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Determination Of Alcohol Content In Blood

Because 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 by sodium dichromate. A sealed glass tube is used, which con­tains the oxidising agent impregnated onto an inert material. The ends of the tube are broken off. One end of the tube is attached 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.

Ethanol in the breath is oxidized as it passes through the column. When ethanol is oxidized, the red-orange oxidizing agent (Cr2O72-) is reduced to green chromic ion. The greater the concentration of alcohol in the breath, the farther the green colour spreads through the tube.

C2H5OH + Cr2O72-- --------> Cr+3 + CH3COOH
          red-orange           green

If the person fails this test — determined by the extent to which the green colour spreads through the tube — a more accurate Breathalyser test is administered. This test also depends on the oxidation of breath ethanol by sodium dichromate, but it provides more accurate results. Here, an accurate volume of breath is bubbled through an acidic solution of sodium dichromate. The concentration of chromic ion is measured precisely using a spectrophotometer.

Contributed by: Mr Devender Singh, dev_chem2002@yahoo.co.in .