Chemistry, Third edition

23 · FORENSIC CHEMISTRY

Variation of the concentration of a

poison with time

Many poisons that are absorbed into the bloodstream are broken down by enzymes

within the body’s organs. The concentration of the poison in the blood then falls

from its peak value. Figure 23.2 illustrates two cases where the fall in concentration is

first order (as with paracetamol) and zeroth-order(as with ethanol).

Fig. 23.2Variation of concentration of poison in blood where the poison decays by (a) first-order

kinetics (b) zeroth-order kinetics.

Not all poisons follow simple laws of kinetics. For example, the poisonous gas carbon

monoxidepermanently binds itself to haemoglobin in the blood and is not elimi-

nated from the body.

Calculations using first-order kinetics

The first-order decay of poisons follows the equation first introduced on page 257:

[A] = [A]o e-kt (23.2)

where [A]ois the peak concentration of poison A (that is, its concentration at t= 0),

kis the first order rate constant (in units of s-1) and [A] is the concentration of poi-

son A at tseconds. This equation was used to plot Figure 23.2(a). A calculation using

this equation is given in Example 23.1.

23.4

23.3

424

Concentration

Time

0

0.0

0.2

0.4

0.6

10 20 30 40 50

0.8

1.0

(a) First-order

(b) Zeroth-order