Chemistry, Third edition

23 · FORENSIC CHEMISTRY

Drinking and driving

Excess alcohol is a major cause of traffic accidents and all countries have taken steps

to legislate against driving whilst over a set BAC limit, although the precise limit

varies. The limit is 80 milligrams of alcohol per 100 ml of blood in the UK. In many

European countries the limit is 50.

UK law allows for a police constable to conduct a screening test (the ‘roadside

breathalyser’) on a driver who is suspected of driving whilst ‘over the limit’. The

screening test was originally based on the reduction of dichromate but this has been

replaced by a fuel cell (see page 106). If the screening test proves positive, the suspect

is arrested and taken to a police station where an instrument based on an infrared

spectrometer is used to provide evidence that may be used in court.

Alcohol is volatile (p.167) and alcohol in the blood soon makes its way into

human breath. The infrared instrument used in police stations analyses the breathof

23.6

426

Table 23.2The physiological effects of alcohol on humans

Blood alcohol concentration Effects

(mg/100ml)

<50 No obvious effect, except perhaps a tendency to a feeling of

well-being and to become more talkative.

50–100 First obvious effects start to show. Some loss of coordination

and sensory perception, possibly some slurred speech.

100–150 More marked loss of coordination, poor sensory perception,

possibly nausea and desire to lie down.

150–200 Drunkenness and probably nausea.

200–300 General inertia, inability to stand, vomiting, probably coma.

>300 Approaching danger limit, coma and anaesthesia, impaired

circulation and respiration, possible death.

>450 Probable death due to respiratory paralysis.

Example 23.2

The BAC of a person on finishing their drinks was 90 mg of

ethanol per 100 cm^3 of blood. Calculate the BAC of the person 4

hours later.

Answer

We symbolize the blood alcohol concentration at t= 0 as [A]o.

[A] = [A]o–kt wherek= 15

[A] = 90 – (15 4) = 30 mg of ethanol per 100 cm^3 of blood.

Therefore, BAC after four hours = 30 mg of ethanol per 100 cm^3 of blood.

A person is found to have a

BAC of 100 mg of ethanol

per 100 cm^3 of blood 6

hours after her last drink.

Estimate her BAC on

leaving the pub.

Exercise 23B