The original version of the Breathalyzer™ included a mouthpiece and two chambers containing liquid connected to a meter that detects a change in color. To use the Breathalyzer™, the subject exhales through the mouthpiece into a test chamber filled with a reddish-orange solution of potassium dichromate (K₂Cr₂O₇).

Figure 4.7 An example of a newer version of a breath testing device

In the Breathalyzer™, alcohol reacts with the reddish-orange potassium dichromate solution and turns green. The degree of the color change is directly related to the level of alcohol in the expelled air.

A photocell compares the difference in colors between the reacted mixture in the test chamber and a reference chamber containing unreacted mixture. The difference in colors produces an electrical current, which can be converted into a quantitative value for the BAC. Read on to understand how the chemical reaction produces the color change.

Figure 4.8 The ethanol vapor in the Breathalyzer™ triggers a chemical reaction with the compounds packed inside, turning the chemicals from orange to green. The more ethanol present, the greater the amount of green color is produced.

A description of different types of breath analyzers and how they work can be found at http://science.howstuffworks.com/breathalyzer3.htm