Chemistry, Third edition

DRINKING AND DRIVING

a suspect. Because of the way that gases inhaled through the lungs are exchanged

with fine capillaries, the alcohol in blood and breath are effectively in equilibrium

(Figure 23.3). The concentration of alcohol in the blood of a person is about 2 300

times greater than the concentration of alcohol in that person’s breath:

[ethanol in blood]

———————– = 2300

[ethanol in breath]

Breath concentrations are usually expressed in units of micrograms ( g) of alcohol

per 100 cm^3 of breath. The breath equivalent of a blood alcohol concentration of 80

mg of alcohol per 100 ml of blood is 35 g of ethanol per 100 cm^3 of breath.

Alcohol may also be detected in urine (Table 23.3).

The Laboratory of the Government Chemist has estimated the uncertainty in the

measurement of ethanol in blood via

breath analysis as

2 mg/100 ml

A measured value of 81 mg/100 ml,

determined using an evidential analyser,

may therefore represent a true value in

the range 79–83 mg/100 ml. In practice, a

more generous allowance of 6 mg/100 ml

is subtracted by the analyst in his report.

For our measured value of 81 mg/100 ml,

the reported concentration would be 81

–6 = 75 and the formal statement from

the Forensic Scientist would be that ‘the

sample contained not less than 75 mg of

ethanol per 100 ml of blood’.

Drivers who are affected by other drugs

(such as amphetamines or cocaine) also

427

Table 23.3Summary of prescribed alcohol limits in the UK

Substrate Concentration limit

Blood (BAC) 80 mg of ethanol / 100 ml of blood

Urine (UAC) 107 mg of ethanol / 100 ml of urine

Breath (BrAC) 35 g of ethanol / 100 ml of breath

Fig. 23.4Measuring alcohol

molecules by infrared

absorption. The instrument is

a Lion Intoxilyzer 6000,

manufactured by Lion

Laboratories, Barry, South

Wales.

Alcohol molecules in breath

(vapour)

Alcohol molecules in blood

Fig. 23.3Alcohol in breath and blood

are in dynamic equilibrium.