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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.