The Breathalyzer contains the following components a system of photocells connected to a meter to measure change in the chemical reaction, a system to sample the breath and two glass vials containing the chemicals.

The exhaled air is bubbled in the glass vial by the mixture of silver nitrate, potassium dichromate, sulfuric acid and water. The following reaction takes place -

In this reaction the reddish-orange dichromate ion changes colour to the green chromium ion in the presence of alcohol in the breath sample. The degree of change in colour is directly proportional to the level of alcohol in the expelled air. A electric meters show the amount of alcohol which compares the vial of reacted and un reacted chemical mixtures in the photocell system.