The colour change from Cr(VI) (orange) in the presence of alcohols to Cr(III) (green) is used in a preliminary determination of the ethanol level in the breath (and therefore blood) of suspected alcohol intoxicated persons, e.g. drivers. If this test proves positve then police officers are permitted to take a more accurate test of blood and urine.
This works because of the diffusion of blood alcohol through the lung into the breath, with a measured ratio of approximately 2100:1 (i.e. 2100ml of breath contains as much ethanol as 1ml of blood). In the most basic version of the test, the participant is asked to blow into a tube containing K2Cr2O7 and H2SO4 supported on powdered silica gel (SiO2) for a time of roughly 10 - 20 seconds. Any alcohol present in the breath is oxidised to acetic (ethanoic) acid. This reaction is shown by the gradual colour change from orange to green along the tube, according to this equation:
2K2Cr2O7 + 8H2SO4 + 3CH3CH2OH = 2Cr2(SO4)3 + 2K2SO4 + 3CH3COOH + 11H2O
If the green develops past the halfway mark then the participant has a blood alcohol concentration above 0.08%, which is considered a criminal offence in many countries. More sophisticated versions of this test exist that use chromatographs and spectrometers.
Some say that smoking, chewing coffee beans or eating garlic befroehand can fool the test this however is false. Are there other ways? More than likely! But it is better to be safe than sorry!
Back to Alcohols