A breath test used by traffic police to check drunken driving works on which chemical reaction?

The breath test for detecting alcohol consumption relies on a simple but effective oxidation–reduction reaction between ethanol (present in a drunk person’s breath) and acidified potassium dichromate. The chemical reaction is used in breath analyzers, which are portable devices carried by traffic police to determine whether a driver has consumed alcohol.

The key chemicals involved are potassium dichromate (K₂Cr₂O₇) and sulphuric acid (H₂SO₄). When a person breathes into the analyzer, any ethanol vapor in their breath reacts with the dichromate solution. In this reaction, ethanol is oxidized to acetic acid (CH₃COOH), while the orange-colored dichromate ions (Cr₂O₇²⁻) are reduced to green-colored chromium(III) ions (Cr³⁺). This visible color change from orange to green is a clear indication of the presence of alcohol in the breath.

Chemical Reaction:
3C₂H₅OH + 2K₂Cr₂O₇ + 8H₂SO₄ → 3CH₃COOH + 2Cr₂(SO₄)₃ + 2K₂SO₄ + 11H₂O

Here, ethanol acts as the reducing agent, and potassium dichromate acts as the oxidizing agent. The intensity of the color change is proportional to the amount of alcohol in the breath, allowing the device to estimate the blood alcohol concentration (BAC).

Why Potassium Dichromate is Used:

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  It is a strong oxidizing agent in acidic medium.

  
  


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  Produces a clear and distinct color change (orange to green).

  
  


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  Reacts specifically with alcohols like ethanol, ensuring reliable results.

  
  

Other Facts:

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  Modern breath analyzers use electronic sensors, but the principle of oxidation of alcohol remains the same.

  
  


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  The test is quick, non-invasive, and provides instant results for law enforcement.

  
  

Thus, the potassium dichromate–sulphuric acid reaction forms the chemical basis of the breath test for drunken driving, effectively detecting the presence of ethanol through a redox reaction and visible color change