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How can a breath analyzer use the reaction between ethanol and potassium dichromate?

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A breath analyzer measures alcohol levels by utilizing an oxidation-reduction reaction between ethanol and potassium dichromate.

When a person exhales into the device, the breath sample passes through a solution containing potassium dichromate and sulfuric acid. During this process, the ethanol in the breath is oxidized to acetic acid, while the orange-colored potassium dichromate is simultaneously reduced to green chromium (III) sulfate.12

The intensity of this color change from orange to green is directly proportional to the amount of ethanol present in the breath sample. Because the concentration of alcohol in the breath is directly related to the alcohol content in the blood, this visible chemical shift allows the breath analyzer to provide an accurate estimate of a person's level of intoxication.12