Blood alcohol concentration: Principle and detection

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• By Jahnavi Zalki M

Alcohol, referred to by the chemical name ethanol is a psychoactive drug. It is found in fermented food and few fruits. It is an active ingredient in beverages, such as wine, beer and distilled spirits. This is one of the most common recreational substances and has equal hazards too. After consumption, it gets absorbed very fast. It appears in blood immediately and gets completely absorbed within an hour. Once absorbed into the blood, it gets distributed through the cardiovascular system. Since the blood-brain barrier is permeable to alcohol, it enters the brain causing central nervous system (CNS) depression, inhibition and slows down reflex and responses. Distribution of alcohol is uniform throughout the body except for hair, fat and bones. Many tests and forensic evidence rely on blood alcohol concentration (BAC). BAC can be calculated from the amount of alcohol consumed and the person’s mass. Widmark factor is the reference to alcohol distribution in the male and female bodies. Men have a Widmark factor of 0.68 while women have 0.55 because of the presence of a high proportion of fatty tissues. BAC is given by C=a/(p*r); where, a is the amount of alcohol consumed, p is the person’s body mass and r is the Widmark factor. BAC can also be analysed using gas chromatography. The concentration of alcohol found in the blood is equal to the concentration of alcohol in the brain. Alcohol is also found in a person’s breath. Breath alcohol is related to BAC according to Henry’s law that states, “For a solution of a volatile substance in liquid, the vapour pressure of the volatile substance will be proportional to its concentration in the liquid.” The blood alcohol to breath alcohol ratio is 2300:1, which is commonly used to check drink and drive cases. It utilises chromium chemistry. Breathalyzer devices use potassium dichromate (orange coloured substance) in the presence of acid to analyse alcohol. This reacts with alcohol to give a bright green colour. Loss of absorbance at 420 nm is measured to know about the breath alcohol concentration, hence deducing blood alcohol concentration.

Keywords: Alcohol, Blood alcohol concentration, Widmark factor, Gas chromatography, Henry's law, Breathalyzer

Image source:
Breathalyser by Piotrpiotrwojcicki, available under a Creative Commons Attribution Licence Zero at https://www.needpix.com/photo/download/739756/breathalyser-the-police-sobriety-the-driver-of-the-the-provisions-of-the-alcohol-control-promile-free-pictures

Citation:
Jahnavi Zalki M. Blood alcohol concentration: Principle and detection. The Torch. 2020. 1 (2).
Available from: https://www.styvalley.com/pub/magazines/torch/read/blood-alcohol-concentration-principle-and-detection/.

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