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Breathalysers: detecting alcohol by colour changes
When someone blows into a breathalyser bag, any alcohol in their breath is turned into acetic acid (vinegar). This chemical reaction changes the colour of the crystals in the blowing tube. The more crystals that change colour, the more alcohol they have in the body.

The first breathalyser was developed by an American doctor, Rolla N. Harger (he called it a Drunkometer'), and it was introduced by the Indianapolis police in 1939. Similar breathalysers began to be widely used by the police in many countries in the 1960s, as a yardstick for judging a driver's ability to drive. A high intake of alcohol dulls the nervous system and slows up coordination.

To begin with, the commonest type of breathalyser was a plastic bag, similar to a balloon, with the crystals in the blowing tube, and the driver was asked to inflate the bag. If the crystals changed colour as far as a level marked on the tube, the driver was possibly 'over the limit', and needed further tests. The crystals used were an orange-yellow mixture of sulphuric acid and potassium dichromate. They turned the alcohol into acetic acid (vinegar), and in doing so they were changed into colourless potassium sulphate and blue-green chromium sulphate.

The breathalysers used by the police today, however, are usually electronic, and much more accurate than the inflatable-bag type. They use the alcohol blown in through the tube as fuel to produce electric current.

The more alcohol the breath contains, the stronger the current. If it lights up a green light, the driver is below the legal limit and has passed the test. An amber light means the alcohol level is near the limit, a red light above the limit, and in both cases the driver has failed the breath test and needs further testing.

This type of breathalyser is about the size of a TV remote control, and contains a fuel cell that works like a battery. Breath from the tube is drawn into the cell through a valve, and meets a platinum anode (a positive plate), which is against a spongy disc impregnated with sulphuric acid.

The platinum causes any alcohol in the breath to oxidise into acetic acid - that is, its molecules lose some of their electrons. This sets up an electric current through the disc, and it flows to a cathode (a negative plate) on the other side.

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