The chemistry of some Breathalyzers involves the reduction of orange chromium(VI) to green chromium(III) by ethanol. In this activity, an orange solution of chromium(VI) is added to four quadrants of a Petri dish. Ethanol is then added to one quadrant until a color change occurs. Differing amounts of ethanol are added to the second and third quadrants, and the color changes are compared to each other and to the standard (control). (NOTE: Most Breathalyzers used by law enforcement agencies now utilize infrared analysis.)

Materials

• 4-Quadrant glass Petri dish (or four separate dishes arranged in a square)
• Overhead projector
• Stirring rod
• 3 Medicine droppers
• Paper towel
• Chromic acid reagent [add 6 g chromium(VI) oxide (CrO 3 ) to 12 mL concentrated sulfuric acid and stir until a smooth paste is obtained. Cautiously dilute the paste with 40 mL distilled water, and stir until a clear orange solution is obtained. Cool to room temperature.]
• Ethanol, 45% [47 mL 95% ethanol (C 2 H 5 OH) per 100 mL solution]

Safety

Handle the chromic acid solution with care. It is a strong oxidizing agent, toxic by ingestion, and an alleged carcinogen. Follow proper disposal procedure (see SourceBook Safety section and your local requirements for disposal).

Procedure

1. Place the Petri dish on the overhead projector with the lamp on (or on a white sheet of paper on the desktop).
2. Add 15 drops chromic acid solution to each quadrant in the Petri dish. Add 15 drops distilled water to each quadrant and mix.
3. Keep Quadrant 1 as a control.
4. Add 1 drop 45% ethanol to Quadrant 2.
5. Add 2 drops 45% ethanol to Quadrant 3.
6. Add 3 drops 45% ethanol to Quadrant 4.
7. Compare the intensity of the green-brown color produced in each quadrant as a function of the amount of ethanol added. Make obser-vations after 1-2 min. [Quadrant 2 contains a light colored precipitate; Quadrant 3, medium-dark; and Quadrant 4, a heavy dark precipitate. The increasing amount of precipitate indicates the increasing amount of ethanol oxidized.]