The science of alcohol
The science of alcohol

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The science of alcohol

2.1 Analysing ethanol by wet chemistry

As you learned in Week 1, the alcohol in alcoholic drinks is ethanol, and a molecule classes as an alcohol if its chemical structure includes the –OH functional group.

The following video demonstrates the chemistry used in roadside testing breathalysers. The breath of a test subject is exposed to an acidified purple solution of a substance known as potassium dichromate. If alcohol is present, the solution changes colour from orange to brown, and finally to green. The depth of the colour change can indicate how much alcohol is present although normally this is not sufficient for quantitative analysis – therefore, the secondary blood or urine test is required for law enforcement reasons.

Download this video clip.Video player: s240_breathalyser_bk1_act4_6.mp4
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Transcript

NARRATOR
Prior to the use of infrared spectroscopy, or fuel-cell technology, the method used for on-the-spot testing for breath alcohol was by chemical reaction. The most common reaction used for this was between ethanol, the alcohol found in alcoholic drinks, and potassium dichromate, a solid-orange compound. The reaction scheme is shown here. The ethanol reacts with potassium dichromate in the presence of sulfuric acid, producing chromium sulphate, potassium sulphate, acetic acid, and water. Silver nitrate is used as a catalyst. The reaction is visualised as a colour change from orange through brown to green. To demonstrate this, we will use the following experimental setup. The left-hand chamber contains ethanol, and the right-hand chamber contains a mixture of potassium dichromate, sulfuric acid, and silver nitrate. The two chambers are attached to each other by a tube that allows air to move from the left-hand chamber into the right-hand chamber. Air is introduced into the left-hand chamber containing the ethanol from the tap on the far left. Upon entering the left-hand chamber, a diffuser is used to bubble or sparge the air through the ethanol. It will bubble vigorously, and the ethanol is evaporated into the air. It is then carried through into the next chamber and blown over the mixture of chemicals. As the ethanol reacts with the potassium dichromate and sulfuric acid, a colour change is observed-- first through brown to, in this case, dark brown with a hint of green. The sample shown here does not turn completely green. This is due to the reaction not being carried to completion. From this, we can't calculate the amount of ethanol present. However, this reaction is specific to alcohols and any observed colour changes indicative of their presence. Therefore, it can be used as a qualitative test for alcohol in breath.
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At this stage it is worth pointing out that sometimes this test does not give the results we would expect.

  • What do you think is meant by the term ‘false positive’?

  • A false positive arises when a positive result is obtained for a substance without this substance being present.

You may be familiar with the concept of drug testing in athletes. Sometimes, a test subject may test positive for a particular substance that the athlete denies ingesting. If the athlete is telling the truth and has not ingested the substance – but the substance is detected anyway – this is a false positive.

The same concept applies to alcohol testing. Several factors can cause false positives in breath tests for alcohol. These tests assume a particular ratio of blood alcohol to breath alcohol. If the concentration of alcohol in the mouth is artificially increased, this ratio is invalid. For example, recent use of mouthwash – which contains ethanol – can lead to false positives. Stomach gases brought up by belching will also contain alcohol. This means that, while these tests are generally qualitative (i.e. tell you whether or not alcohol is present), quantification (determining the exact amount of alcohol) is required by law.

The wet chemistry principle (that is, using a solution to test breath) described here is one very simple method of detecting alcohol and has been in use since 1938 when the first breathalyser – known as a ‘drunkometer’ – was produced. In recent years, however, this technology has been superseded by more advanced fuel cell technology, capable of accurate quantification of blood alcohol levels. These fuel cells systems are small, portable and highly accurate and consequently now represent the gold standard for roadside alcohol testing. However, the chemistry you have learned about here is still often used in commercial over-the-counter single-use test kits.

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