Chemistry Reference
In-Depth Information
better, the ratio above is redefined as the
apparent partition coefficient
, to
differentiate it from the partition coefficient of the unionised species, which
is now termed the
true partition coefficient
.
The apparent partition coefficient (
P
app
) is dependent on the propor-
tion of substance present in solution, which in turn depends upon the pH
of the solution, or
P
app
P
f
unionised
(2.2)
where
f
unionised
equals the fraction of the total amount of drug unionised at
that pH. It follows that if
f
unionised
1 then
P
app
P
true
and the compound is
unionised.
To illustrate the effect of ionisation, consider again the drug in the
example above. If the pH of the aqueous phase is adjusted so that the drug
becomes 66.7% ionised, only 40 mg of the drug partitions into the organic
phase (since the ionised drug will be less soluble in the organic solvent), and
the partition coefficient can be recalculated (see Figure 2.2).
Organic (50ml)
Aqueous (50ml)
40mg (2)
(unionised)
20mg (1)
(unionised)
40mg (2)
(ionised)
Figure 2.2
The partition of ionised drug.
The mass of drug in the water phase
100
40
60 mg.
The mass of ionised drug in the water phase
total mass
fraction
ionised, which is 60
40 mg.
The mass of unionised drug in the water phase
0.666
60
0.333
20 mg.
The concentration of
drug in the organic phase
40/50
0.8 mg mL
1
.
The concentration of unionised drug in the water phase
20/50
0.4 mg mL
1
.
The concentration of
total drug in the water phase
60/50
1.2 mg mL
1
.
