes the solid -> liquid transition right in front of your eyes, and it’s rapidly reverses when you (carefully) release the plunger. a similar setup might be applicable to your extraction

  3. I was an assistant for “Travaux Pratique” organic lab in France and we did the limonene extraction by refluxing orange peels in water and collecting the limonene….boring…I will suggest they change to this for next year, minus explosions though!

  4. To make it more reproducible, safer and work better would be quite easy. Get a container you can put a septa on. Pierce a disposable syringe through the septa, and hook it up to a vacuum hose line. Connect the other end of the hose to a makeshift bubbler (all of this should sound familiar if you have done much schlink line work). Now, this bubbler needs to be really deep, and have it going through heavy mineral oil. You can then start the experiment with just a little oil in the bubbler, and as the gas escapes rapidly slowly add more and more oil to increase the pressure from the oil and thus the pressure required to push it out of the way to get out of the container. Once you find the sweet spot, mark the spot on your bubbler and you are golden.

    This will require HEAVY mineral oil (not the kind typically used in bubblers for schlink line work). And a really tall makeshift column.

    You then have a makeshift one way pressure regulator.

    • Once you go down that path it just makes 10 times more sense to buy a small pressure regulator.

      • Not really. Most chemistry departments have a supply of glass tubing of various diameters for making stuff. It would be quite trivial to melt/close one end of the larger diameter one and stick the smaller one in there. And again heavy mineral oil should be something trivial to find in a chemistry department. I would be shocked that any school with undergraduates and more then 30 chemistry students a semester would not have all of these supplies in the stock room.

        Students also get to learn about pressure this way! You could if you want, give them the physical constants of the mineral oil, and the height of the oil, and they could then calculate the pressure at which the gas is able to just escape, etc.

  5. I’m not sure you can apply the ideal gas law in this case. While by opening the tube you lower the pressure, you also increase the volume drastically, in fact to infinity.
    I suppose in the case of the very non-ideal CO2, you’re witnessing the Joule-Thomson effect ( This is related to the heat capacity of the gas.

    • Abstractly, if he was anywhere near equilibrium:

      An infinite volume would make a gas behave more ideally. 😉

      As the volume goes to infinity the Van der Waals’ term (V – nb) approaches the ideal gas law’s term (V).

  6. I just realized you had a pressurized system surrounded by glass with a mercury thermometer. Everything is fine with that setup until it isn’t.

  7. This is how they decaffinate coffee.

    Your current setup is probably unsafe. Replace the mercury thermometer with a digital probe of some sort. Why risk spreading mercury about?

    I would probably entertain the notion of finding some heavy wall Pyrex tubing. There are nomographs on the net that detail exactly how much pressure a tube of so much diameter and wall thickness can take. You can also submerge the tube in a water bath, that way if there is an explosion, its energy dissipates into water. Obviously it is related to the surface area of the inside of the tube.

    If not glass, why not flanged Swagelock stainless steel tubing?

    On the topic of disastrous over-pressurizations:

    in particular the incident on page 3 and 4. It could easily have been far worse if the cesium had ignited as well.

    • I think the only thing I would have done differently is take out the mercury thermometer. In retrospect, that was probably a bad idea. It was the only one in my thermometer drawer in the right temperature range.

      As to the overpressurization, that was my main concern. But having run through the demo probably a dozen times, now, I really don’t think the pressurization hazard was as big as it could have been. When done correctly*, as the pressure builds it simultaneously releases through the threads. You can see the bubbles in the water bath formed from escaping CO2 in the pictures.

      The authors think the conditions approximated the triple point, based on the cold temperature of the tube during and after the demo and the presence of all three phases simultaneously. That puts the system at about 5.1 atm or so. Nothing to handle casually, but I think the cap would have blown off before the tube exploded.

      *(famous last words)

  8. not a science pro like everyone else. but i was in the audience, and it looked pretty cool. too bad the orange peels didn’t give us something, but it was still a fun thing to look at before the lab itself. keep it up!

  9. Jan Wade Gilbert, DMD says:

    It seems as though the major problem with coal-fired energy plants is the production of CO2. Why can’t the CO2 be split into carbon and oxygen? It seems as though the time, effort and money that is being used to capture and store the CO2 can be used to split the molecule resulting in usable products.

  10. Pingback: Chemistry Blog » Blog Archive » Chemisry Lab Demonstrations: Nylon Rope Trick

  11. I just realized you had a pressurized system surrounded by glass with a mercury thermometer. I would recommend using a stoppered sink for this experiment. This is very much dangerous to use this in LAB.

  12. Hello, This is a great article and I know this is an old thread, but if someone reads this great. It’s only my speculation, maybe the reason why no limonene was extracted is because unless the super critical stage is reached, the liquid co2 itself will not act as a solvent and extract the oil. So, the pressure may have been enough (5.1 bar) turn to the solid into liquid, but the the temp was not sufficient to turn the mixture of gas/liquid in the tube super critical and thus no limonene.

    • Thanks for your comment. I don’t think supercritical conditions are necessary. I think liquid CO2 should suffice.

      I think the real reason it didn’t work was quality of reagents. The orange peel was a day old. While all the limonene won’t have diffused, the amount left in the peel was probably far less than optimal. I also probably didn’t use as much orange peel as I was supposed to, and I only did 1 extraction instead of the recommended 3.

  13. this was exactly what i was looking for, great explanation guys.
    i work in essential oils and im trying to build an industrial co2 extraction set up.
    im part of a collective of scientific glass blowers and began to manufacture 4 foot long glass tubes with rotovist glass on glass screw on glass chemical filters with valves for tanks.
    iv purchased a 20lb tank with a dip tube inside and a 2 way regulator valve for welding “because it had a 1/4 tube atachment”.

    is it important to submerge my extractors in warm water to make the co2 super critical for the best results as a solvent. what factors can i tweak to make the solvent more super critical and yield more oils?

    if anyone is interested in these extractors id love to share any information.

  14. We did this experiment in my organic lab. It was relatively easy to see how tight to put the cap on. Me and my lab partner did the process twice and got a very good amount of product. We put our tube in a plastic graduated cylinder filled halfway with water that was room temperature and we had no accidents. I found it key to pack the dry ice as tightly as possible. And yes, visually, this project is soo cool!

  15. I’m doing a experiment in which I will react salicylic acid with methanol to get methyl salicylate. My question is why when doing the extraction why might the layers switch ?

  16. I would really like to talk to you about the supercritical extraction process. I have done some basic experiments with essential oils and am going to construct a larger model.

  17. Hello, I’m a government contractor and I’d like to use one of your images in a report for the Department of Energy. If possible, please e-mail me and I will provide the details (name of the report, synopsis, etc.). Thanks!

  18. good job , cab you extract co2 from sea water

  19. how can you verify that the liquid you extracted is really limonene? and what type of molecules will carbon dioxide extract

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