Testing Blood Alcohol Levels

The chemistry behind the Breathalyzer is based on the reaction between alcohol in the breath and the chromate or dichromate ion.

3 CH3CH2OH(g) + 2 Cr2O72-(aq) + 16 H+(aq) ---->3 CH3CO2H(aq) + 4 Cr3+(aq) + 11 H2O(l)

The instrument contains two ampules that hold small samples of potassium dichromate dissolved in sulfuric acid. One of these ampules is used a reference. The other is opened and the breath sample to be analyzed is added to this ampule.

If alcohol is present in the breath, it reduces the yellow-orange Cr2O72- ion to the green Cr3+ ion.

The extent to which the color balance between the two ampules is disturbed is a direct measure of the amount of alcohol in the breath sample.

Measurements of the alcohol on the breath are then converted into estimates of the concentration of the alcohol in the blood by assuming that 2100 mL of air exhaled from the lungs contains the same amount of alcohol as 1 mL of blood. Print This Post