Chemistry and Crime

In the USA, the most common source of injuries and deaths related to alcohol is drunk-driving automobile accidents. Drunk driving fatalities account for about a third of total deaths due to traffic crashes. These have decreased by over 50% since 1982 when the first drunk-driving statistics were collected, of 21113 deaths. In 2010, 32,885 people died in traffic crashes of which 10,228 were thought to be alcohol related. In 2011 there were 9,878 fatal alcohol related crashes whilst in 2012, 33,561 people died in traffic crashes (latest figures available), including an estimated 10,322 people who were killed in drunk driving crashes involving a driver with an illegal BAC (.08 or greater). Among the people killed in these drunk driving crashes, 65% were drivers (6,688), 27% were motor vehicle occupants (2,824), and 8% were non-occupants (810), with an average of one person dying in a drunk driving fatality every 51 minutes. Additionally, around a quarter-million people are injured each year.

Law enforcement officers and prosecutors around the world rely on breath alcohol testing to investigate and/or prove their "driving under the influence", DUI and "driving while impaired (intoxicated)", DWI cases. They use preliminary breath testing devices (also known as prearrest breath testing devices or "PBTs") and passive alcohol screening devices to identify impaired drivers, evidential breath testing devices (EBTs) to prove their guilt, and ignition interlock devices to ensure that they do not drive again under the influence.

Alcohol legal limits by country

The Breathalyser
It is generally accepted that Robert Borkenstein (1912-2002), a captain with the Indiana State Police and later a professor at Indiana University at Bloomington, was the first to create a device that could measure a subject's blood alcohol level based on a breath sample. In 1954, Borkenstein invented his breathalyzer that used chemical oxidation and photometry to determine the alcohol concentration.

The invention of the breathalyzer provided law enforcement with a non-invasive test providing immediate results to determine an individual's breath alcohol concentration at the time of testing. Given that research by a physician (Francis Edmund Anstie) into the possibility of using breath to test for alcohol in a persons body dates as far back as 1874, this was very slow in coming. Anstie is remembered for Anstie's limit which is the amount of alcohol that he proposed could be consumed daily with no ill effects. It is ~ 45 mL (1.5 ounces) of pure ethanol or two to three "standard" drinks (depending on country).

Note that breath analyzers do not directly measure blood alcohol content (BAC) or concentration, which requires the analysis of a blood sample. Instead, they estimate BAC indirectly by measuring the amount of alcohol in one's breath. Two breathalyzer technologies are prevalent. Desktop analyzers generally use infrared spectrometry, electrochemical fuel cell technology, or a combination of the two. Hand-held field testing devices are generally based on electrochemical platinum fuel cell analysis and, depending upon jurisdiction, may be used by officers in the field as a form of "field sobriety test" or PBT.

Coin operated Breathalyser Breathalyser Keyring
Coin operated and simple keyring breathalyser for personal use.

How do they work?
Henry's law states that when an aqueous solution of a volatile compound is brought into equilibrium with air, there is a fixed ratio between the concentration of the compound in air and its concentration in water. This partition ratio is constant at a given temperature. The human body is 37° Celsius on average. Breath leaves the mouth at a temperature of 34° Celsius. Alcohol in the body obeys Henry's Law as it is a volatile compound and diffuses in body water. Breathalyzers assume that all subjects being tested have a 2100-to-1 partition ratio. It has been claimed, however that the actual partition ratio may vary between 1300:1 to 3100:1 or wider among individuals and even for the same individual over time.

Studies have suggested that about 1.8% of the population have a partition ratio below 2100:1. Thus, a machine using a 2100-to-1 ratio would overestimate the BAC. As much as 14% of the population has a partition ratio above 2100, thus causing the machine to under-report the BAC. Further, the assumption that the test subject's partition ratio will be average, i.e. that there will be 2100 parts in the blood for every part in the breath, means that accurate analysis of a given individual's blood alcohol by measuring breath alcohol is difficult, as the ratio may vary considerably. Other studies suggest that women on average have a lower partition ratio than men, leading some to suggest that breathalyzers might be inherently biased against women, since the lower the actual partition ratio the higher the recorded result.

The first practical roadside breath testing devices intended for use by the police were the Drunkometer (Rolla Harger) and Intoximeter (Glen Forrester). The drunkometer was developed in 1931 and patented in 1936. It was used to directly collect a motorists breath sample in a balloon inside of the machine. The breath sample was then pumped through an acidified potassium permangate solution. If there was alcohol in the breath sample, the solution lost colour. The greater the colour change, the more alcohol there was present in the breath. The intoximeter used permanganate reactions as well.

Rolla Harger describes his Drunkometer in the Journal of Criminal Law and Criminology (1931-1951), Vol. 40, No. 4 (Nov. - Dec., 1949) (pp. 497-506)
"Since many police laboratories do not have graduate chemists, we felt that there was a need for a simpler method which could be handled by an intelligent police technician. The method we developed for this purpose analyzes breath, and the reagent for alcohol is standard permanganate in 56 per cent sulfuric acid. In this concentration of sulfuric acid, which is much higher than that commonly used by chemists for analyses with permanganate, alcohol reacts rapidly and quantitatively with the permanganate at room temperature. In the procedure we also determine the weight of carbon dioxide in the sample of breath required to decolorize the permanganate. This enables us to calculate the volume of lung air in this sample of breath.

