Breathalyzer Test
This demonstration shows the reaction in a police
breathalyzer test.
Materials
Breathalyzer Solution
2 large test tubes
1 ml 95% ethanol
3 ml distilled water
Hot plate
250 ml beaker
To Prepare Breathalyzer Solution:
5g K2Cr2O7
500ml 1M H2SO4
Where to Find
Shelf 2G
Drawer M1
Flammables cabinet
Faucet next to sink
Cabinet B4
Cabinet D4
Shelf 4B
Shelf 5F
Set Up
Add 5 ml of breathalyzer solution (K2Cr2O7 in H2SO4) to each test tube. Add 1 ml of 95%
ethanol to one test tube followed by 1 ml of distilled water. Next, add 2 ml of distilled
water to the other test tube (control). Label the test tubes to distinguish the one containing
ethanol. Put test tubes in the 250 ml beaker with water as a water bath.
To make Solution



Pour 250 ml of 1M H2SO4 into 500ml volumetric flask.
Add 5g K2Cr2O7
Add 1M H2SO4 until solution reaches 500ml mark.
Safety
Do not ingest the ethanol or breathalyzer solution. Do not touch the hot plate.
Disposal
Pour solutions into aqueous waste.
Professor/Lecturer
Chemistry
The breathalyzer is a test commonly used by the police to detect the presence of alcohol
on the breath. A strip in the device contains potassium dichromate in sulfuric acid. The
water vapor from the breath activates the reaction. The potassium dichromate in H2SO4
oxidizes the alcohol to an aldehyde, changing the strip’s color, thus indicating the
presence of alcohol. The equation for the oxidation of alcohol to an aldehyde with
potassium dichromate is:
Cr2O72-(aq) + 3 C2H5OH(aq) + 8H+(aq) → 2 Cr3+(aq) + 2 C2H4O(aq) + 7 H2O(l)
The alcohol reduces the dichromate ion, which has a yellow color, to Cr3+ ion, which has
a green color, and the alcohol is oxidized to an aldehyde. A false positive can occur if the
person receiving the breathalyzer test is diabetic, as diabetics have aldehydes in their
breath. Other compounds also give false positives.
Presentation
Place the beaker on the hot plate to heat the solutions. The solution in the test tube with
ethanol should change the solution color from yellow to green.
Search Items
Alcohols
Redox Reactions
Aldehydes
Rate of Reaction
Suitable for Recitation
Yes