principles of alcohol detector?

Answer 1

there are 2 types of "breathalyser",
1) the ones based on colour change. orange dichromate crystals turns green when oxidised by ethanol.

2) Ones based on a simple fuel cell.

The first are simple chemical reactions where the orange dichromate IV ions are reduced to green chromate IV ions, as ethanol is oxidised into ethanal. the more ethanol in the breath, the greater the colour change.

the second are a bit more complex, as they measure a voltage produced between two electrodes where 2 chemical reactions occur. Oxygen is reduced to water at one electrode while the ethanol is oxidised into ethanoic acid at another. The greater amount of alcohol in the breath, the greater the voltage produced.

No alcohol = no voltage = green light
Some = a small current = amber light
Too much = large current = red light.

And before you ask there is no way to fool either method. If you have alcohol in your system it will be picked up.

Answer 2

Works on the colour change which occurs when Cr2O7(2-) is reduced.