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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)

phasediagramco2

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.

before3phasesliquidco2

By February 4, 2009 31 comments fun

Chemistry Lab Demonstrations: Check-In Day

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I’m TA’ing an undergrad organic lab this semester, and in order to keep myself and the students entertained, I’m trying to have some kind of demonstration at the beginning of each lab.  The goal is to have the demonstration have something to do with the theme of the lab.  The first lab of the semester was this week.  On the first day, all the students do is check in their glassware.  The lab doesn’t acutally have anything to do with organic chemistry.  So that means the demonstration doesn’t have to have anything to with organic chemistry, right?

This week, I did my favorite demonstration, the oscillating clock reaction.  Formally known as the Briggs-Rauscher reaction, mixing three colorless solutions together creates a solution which oscillates between blue, amber, and colorless for several minutes.

The explanation for what’s happening is actually really complex.  I’ll quote from the University of Leeds website:

In the Briggs-Rauscher reaction reaction the evolution of oxygen and carbon dioxide gases and the concentrations of iodine and iodide ions oscillate.  The somewhat simplified mechanism of this reaction can be represented by the following overall transformation:

IO3 + 2 H2O2 + CH2(COOH)2 + H+==> ICH(COOH)2 + 2 O2 + 3 H2O           (11.1)

This transformation is accomplished through two component reactions:

IO3 + 2 H2O2 + H+==> HIO + 2 O2 + 2 H2O                              (11.2)

HIO + CH2(COOH)2==> ICH(COOH)2 + H2O                            (11.3)

The first of these two reactions can occur via two different processes, a radical process and a nonradical process.  Which of these two processes dominates is determined by the concentration of iodide ions in the solution.  When [I] is low, the radical process dominates; when [I] is high, the nonradical process is the dominant one.  The second reaction (eq. (11.3)) couples the two processes.  The reaction consumes HIO more slowly than that species is produced by the radical process when that process is dominant, but it consumes HIO more rapidly than it is produced by the nonradical process.  Any HIO which does not react by eq. (11.3) is reduced to I by hydrogen peroxide as one of the component steps of the nonradical process for reaction (11.2).  When HIO is produced rapidly by the radical process, the excess forms the iodide ions, which shut off that radical process and start the slower nonradical process.  Reaction (11.3) then consumes the HIO so rapidly that not enough is available to produce the iodide ion necessary to keep the nonradical process going, and the radical process starts again.  Each of the processes of reaction (11.2) produces conditions favourable to the other process, and, therefore, the reaction oscillates between these two processes.

You can read more about the reaction and learn how to do it yourself from about.com.

Here’s a video of the oscillating clock reaction.  No, this is not me from lab the other day.

By January 29, 2009 6 comments fun

The Periodic Table of Videos

I’d like to share this link with you: The Periodic Table of Videos (http://www.periodicvideos.com/).

A group of lecturers around Prof. Martyn Poliakoff from the University of Nottingham have compiled this “Periodic Table”, which contains a video of a few minutes for every (!) known element. Sometimes it’s an explosive experiment (e.g., Hydrogen), in other cases they simply show a sample, while giving some information about the element. I must say, I am impressed how they manage to come up with something for all the lanthanoids and actinoids. For Ytterbium, Yttrium, Terbium and Erbium, they even went on a trip to Ytterby (Sweden), the town these elements are named after. A great distraction for rainy days, and also a useful video collection for teaching!

By January 27, 2009 4 comments fun, general chemistry

Science Podcasts

I got a new iPod Touch from Sigma-Aldrich for Christmas this year (no, really).  Probably the best part is the wi-fi capabilities.  I tried keeping up with podcasts before on other iPods, but it was too much work to plug the iPod into the computer, transfer over the new episodes, and repeat.  Now I can grab the new episodes directly from the iTouch, and it’s awesome.  (btw, does anyone know how to subscribe to the podcasts so the iPod will update itself with new episodes automatically?)

Anyway, I’ve been surveying the chemistry podcasts over the past few weeks.  They’re great to listen to while running a column or doing other tasks that don’t require a lot of mental energy.  Most of the major journals have a podcast, as do most news outlets and some other random sites.  I’ll tell you about some of the ones I liked below the jump if you’re interested in giving some of them a listen.

Read more ›

By January 26, 2009 7 comments fun, science news