Chemistry Lab Demonstrations: Homemade Breathylizers…Sorta
by azmanam on Mar 19 2009 (3164 Views)*For more cool stories, pictures, and videos of chemistry demonstrations, click here*
Undergrads were on spring break last week, so no lab last week. This week, we performed the Jones oxidation of isoborneol to camphor. The crude product was put in the bottom of a Hirsch funnel with a cold finger, and the product was purified by sublimation. One of my favorite chemistry trivia facts is that the opposite of sublimation is deposition. Now you know.
I must say, I am really, really disappointed with the way this week’s demo turned out. Plan A was following a patent prep to immobilize Jones reagent on silica gel. That actually worked really well. No problems there. I had a nice orange granular solid. The next step was to pack it into 5″ pipets between glass wool and plain silica gel to create a tall, narrow column of Jones reagent. That also worked really well. I meant to take a picture of the setup before I dumped everything, but I was pretty down after it didn’t work and just poured it all into my chromium waste bottle. Sorry.
Anyway, the plan was to ask if anyone came to lab drunk that day (no one admitted they did), then have someone volunteer to swish around some Listerine for a while. Then attach a clean drinking straw to the pipet and blow. See, Listerine is 21% alcohol, so there’d still be some ethanol vapor in the breath which should flow past the Jones reagent. The Jones reagent will oxidize the ethanol to acetic acid and will itself be reduced. The reduced chromium reagent is greenish brown. With the pipet system, the orange color would slowly change over to green from the start to the finish. The amount of ethanol in a person’s breath (and therefore the BAC) can be determined by seeing how far up the column the green color extends. The more solid that is reduced to green, the more wasted the person.
I know there are several potential safety issues with this setup. Chromium is toxic and shouldn’t be ingested or released into the environment. The goal was to have only me handle the glass and only the student with clean hands touch the drinking straw – which was not ot be unwrapped until just before use. When I was testing this, I don’t know what was going on, but I could get no air pressure through the pipet. With breath not flowing through the pipet, there’s no chance of the Jones reagent being reduced, so no demo.
See, they really did used to use Jones reagent in breathalyzers. They don’t anymore because chromium is toxic, and well, lots of things can reduce chromium… not just ethanol. This leads to false positives. Now they use a fuel cell for better results. The best part of the Jones reagent story is that drunk people would blow through a solution of Jones reagent, which would reduce some of the chromium. A UV/Vis detector could measure the absorption of the solution before and after the test. The absorption is related to the amount of ethanol because the measured absorption (A) is equal to the product of the concentration of chromium (c), the path length of the cell (l), and a constant unique to chromium (ε). A =εcl. This relationship, ironically enough, is known as Beer’s law. That’s right. Beer’s law can be used to tell how drunk a person is.
Plan B was to have a row of test tubes with a solution of Jones reagent to which was added increasing amounts of ethanol. This would gradually change the color from yellow to greenish. A separate test tube with a solution of Jones reagent in lab would have an arbitrary amount of ethanol added to it. It could be titrated against the standards to show the color change and determine the level of drunkenness. Problem is the color change is very subtle in dilute solutions. Too subtle to really see. No demo again.
I won’t bore you with plan C, but suffice it to say – fail.
So finally, plan D. Same as plan B, but with a surrogate for Jones reagent… and a surrogate for ethanol. Basically none of the actual reagents, but would still give a color change. I went with acid/base chemistry. A row of test tubes was filled with an acidic solution of bromocresol green. The acidic solution is yellow. To each tube was added an increasing amount of sodium hydroxide so the color would gradually change to a deep blue. The first test tube was yellow, then gradually orange, then muddy brown, then blue. In fact, the series hit just about every color EXCEPT green (the image looks better. Mine was not nearly so nice. I was in a hurry). Anyway, then an arbitrary amount of NaOH was added to a separate tube with an acidic solution of bromocresol green and titrated against the standard.
It wasn’t a great demo, but it looked like it was supposed to look.
What ever happened to the paper cones (utilizing paper chromatography?) that are placed over the eyes to absorb the alcohol vapors emanating from your cornea and/or tears? I found a patent that is similar, but not quite. http://www.google.com/patents?id=o6gcAAAAEBAJ
http://www.youtube.com/watch?v=7TY9womdTz8