How a Breathalyser Works

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When a breathalyzer test is administered, the subject breaths into the device, where any ethanol in the breath becomes involved in a redox reaction with acidified potassium dichromate(VI) K2Cr2O7. Any alcohol is oxidized into ethanoic acid. Orange Cr(VI) is reduced to green Cr(III).

Cr2O7-2 + 14H+ + 6 e- -> 2Cr3+ + 7H2O
C2H5OH + H2O -> CH3COOH + 4 e- + 4H+
2Cr2O72- + 3C2H5OH + 16H+ -> 4Cr3+ + 3CH3COOH + 11H2O

The breathalyzer operates through a redox reaction, where orange chromium(VI) is reduced to green chromium(III).

Cr2O7-2 + 14H+ + 6 e- -> 2Cr3+ + 7H2O
C2H5OH + H2O -> CH3COOH + 4 e- + 4H+
2Cr2O72- + 3C2H5OH + 16H+ -> 4Cr3+ + 3CH3COOH + 11H2O

The redox reaction generates an E.M.F. that is digitized into a reading on the device.

The data shows the B.A.C. (blood alcohol concentration) in the subject's blood (mg/cc). Data is acquired from an analog to digital conversion of E.M.F. Readings, and is usually relatively inaccurate compared to results obtained through gas-liquid chromatography.

Breathalyzer - A scenario

The breathalyzer is often referenced in popular culture and has become a symbol of law enforcement. But how does this iconic device work? Let's go through the process of taking a breathalyzer test.

You swerved around a pothole, and a police officer thinks you have been drinking. Therefore, you are pulled to the side of the road. The police officer orders you to blow into a breathalyzer. In this case, you are drunk over the legal limit. But you had a drink an hour ago, so how is there alcohol still in your breath?

Alcohol is highly penetrating within the body. It gets just about everywhere via the blood stream, even inside of your lungs. This alcohol is breathed out when you exhale, and this is what the breathalyzer detects.

Your breath with the alcohol in it enters the breathalyzer. Inside the breathalyzer is a chemical that reacts with alcohol. This reaction can be detected by a computer embedded within the breathalyzer.

The computer inside the breathalyzer produces a reading on the screen. The police officer reads it, and it indicates that you have too much alcohol in your blood. Go directly to jail. Do not pass go, do not collect $200.00.


The breathalyzer is certainly a powerful tool for courts and law-enforcement alike. This chemical masterpiece of technology has allowed our society and legal system to evolve to a point never before seen.

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