How a Breathalyzer / Alcoblow works and why it may not be possible to trick or beat it

The term “alcohol” to a chemist means an organic compound that contains an OH group, but as far as the public is concerned “alcohol” refers to one specific compound, namely, ethanol. It is ethanol that we consume in wine, spirits or beer, and when we measure blood alcohol content (BAC), we’re really measuring blood ethanol content.

Alcohol that a person drinks shows up in the breath because it gets absorbed from the mouth, throat, stomach and intestines into the bloodstream.

Alcohol is not digested upon absorption, nor chemically changed in the bloodstream. As the blood goes through the lungs, some of the alcohol moves across the membranes of the lung's air sacs (alveoli) into the air, because alcohol will evaporate from a solution -- that is, it is volatile. The concentration of the alcohol in the alveolar air is related to the concentration of the alcohol in the blood. As the alcohol in the alveolar air is exhaled, it can be detected by the breath alcohol testing device. Instead of having to draw a driver's blood to test his alcohol level, an officer can test the driver's breath on the spot and instantly know if there is a reason to arrest the driver.

Because the alcohol concentration in the breath is related to that in the blood, you can figure the BAC by measuring alcohol on the breath. The ratio of breath alcohol to blood alcohol is 2,100:1. This means that 2,100 milliliters (ml) of alveolar air will contain the same amount of alcohol as 1 ml of blood.

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.

The police often use a device called a breathalyzer to test drivers suspected of being drunk. The chemical basis of this device is a redox reaction. A sample of the driver’s breath is drawn into the breathalyzer, where it is treated with an acidic solution of potassium dichromate. The alcohol (ethanol) in the breath is converted to acetic acid as shown in the following equation:

3CH3CH2OH   +   2K2Cr2O7   +   8H2SO4   → 3CH3COOH + 2Cr2 (SO4)3   +   2K2SO4   +   11H2O

 

Ethanol + Potassium Dichromate + Sulfuric Acid →  Acetic Acid + Chromium (iii) Sulfate + Potassium Sulfate +11H2O

                (Orange yellow)                                                         (Green)


In this reaction, the ethanol is oxidized to acetic acid and the chromium (VI) in the orange-yellow dichromate ion is reduced to the green chromium (III) ion. The driver’s blood alcohol level can be determined readily by measuring the degree of this color change. The current legal limit of blood alcohol content in most states is 0.1 percent by mass. Anything higher constitutes intoxication.



The alcohol in the driver’s breath is reacted with a potassium dichromate solution. The change in the absorption of light due to the formation of chromium (III) sulfate is registered by the detector and shown on a meter, which directly displays the alcohol content in blood. The filter selects only one wavelength of light for measurement.



A breathalyzer doesn’t measure the alcohol in your mouth; it measures the alcohol (ethanol) in your breath, which comes straight from your lungs. Ethanol is a molecule which can slip through all kinds of membranes and end up anywhere in your body. So alcohol in your blood moves to your lungs.

There are a few situations in which a breathalyzer may fail to measure BAC accurately. Notably, individuals with higher-than-normal levels of acetone in their breath may have it detected as ethanol. This could include diabetics, those on fasting diets, or those adhering to a ketogenic diet. There are a few other substances that could interfere with the chemistry of a breathalyzer, but not ones that you’re too likely to have in your bloodstream, thankfully.


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