Breathalyzers use the redox reaction of alcohol to detect ethanol in a driver’s breath. Ethanol reduces potassium dichromate, causing it to change color, which is used to calculate the blood alcohol content.
Who hasn’t drowned their worries in a glass of alcohol? Alcohol seems to be an inseparable part of our increasingly frenetic lives. However, behind its powerful appeal lies the danger of drunk driving. Many politicians and celebrities are no exception to this, as driving under the influence is potentially dangerous enough to be considered attempted murder. While many deny that they’ve been drinking, a palm-sized breathalyzer can always suspend or take away your license. So how do these breathalyzers work?
The answer is the redox reaction of alcohol. When a drunk person blows into a breathalyzer, the alcohol it contains oxidizes inside the device, reducing the orange potassium dichromate (K2Cr2O7) to green chromium sulfate (Cr2(SO4)3). All the officer has to do is watch the meter change color. Here’s a detailed explanation of this process
It’s no secret that ethanol is the main component of alcohol. Ethanol is a substance that exists as an ethyl group (C2H5-) with two carbons and a hydroxyl group (-OH) attached to it (C2H5 + OH = C2H5OH). When we consume ethanol, our central nervous system is inhibited, causing us to feel languid, excited, dizzy, and lightheaded, while endorphins are produced in the brain to make us feel good. This is why alcohol has been the main beverage throughout human history.
Alcohols, including ethanol, share several common characteristics due to their hydroxyl group, including the aforementioned redox reactions. Oxidation is the process by which an electron-loving atom, such as oxygen or a halogen (a group 17 element in the periodic table), is added to a molecule, or an electron-hating atom, such as hydrogen, is removed from a molecule. Reduction is the opposite process and occurs simultaneously with oxidation, such as in the breathalyzer process outlined above. In the case of ethanol, it is oxidized under certain conditions to a substance called acetaldehyde, which is then oxidized once more to acetic acid.
Of course, in order for ethanol to be oxidized in this way, it needs to be reduced (potassium dichromate). Chromium (Cr), atomic number 24, has an orange color when it is in its most oxidized form, Cr(Ⅵ), and, unusually, a dark green color when it is reduced to Cr(Ⅲ). Chromium can form a variety of compounds, one of which is potassium dichromate, which is orange in color, Cr(Ⅵ). This potassium dichromate is attached to silica gel and placed in a powdered form inside the tube of the breathalyzer, and when the driver blows into the tube, the ethanol in the breath causes the following reaction
2K2Cr2O7 (orange) + 8H2SO4 + 3C2H5OH (ethanol) → 2Cr2(SO4)3 (green) + 2K2SO4 + 11H2O + 3CH3COOH (acetic acid)
As you can see from the reaction above, ethanol oxidizes to acetic acid, and the orange potassium dichromate turns to green chromium sulfate. The result is a gradual change in color from orange to green inside the breathalyzer tube. What this visual result tells us is that the driver’s breath contains ethanol. About 10% of the alcohol we drink is not digested and is absorbed into the blood and diffused throughout the body, which contains alcohol in the breath at a rate of 1/2100 of the alcohol in the blood (i.e., the amount of alcohol in 1 ml of blood is equal to the amount of alcohol in 2100 ml of exhaled breath). The breathalyzer takes this ratio into account to calculate your blood alcohol content. If more than half of the meter tube turns green, it means that the driver’s blood alcohol concentration is 0.08% or more, and in Korea, 0.05% or more corresponds to drunk driving, so it is possible to test for alcohol in this way.
A simple breath test is performed as shown above, and drivers who are determined to be drunk through this test are subjected to a blood test to obtain a more accurate reading. Separating the blood through gas chromatography allows alcohol and other volatile substances to be filtered out, and gives an accurate reading of the blood alcohol content. This determines the fine or jail time.
Recently, a new anti-drunk driving technology being developed by the National Highway Traffic Safety Administration (NHTSA) has been the talk of the town. Called DADSS, it uses a system installed in the car’s body to detect alcohol on the driver’s breath or on the driver’s fingertips when they touch the car to start it. Once the technology is fully developed, it is being considered for mandatory installation in new cars manufactured in the United States. The technology to stop this potentially lethal behavior is improving every day. These measurements, which use chemical reactions and spectroscopic methods, are not something that can be avoided by smoking, drinking coffee, or eating garlic right before a DUI stop. While we all have occasions to drown our worries with a drink, the smart thing to do is to avoid drinking and driving at all costs.
