Friday, September 16, 2011

Simple Distillation is Not So Simple


 Simple distillation is a deceptive name.  This week I conducted a demonstration of a simple distillation for my honors class as part of the Foul Water Lab from the ChemComm curriculum.   (I don’t use ChemComm, but I like this lab from the book.)  I used Demo 9.10 Separating Liquids: Fraction Distillation found in volume 3 of Shakhashiri’s Chemical Demonstrations, A Handbook for Teachers of Chemistry as a reference for the set-up and discussion. 

Here's the simple distillation apparatus.
The distillation demo was the final step in a water purification lab conducted by my Honors students.  They were challenged to purify a water sample that contained oil, coffee grounds, garlic powder, and salt.  I used the “Foul Water Lab”, which is an excellent introduction to separating mixtures, chemistry lab techniques, and the physical properties of matter.  The students did a series of purification steps:  separating the oil from the water, sand/gravel filtration to remove large particles, charcoal absorption to remove odors, and a distillation to remove the salt (and the rest of the impurities that they didn’t get out before).  I conducted the final step of the experiment as my demo this week because my lab tables don’t have sinks (very annoying) and we don’t have enough condensers (they are useless without sinks, by the way). 

My colleague suggested that I distill Cherry Coke first to introduce the technique, something I had never tried.  This was a lot of fun because the kids always love testing something they might actually eat or drink.  I decided to set up the apparatus during class (some of the groups were still finishing up the filtration, so half the class had some down time).  Most of the students had never seen a distillation.  I handed the tubing to a couple of boys and asked them to connect it for me, while I attached clamps to ring stands.  I asked for more helpers to hold the round bottom flask and condenser in place while I clamped it all together.  The set-up was a distant cousin to the fractional distillation apparatus in my college organic chem lab, with glassware that all fits together and bars in the back of the hood for clamps.  After a lot of adjusting, I managed to assemble the apparatus with the Coke in the flask, when I realized that I had forgotten to add the boiling chips.  So I loosened some clamps, lowered the round bottom flask, added the boiling stones, and manipulated the apparatus back together.  My students watched, gathered around the front bench, a few lending a hand to help me steady everything as I readjusted.   At the same time I explained how the system works to separate liquids with different boiling points.  Finally we were ready to light the Bunsen burner.  (As a side note, the first time they see me light the burner is a big deal every year.  Soon enough the magic is gone and this becomes just part of the regular lab routine.)

The first thing we observed was rapid bubbling as we watched the carbon dioxide leaving the solution.  I bubbled the gas through limewater to confirm the presence of CO2, another suggestion from my colleague.  Very cool so far.  Then we started collecting a liquid.  I wish that I had remembered to put a thermometer in the flask to note the boiling point of the distillate, but I didn’t think about that until it was too late.  It’s been a really long time since I’ve done a distillation.  It all started coming back to me as we were boiling the Coke.  Maybe I should have spent a little longer reading Shakhashiri, because a thermometer is clearly labeled in the diagram!  We collected a clear, colorless liquid form of the cherry/vanilla flavoring.  The strong odor of cherry coke was overpowering in the small sample we collected.  The second liquid we collected was also clear and colorless with a faint smell of cherry coke; this time I think we collected water with a small amount flavoring contaminate.  After we had about 100 mL of distillate, I removed the pot and let the students observe the smell of the liquid left behind, flat and flavorless Coke.  They enthusiastically passed around the three samples and commented on their odor and color.

 Distilling the foul water sample, the last step in the purification lab.

Next we distilled their foul water samples.  I combined several groups’ samples into the pot because we were only going to do this once.  The brownish, electrically conductive water sample in the pot started to boil vigorously as the bell rang ending the period.  No!  My kingdom for 20 more minutes.  We were already collecting distillate, so I decided to let it continue to distill after the period ended.  I saved the purified water sample into an Erlenmeyer flask and saved the remaining pot liquid.  During the next class period, I was a bit nervous to test the conductivity of the solution.  What if it didn’t work?  How would we explain the presence of salt in our purified sample?  I decided to let the anticipation build by showing the class a control test of the conductivity of distilled water (no light) and salt water (bright light).  We were all pumped up when the light did not come on when we submerged the electrodes in our purified water. The only disappointment was the lingering garlic odor in our water sample.  The purification would have been better with a fractional distillation set-up rather than a simple distillation (a second condenser above the pot makes all the difference), possibly separating out the organic substance in the solution responsible for the lingering odor.  However, we did the job we set out to accomplish; the salt was definitely gone from the final product.

 Compare the sample of untreated foul water to our purified sample.  Looks nice, but don't smell it!
 Here's the distillate we collected and the liquid that was left in the pot at the end of the distillation.

3 comments:

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    1. Sorry that I removed your comment. I didn't mean to do that. I find that the distillation of cherry coke goes well as long as you leave enough of the soda solution in the pot. That will help avoid burning the sugar in the pot. The flavoring that comes over first is concentrated, but not pure (at least with my apparatus). If you have a distillation column you could get a much more pure sample of the cherry flavor.

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