Chemistry Reference
In-Depth Information
Amount of drug in the total system
70 lg
80 lg
150 lg
(b)
P
app
can be calculated from the concentration data since the
drug is ionised in the plasma.
[drug] in ether
P
app
————-----------
total [drug] in plasma
7 lg mL
1
——------
16 lg mL
1
0.44
(c)
The partition coefficient,
P
, is given by
P
app
P
————-------
fraction unionised
0.44
——
0.112
3.93
It should be noted that
P
is greater than
P
app
since the drug will be
ionised at the pH of measurement of
P
app
and hence will be less
soluble in the ether phase.
(d)
The percentage of drug extracted is given by
Mass of drug in ether
——————------------
Total mass of drug in system
70 lg
——-
100
150 lg
47%
(e) Since the drug is an acid, lowering of the pH of the plasma
sample (e.g. by the addition of a small volume of strong acid) will
suppress ionisation of the drug and allow more drug to partition
into the ether phase.
(f)
3.93, the
mass of drug extracted when it is essentially unionised can be
calculated. The mass of drug remaining in the plasma after extrac-
tion under the new conditions is
If the plasma sample is acidified so that
P
app
P
x
lg. Therefore, the mass
extracted into the ether
(150
x
) lg.
