By Kalwinder KaurJun 22 2012
Image Credit: Andrey_Popov/Shutterstock.com
In 2008, Britain saw approximately 8,620 road accidents related to drunk driving. Out of the 8,620 reported incidents, 2,020 were fatal. Recent statistics show that the number of reported casualties or deaths from drunk driving accidents in the United Kingdom in 2016 was at 9,040, seven percent more than the statistics from the previous year. This demonstrates the increasing trend of drunk driving incidents over the past years and how standard alcohol detectors have become increasingly important to help combat drunk driving culture.
The most standard alcohol breath test is performed using a breathalyzer. This device is made up of two vials composed of a photocell indicator and a series of chemicals that work together to calculate blood alcohol concentration (BAC). A mouthpiece is attached to the breathalyzer, that allows the suspect to blow into the device. During a routine breath test, the breath passes through the vessel to the measurement component of the detector and collects into a vial containing sulfuric acid, potassium dichromate, silver nitrate, and water.
Chemistry
When users exhale into a breathalyzer, traces of ethanol react with sulfuric acid and potassium dichromate. These are then oxidized into chromium sulfate and acetic acid. The silver nitrate in the vial catalyzes this chemical reaction:
2 K2Cr2O7 [Potassium dichromate] + 3 CH3CH2OH [Ethyl alcohol] + 8 H2SO4 [Sulfuric acid] + 2 Cr2(SO4)3 [Chromium sulfate Green]+ 2 K2SO4 [Potassium sulfate] + 3CH3COOH [Acetic acid]+ 11 H2O
A photocell system is then applied to compare the amount of unreacted potassium dichromate left in the vial to the amount of the same chemical used to oxidize ethanol, which will provide a measurement of alcohol content from the breath sample. The photocell system bases its alcohol content measurement on the absorption of light by the potassium dichromate. The amount of light absorbed by this chemical will be proportional to the amount of alcohol in the sample cell.
Comparing the reference sample in the breathalyzer and the suspect sample generates an electrical current that moves a needle on an indicator meter. One of the main problems with this test is that it consumes the chemicals in the reaction vessel, implying a constant need to calibrate the device and replenish it with the right amount of chemicals to oxidize ethanol so that an accurate reading of alcohol concentration can be produced.
Types of Alcohol Breath Tests
Based on the principle of molecular exchange through the alveoli in the lungs, by measuring a constant ratio of BAC, an equivalent BAC can be calculated. The average ratio of breath alcohol to the BAC is 2,100:1. By volume, it can be assumed that 2,100 milliliters of the air contained within an alveolar sac in the lungs will contain a similar concentration of alcohol compared to 1 milliliter of blood.
The modern-day electronic breath testing device is made of two platinum electrode components that surround acid-electrolyte material. Where air passes through the electronic device, the platinum electrodes begin to oxidize the ethanol molecules, and via catalysis, this reaction converts the ethanol into acetic acid and a collection of residual protons and electrons.
The protons and electrons generated as a product of the chemical reaction flow through the electric meter over to the platinum electrode at the opposite end of the meter where they combine oxygen atoms and the residual electrons to form water molecules. Based on the proton-electron exchange in a breathalyzer, more alcohol molecules will result in a greater number of electrons generated from a catalytic chemical reaction. The electrons generated then drive the electrical current that is measured by a microprocessor to indicate the amount of alcohol content proportional to BAC.
Intoxilyzer
In the United States, intoxilyzers have replaced breathalyzers for measuring alcohol content. A principle of the intoxilyzer involves measuring the infrared radiation (IR) to a wavelength that reacts with alcohol molecules. The amount of alcohol in a suspect’s sample is directly proportional to the amount of energy it takes to bend each covalent bond between atoms in each molecule.
The intoxylizer is far more accurate than a traditional breathalyzer as it will measure the alcohol content in deep lung air. There is a spring valve to a tube that connects to the testing chamber that opens in response to a large exhale of air pressure, to ensure that deep lung air is entering the valve. By using this IR breath test, the chances of receiving a false-positive result decrease even if the suspect has little traces of alcohol content only in the mouth (i.e., from the use of mouthwash) because the main sample to be tested can only come from a deep exhale delivering air content from the lungs.
During a standard intoxilyzer test, the energy from a suspect’s breath travels through a confined sample chamber to an IR filter with a wavelength band that will be able to detect ethanol molecules. As the IR energy travels through the filter, it reaches a detector that converts the energy into an electrical voltage calculated by a microprocessor.
How Intoxilyzer Breathalyzers Work
Fuel-Cell Alcohol Sensors
Also called, “alcosensors”, fuel-cell alcohol sensors are considered the most prevalent method for testing alcohol content. The alcosensor device is engineered with fuel cells consisting of a porous layer coating, a platinum sheet covered with an acidic chemical and works on the principle of electrochemical oxidation. The thin sheet of platinum is enclosed in a plastic tube. During a breath test, the suspect exhales air into the plastic tube and transfers the air molecules to the platinum layer.
The fuel cells picking up the alcohol, convert the alcohol molecules into acetic acid. Electrons are generated as an end product of alcohol oxidation (2 electrons are generated per molecule of ethyl alcohol). Hydrogen ions are also generated and combine with free oxygen atoms on the fuel cell where both elements combine to form water. Excess electrons at the upper surface of the fuel cell generated from the oxidation process can then be compared to the number of electrons at the lower end of the fuel cell.
As both platinum surfaces are electrically wired, the electrical voltage flowing through a circuit is directly proportional to the alcohol content utilized by a fuel cell. Fuel-cell alcohol sensor technology is ideal for small-scale screening based on the low power resource required to function this device.
Sample Contamination: False-Positive Tests
Many products can interfere with alcohol breath tests, such as:
- Mouthwash (which contains 27% alcohol). In this instance, the breath testing device is calculating the alcohol content from the mouthwash as though it is a direct sample from the lungs and thus providing a false 2100: 1 ratio of BAC.
- Acetone, a naturally occurring chemical produced by the body in response to incomplete digestion, is a potentially interfering substance.
- Physical activity and hyperventilation have been found to reduce BAC which will interfere with the suspect sample reading.
Sources and Further Reading
This article was updated on 17th February, 2020.
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