The first devices used for the detection of ethyl alcohol in the breath were based in oxidation-reduction reactions. The next experiment uses an acid dissolution of potassium dichromate as an indicator. The passage of ethanol to acetic acid (oxidation), will produce the reduction of dichromate (orange) to Cr3 +, (green colour). Thus, the shift from Orange to green will reveal the presence of ethanol.
The experiment shown in the video there is a first gas washer bottle containing a small amount of ethanol, which simulates the ethanol content in the breath. Blowing, part of this alcohol, will go to the dissolution of dichromate (in the second washer bottle of gas) producing his reduction to Cr3 + and therefore the turning from Orange to green, according to the reaction: