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Most law enforcement agencies use either the Intoxilyzer or the Intoximeter to determine if a driver is driving under the influence of alcohol. The Intoxilyzer operates on the principles of IR spectroscopy while the Intoximeter uses gas chromatography. However, before these devices many law enforcement agencies used devices which involved the reduction of the dichromate ion to chromium (III). The reaction is given below:
8H1+ + Cr2O72- + 3C2H5OH -----------> 2Cr3+ + 3C2H4O + 7H2O
This reaction is still used today in the device Final Call (available from U.S. Alcohol Testing of America, 10410 Trademark St., Rancho Cucamonga, CA 91730). I recently presented my laboratory assistant (Heather O'Neill) with a picture of the Final Call device and asked her to see if she could devise a simple demonstration version of the device to be used in my classes. The device Heather created worked very well the first year I tried it. However, I have since found that it is not very reproducible. Her original procedure is reproduced below:
Approximately 5 G of silica gell was soaked in a solution of 20 mL of 0.1 M potassium dichromate and 10 mL of 6 M sulfuric acid. The gel was allowed to soak in the solution overnight. After soaking the the silica gel was separated from the dichromate-acid mixture by filtration. The yellow colored crystals of silica gel were then air dried in a hood overnight.
Glass tubing, with a diameter of 1 cM, was cut into a length of 10 cM and fire polished. Cotton was loosely packed into one end of the glass tubing. Approximately one gram of the dried silica gel was placed into the tube followed by a second piece of loose cotton. (The potassium dichromate laden silica gel is sandwiched between the two pieces of cotton.)
I demonstrated the operation of the device by placing one drop of ethanol into a balloon, blowing up the balloon and slowly releasing the "artificial breath" into the device. The yellow silica gel started to turned green after only about half of the air had been released into the device. I was impressed by its sensitivity.
I believe that the concentrations of the potassium dichromate and sulfuric acid can be reduced dramatically. The next time I prepare these demonstration devices I will use 0.01 M potassium chromate and 1 M sulfuric acid.
When I demonstrate this before my class I indicate that the one drop of alcohol (95% ethanol) I placed in the balloon is about equal to 2 drops of most vodka's or whiskey's (48% ethanol), 8 drops of wine (12% ethanol), and 32 drops of beer (3% ethanol). Hopefully, this will make some of them think twice about drinking and driving.
I have also discovered that the type of silica gel affects how this demonstration works. I have tried several gel's and the best seems to be a type labeled for Thin-Layer Chromatography. Care must be taken on the environment that the gel is dried in. Laboratory vapors seem to cause the absorbed dichromate to react, and be reduced, during the drying process.