How Breathalyzers Work

Sometimes a driver can pass the roadside sobriety test that the officer gives them, but still be over the legal limit and have their driving skills impaired (Freudenrich 1). For this reason, many police officers and state troopers use portable breathalyzers, like the one shown here:
Portable breathalyzer
There are also bigger units at the station that are used when the suspect is taken there. Blood alcohol contain concentration can be measured with blood, urine, saliva, or breath, but breath is the quickest and easiest way for an officer to test it in the field (Goldberg 41). Alcohol testing devices of the breath were first developed in the 1940s for police use (Freudenrich 2). There are three main types of breath alcohol testing devices: the Breathalyzer, Intoxilyzer, and the Alcosensor III or IV (Freudenrich 2).
The mouth, throat, stomach, and intestines absorb alcohol into the bloodstream. When the individual's blood passes through the lungs' air sacs, the alcohol moves across them as well, thus the concentration of alcohol in the breath is directly proportional to the concentration of it in the blood (Freudenrich 2). The breath alcohol to blood alcohol contain ratio is 2,100:1 (Freudenrich 2). This means that every 1 ml of blood has the same amount of alcohol in it as 2,100 ml of exhaled breath.
Dr. Robert Borkenstein invented the Breathalyzer, one type that is used by police today (Freudenrich 2). The Breathalyzer has 4 basic parts: the mouthpiece, which is a tube that the person blows air in, a sample chamber for the air, two glass vials, and a system of photocells (Freudenrich 2). The two glass vials contain a mixture of water, sulfuric acid, silver nitrate, and potassium dichromate (Freudenrich 2). The sample of breath that the person breathes in is bubbled through this mixture. The photocells are connected to a meter that measures the chemical reaction through a color change (Freudenrich 3).
The way the Breathalyzer works is though a fairly simple chemical reaction. When the person breathes into the mouthpiece, the air is collected in the sample chamber. From here, the air is bubbled through the mixture into the two glass vials where the following chemical reaction occurs
Breathalyzer chemical reaction
The sulfuric acid takes the alcohol from the air and puts it into a liquid solution, and then the alcohol has a chemical reaction with potassium dichromate (Freudenrich 3). This reaction produces water, potassium sulfate, chromium sulfate, and acetic acid (Freudenrich 3). The silver nitrate makes the reaction happen faster, but has no participation in the reaction (Freudenrich 3). When the dichromate ion reacts with the alcohol, it changes to a green chromium ion (Freudenrich 3). The level of alcohol in the expelled breath is measured by the degree of color change (Freudenrich 3). A vial of unreacted mixture in the photocell system is compared to this color-changed mixture to determine the amount of alcohol that was in the expelled breath (Freudenrich 3). The photocell system causes the needle in the meter to move by an electric current (Freudenrich 3). The person operating the Breathalyzer moves the needle back to its original position and then reads the number on the knob (Freudenrich 3). This tells how much alcohol was in the breath. The more the knob is turned, the higher the level of alcohol (Freudenrich 3).
The Intoxilyzer is based on the principles of IR (infrared) spectroscopy (Dombrink 135). This device has a greater accuracy in testing the presence of alcohol than the Breathalyzer, thus more states are beginning to use it (Dombrink 135). IR spectroscopy is based on the way different molecules absorb infrared light (Freudenrich 4). The vibrations of molecules change when IR light is absorbed and this causes the various bonds within a molecule to bend and stretch (Freudenrich 4). IR absorption happens at different wavelengths for each bond (Freudenrich 4). By knowing the wavelengths of the bonds in ethanol (drinking alcohol), the absorption of IR light can be measured (Freudenrich 4). From this measurement, the amount of alcohol in the breath can be computed (Freudenrich 4). There are four basic parts to the Intoxilyzer: the lamp, the broadband IR beam, the filter wheel, and the electrical pulse (Freudenrich 4).
A) Quartz Lamp D) Sample Chamber H) Microprocessor
(IR Source) E) Lenses
B) Breath Input F) Filter Wheel
C) Breath Outlet G) Photocell
The IR beam is generated by the lamp and this beam goes through the sample chamber to a lens where it's focused onto the filter wheel (Freudenrich 4). The filter wheel detects the wavelengths of the ethanol bonds, and then the light that passed though the wheel is detected by the photocell and transformed into the electrical pulse (Freudenrich 4). This pulse is then sent to be interpreted by the microprocessor and the blood alcohol contain is calculated based on this interpretation of the absorption of the IR light (Freudenrich 4).
The Alcosensor III or IV is based on fuel cell technology (Freudenrich 4). The fuel cell consists of two platinum electrodes with a acid-electrolyte substance in between them (Freudenrich 4). When the person's exhaled breath flows part of the fuel cell, the alcohol is oxidized by the platinum into protons, electrons, and acetic acid (Freudenrich 5). The electrons produced flow through a wire to a current meter and on through to the platinum electrode (Freudenrich 5). The protons combine one the other side of the fuel cell with oxygen and electrons and form water (Freudenrich 5). The greater the current is, the more alcohol that was in the person's breath (Freudenrich 5). The blood alcohol contain concentration is then computed by the microprocessor (Freudenrich 5).
All forms of breathalyzers are required to be used only by people trained to use them if the evidence is to be used in court (Freudenrich 5). This is why officers usually use a full-sized breath testing device back at the station (Freudenrich 5).

sources
Dombrink, Kathleen J. "A Commercial Device Involving the Breathalyzer Test
Reaction." Journal of Chemical Education 73, no. 2 (1996) : 135.
Freudenrich, Craig C., Ph.D. "How Breathalyzers Work." 2000. HowStuffWorks, Inc. 11
April 2005 http://science.howstuffworks.com/breathalyzer.htm/printable.
Goldberg, Raymond. Taking Sides: Clashing Views on Controversial Issue in Drugs and

Society, Sixth Edition. Guilford, Connecticut: McGraw-Hill/Dushkin, 2004.