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Measuring alcohol levels

Award Level Gold

 

Before you start your investigation you should carry out a risk assessment and have it checked by your teacher. For help with this, read through our health and safety information and look out for health and safety warnings in the text.

In the UK it is illegal to drive if the alcohol level in your blood is above 80 mg per 100 ml, about 0.08%. In Scotland the legal limit is 50 mg of alcohol per 100 ml of blood. We therefore need ways to determine whether a person is ‘over the limit’. The aim of this project is to find out how a suspect’s alcohol level can be measured. You will investigate simple chemical tests, and more sophisticated methods.

Getting started

You could start with simple aqueous solutions of ethanol, and progress to testing for ethanol dissolved in artificial urine, and possibly in animal blood.

Ethanol is HIGHLY FLAMABLE.

You probably already know that ‘alcohol’ means any of a whole family of organic compounds containing the –OH functional group. This project concerns ‘ethanol’, the alcohol present in drinks. However, the ethanol you use for testing has had methanol added. It is toxic and is not drinkable.

Local police may be willing to show you their breath alcohol meters. You should also link up with an organisation that uses appropriate instrumental analysis (though not necessarily for ethanol), so that you can see the instruments in use, and maybe analyse some of your own samples.

Qualitative tests

Look up, and try out, chemical tests for alcohols, including reduction of dichromate ions.

  • Are any of these specific to ethanol, or do they detect alcohols in general?
  • By progressive dilution, determine the lowest concentration of aqueous ethanol that you can detect. Is it low enough to detect the legal limit – 80 mg per 100 ml in blood (50mg in Scotland) and 107 mg per 100 ml in urine (67 mg in Scotland).
  • What would be the problems in applying such tests to a driver’s blood or urine sample? You could try testing alcohol dissolved in artificial urine instead of in water – recipes for artificial urine can be found on the internet.

 

Dichromate ions are TOXIC. Check for hazards of the chemical tests and consider what control measures are necessary to reduce the risk.

Quantitative tests

Devise, and try out, a method of adapting one of the above tests to measure the concentration of ethanol in an aqueous solution, not just detect it. Colorimetry might help.

  • Again, test whether you could measure concentrations at least down to the legal limits
  • Explain whether the method could be used with blood or urine samples.

The reduction of dichromate was used as the basis of the original breath-in-a-bag ‘Breathalyser’. Find out how it measured, rather than just detected, the ethanol in breath. The dichromate reaction is not specific to ethanol. Why is it reasonable to assume that ethanol is the only alcohol likely to be present in a person’s breath? The legal limit for ethanol in breath is 35 μg per 100ml or 22 μg in Scotland (μg = microgram). Some people claimed that eating pickled onions would beat the Breathalyser – devise a way to test this theory. Don’t eat pickled onions in the laboratory and don’t drink alcohol.

Instrumental methods

Measuring ethanol concentration in blood or urine is far more complex than in a simple aqueous solution.  It needs a quantitative method that is specific to ethanol and unaffected by other substances in the mixture. Miniaturisation and electronics have allowed some laboratory techniques to be incorporated into portable alcohol meters. These are used not only by police, but also in the rail and airline industries and other workplaces.

Through your external link person, arrange visits to see appropriate instruments in use – multi-purpose laboratory instruments and/or alcohol meters themselves. These might involve:

  • Chromatography
  • Infra-red spectrometry
  • Mass spectrometry
  • Ethanol-sensitive electrodes
  • Fuel cell technology.

Before your visit you should read some background information about each technique, so that you understand what they tell us about a sample. You do not need to understand details of how the instruments work, but should know the principles on which each is based. You should then be able to decide which techniques are likely to be most useful for detecting and measuring ethanol in breath, blood or urine.

An analytical challenge

Having investigated various methods of measuring ethanol concentration, your challenge is to devise, construct and test an alcohol analyser. This could be a portable breathtester, or a piece of laboratory apparatus, such as a simple gas chromatograph. Remember you must not consume alcohol.

Initially, aim to distinguish between ‘pass’ and ‘fail’, for example test samples with ethanol levels half and double the legal limit. The next step would be to calibrate your device, and finally to measure samples with concentrations known to your teacher, but not to you.

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