Chemistry Reference
In-Depth Information
The true partition coefficient is the partition coefficient for the
unionised molecule.
P
true
is a constant for a given drug and allows comparison
of
P
values for different molecules. The apparent partition coef-
ficient is the partition coefficient measured in the laboratory. If
the drug in question ionises,
P
app
will vary with the pH of
measurement.
This may be measured either by a shake flask method (e.g. using
ether and pH 7 buffer) or by chromatography.
P
app
would be
measured first and
P
true
calculated using equation (2.2):
P
app
A2.1
f
unionised
The first step is to calculate
P
app
using equation (2.2):
P
app
P
125
0.0156
P
app
1.950
Using equation (2.3),
(
—----
)
W
n
A
n
—-
WPS
A
(
—----------
)
W
2
4
2
—
W
(1.95
5)
4
W
2
—-
0.0846
W
The concentration in chloroform is 15.8 lgmL
-1
in 2 mL and
therefore
Amount in chloroform
31.6 lg
Since the fraction remaining in the aqueous phase is 0.0846,
Fraction extracted
(1
0.0846)
0.9154
Therefore, 31.6 lg
91.54% of total, so
31.6
Total amount
—---
34.52 lg
0.9154
