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Types of Devices: Breathalyzer

There are three major types of breath alcohol testing devices, and they're based on different principles:

Breathalyzer - Uses a chemical reaction involving alcohol that produces a color change
Intoxilyzer - Detects alcohol by infrared (IR) spectroscopy
Alcosensor III or IV - Detects a chemical reaction of alcohol in a fuel cell

Regardless of the type, each device has a mouthpiece, a tube through which the suspect blows air, and a sample chamber where the air goes. The rest of the device varies with the type.

Breathalyzer

The Breathalyzer device contains:

A system to sample the breath of the suspect
Two glass vials containing the chemical reaction mixture
A system of photocells connected to a meter to measure the color change associated with the chemical reaction To measure alcohol, a suspect breathes into the device. The breath sample is bubbled in one vial through a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The principle of the measurement is based on the following chemical reaction:

Types of Devices: Intoxilyzer

This device uses infrared (IR) spectroscopy, which identifies molecules based on the way they absorb IR light.

Molecules are constantly vibrating, and these vibrations change when the molecules absorb IR light. The changes in vibration include the bending and stretching of various bonds. Each type of bond within a molecule absorbs IR at different wavelengths. So, to identify ethanol in a sample, you have to look at the wavelengths of the bonds in ethanol (C-O, O-H, C-H, C-C) and measure the absorption of IR light. The absorbed wavelengths help to identify the substance as ethanol, and the amount of IR absorption tells you how much ethanol is there.

Types of Devices: Alcosensor III or IV

Modern fuel-cell technology (which may power our cars and even our houses some day) has been applied to breath-alcohol detectors. Devices like the Alcosensor III and IV use fuel cells.

The fuel cell has two platinum electrodes with a porous acid-electrolyte material sandwiched between them. As the exhaled air from the suspect flows past one side of the fuel cell, the platinum oxidizes any alcohol in the air to produce acetic acid, protons and electrons.

Oxidation of Alcohol

If you strip off hydrogens from the right carbon of ethanol in the presence of oxygen, you get acetic acid, the main component in vinegar. The molecular structure of acetic acid looks like this:

O
||
h1C - C - O - H

where C is carbon, H is hydrogen, O is oxygen, the hyphen is a single chemical bond between the atoms and the || symbol is a double bond between the atoms. For clarity, the bonds of the three hydrogen atoms to the left carbon atom are not shown. When ethanol is oxidized to acetic acid, two protons and two electrons are also produced.

The electrons flow through a wire from the platinum electrode. The wire is connected to an electrical-current meter and to the platinum electrode on the other side. The protons move through the lower portion of the fuel cell and combine with oxygen and the electrons on the other side to form water. The more alcohol that becomes oxidized, the greater the electrical current. A microprocessor measures the electrical current and calculates the BAC.