Breathalysers

• Suspect breathes into apparatus and the air passes through a tube.
• Any ethanol is oxidised to ethanoic acid, reducing the orange potassium dichromate to green chromium(III). A meter detects the colour change.
• An electric current causes the needle in the meter to move.
• A knob is then rotated to return the needle to its resting place so the operator can read the alcohol level.
• The more the operator has to turn, the higher the alcohol level.

• As the sample flows along, a piece of platinum oxidises any alcohol to produce ethanoic acid, H+ ions and electrons.
• An external circuit containing a current meter is attached to both electrodes.
• The H+ ions combine with oxygen and the electrodes on the other side to form water.
• The more ethanol oxidised, the larger the electric current.
• A microprocessor measures the current and calculates the blood alcohol content.

CH3CH2OH(l) + O2(g) --> CH3COOH + H2O(l)

Used for preliminary breath testing. They give an idea of the alcohol level, but they can't be used in court because there are too many potential sources of error.

• IR radiation is passed through the sample and certain wavelengths associated with C-O, O-H, C-H and C-C bonds are absorbed.
• A spectrum allows the concentration of ethanol to be measured accurately by a microprocessor.

IR spectrometry gives very accurate results, so evidence from this type of breathalyser can be used in court. However, the instrumentation is less portable than other types.

• Breathalysers must be calibrated regularly to ensure that they give accurate readings.
• They only give the composition of air breathed out. It is assumed that the air exhaled is from within the lungs but it could come from other parts of the body as well. The operator has to leave the suspect for at least 15-20 minutes before taking a further reading.