Normal lung air regularly contains close to 5.5 per cent of carbon dioxide. Then, since it has been found that 2000 cubic centimeters of lung air contain practically the same weight of alcohol as 1 cubic centimeter of blood, one can use the breath alcohol result to calculate the level of blood alcohol. What we really determine in this method is the weight of alcohol in the breath which accompanies the weight of carbon dioxide found in 2000 cubic centimeters of lung air, that is, the alcohol-carbon dioxide ratio of the subject's breath. Our breath method was first announced in 1931 and, after several years of trial during which we compared the results with direct analyses of blood, it was published more fully in 1938. When asked what name we had chosen for the portable apparatus used for the test we rather jokingly called it a "Drunkometer". The name is admittedly crude but fairly expressive, and it has stuck."

4 KMnO4(aq) + 5 CH3CH2OH(aq) + 6 H2SO4(aq) ↔ 5 CH3COOH(aq) + 4 MnSO4(aq) + 2 K2SO4(aq) + 11 H2O(l)

The Alcometer was developed in 1941 by Leon Greenberg and employed a process whereby iodine vapour, starch and potassium iodide were used. The chemicals reacted with a motorists breath sample and changed colour depending on the level of alcohol present. It proved to be operationally much less stable, and thus less reliable than the other two first-generation instruments.

Early photovoltaic assays worked by using photocells to analyze the colour change of a redox reaction. A breath sample was bubbled through an aqueous solution of sulfuric acid, potassium dichromate, and silver nitrate. The silver nitrate acted as a catalyst, allowing the alcohol to be oxidized at an appreciable rate. The sulfuric acid provided the acidic conditions needed for the Cr(VI)/Cr(III) redox reaction. In solution, the ethanol reacted with the potassium dichromate, reducing the dichromate ion to the chromium (III) ion. This reduction resulted in a change of the solution's colour from red-orange to green. The reacted solution would be compared to a vial of non-reacted solution by a photocell, this created an electric current proportional to the degree of the colour change and was monitored by the movement of the needle on a dial that indicated BAC.

3 CH3CH2OH(g) + 2 K2Cr2O7(aq) + 8 H2SO4(aq) ↔ 3 CH3CO2H(aq) + 2 Cr2(SO4)3(aq) + 11 H2O(l)

Like other methods, breath testing devices using chemical analysis are somewhat prone to false readings. Compounds that have compositions similar to ethanol, for example, could also act as reducing agents, creating the necessary colour change to indicate increased BAC.

Electrochemical Fuel Cells
The fuel cell consists of a deposit of gold and platinum on a porous disc. Fuel cell technology is particularly suitable for portable screening devices, due to the small size of the cells and the low power requirements of the technology.

When the user exhales into a breathalyzer, any ethanol present in their breath is oxidized to acetic acid at the anode:
CH3CH2OH(g) + H2O(l) ↔ CH3CO2H(l) + 4H+(aq) + 4e-

At the cathode, atmospheric oxygen is reduced:
O2(g) + 4H+(aq) + 4e- ↔ 2H2O(l)

The overall reaction is the oxidation of ethanol to acetic acid and water.
CH3CH2OH(l) + O2(g) ↔ CH3COOH(l) + H2O(l)

The small electrical current produced by these reactions is measured by a microprocessor, and an approximation of overall BAC is displayed.

Infrared Optical Sensor
Breathalysers utilising infrared optical sensor technology provide much better accuracy but the early technology was more suited for desktop evidential breathalyser machines due to their bulkiness.

Changes in electronics and instrumentation over the past twenty years though has meant that miniaturisation is possible and so portable instruments are now in operation.

Non-dispersive Infrared (NDIR) sensors are simple spectroscopic devices often used for gas analysis. The key components are an infrared lamp, a sample chamber or light tube, a wavelength filter, and an infrared detector. The gas is pumped or diffuses into the sample chamber, and the gas concentration is electro-optically measured by its absorption at a specific wavelength in the infrared (IR).

Non-dispersive Infrared
Non-dispersive Infrared Sensor
See the YouTube video.

Dual Sensor Systems
For an account of the changes that have taken place in measuring alcohol in the breath see a brochure by a manufacturer. Their recent models use two measuring systems with different analytical specificity so the device is capable of reliably detecting any interfering substances that may be present in the exhaled air and that might influence the result in any way, such as petrol or paint vapours, acetone or solvent vapours.
Dual Sensor Breathalyser
Dual Sensor Breathalyser with printout

Breathalyser Myths and potential errors


UWI students can see OurVLE for copies of some of the reports mentioned above.



Modern Breathalysers

Much of the information in these course notes has been sourced from Wikipedia under the Creative Commons License. Students taking this course will be expected to contribute to Wikipedia as a part of their course assignments.
Continue to Narcotic Reagent Kits or return to CHEM2402 course outline.

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The Department of Chemistry, University of the West Indies,
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Created October 2011. Links checked and/or last modified 7th November 2014.