Chemistry Reference
In-Depth Information
(
—--
)
100
56.5%
340
% Extracted
601.5
Q
2 5.0 mL of a plasma sample (pH 7.4) containing an
acidic drug (p
K
a
6.5) was extracted with 10.0 mL of ether.
The concentration of drug in both layers was determined and
the results obtained were:
The total concentration (unionised
ionised) in plasma
16 lgmL
1
The concentration in ether
7 lgmL
1
From these data determine the following:
(a) The distribution of the drug between the two phases at
equilibrium
(b) The apparent partition coefficient,
P
app
(c) The partition coefficient,
P
(d) The percentage extracted
(e) How the efficiency of the extraction might be improved
by modification of the pH
(f)
The percentage extracted under the modified conditions
A
2(a) From the p
K
a
value and the pH it can be seen that the acidic
drug will be ionised at plasma pH.
100
% Ionised
—————----------
1
antilog(pH
p
K
a
)
100
—————---------
1
antilog(6.5
7.4)
88.8%
Thus, the fraction ionised (
f
i
)
0.888. Therefore, the fraction
unionised (
f
u
)
1
0.888
0.112.
Mass of drug in ether
7 lgmL
1
10 mL
70 lg
Mass of drug in the plasma
16 lgmL
1
5 mL
80 lg
Amount of drug ionised
0.888
80 lg
71 lg
Amount of drug unionised
0.112
80 lg
9 lg
