As blood passes through the arteries in the lungs,an equilibrium is established between the alcohol in one's blood and the alcohol in one's breath. So if the concentration of one is known, the concentration of the other can be estimated. The test that law enforcement agencies use to approximate a person's blood alcohol level is based on the oxidation of breath ethanol. The test employs a sealed glass tube that contains the oxidizing agent ( sodium dichromate dissolved in sulfuric acid ) impregnated onto an inert material. The ends of the tube are broken off, and one end of the tube is attracted to a mouthpiece and the other to a balloon-type bag. The person undergoing the test blows into the mouthpiece until the bag is filled with air.
Any ethanol in the breath is oxidized as it passes through the column. When ethanol is oxidized, the red-orange dichromate ion is reduced to green chromic ion .
Thr greater the concentration of alcohol in the breath, the farther the green color spreads through the tube.
If the person fails this test determined by the extent to which the green color spreads through the tube a more accurate Breathalyzer test is administered. The Breathalyzer test also depends on the oxidation of breath ethanol, but it provides more accurate results because it is quatitative . In the test , a known volume of breath is bubbled through an acidic solution of sodium dichromate, and the concentration of the green chromic ion is measured precisely with a spectrophotometer.
