Chemistry Reference
In-Depth Information
may be present in drug samples. A sample of the drug is reacted to produce
a colour (usually) and the intensity of the colour is compared with that
obtained from a known amount of standard drug. The colour obtained
from the standard sample represents the absolute upper limit (hence the
name of the technique) of impurity permitted in the sample of drug.
A typical example of a limit test is the test for salicylic acid in a
sample of Aspirin BP. Salicylic acid is formed by hydrolysis of aspirin (or
may be an impurity from the synthesis). The test involves comparing the
violet colour produced when the sample is reacted with ferric chloride with
that obtained from a standard salicylic acid solution.
The procedure is as follows.
Dissolve 0.1 g of the sample in 5 mL of ethanol (96%) and add 15 mL
of iced water and 0.05 mL of a 0.5% w/v solution of iron(III) chloride
hexahydrate. After 1 minute the colour of the solution is not more
intense than that of a solution prepared at the same time by adding a
mixture of 4 mL of ethanol (96%), 0.1 mL of 5
M
acetic acid, 15 mL
of water and 0.05 mL of a 0.5% w/v solution of iron(III) chloride
hexahydrate to 1 mL of a 0.0050% w/v solution of salicylic acid in
ethanol (96%).
The absolute limit for salicylic acid in Aspirin BP is 500 ppm, as can be
shown below.
1 mL of 0.0050% w/v solution of salicylic acid
0.1 g aspirin
1 mL of 0.005 g/100 mL solution of salicylic acid
0.1 g aspirin
1 mL of 0.00005 g/mL solution of salicylic acid
0.1 g aspirin
0.00005 g salicylic acid
0.1 g aspirin
50 lg salicylic acid
0.1 g aspirin
500 lg salicylic acid
1.0 g aspirin
500 ppm
Problems
Lithium carbonate (Li
2
CO
3
,
M
r
73.9) is a drug widely used in
the treatment of depression. The BP assay for lithium carbonate
involves the addition of an excess of hydrochloric acid to a sample
of the drug and back titration of the unreacted hydrochloric acid
with sodium hydroxide.
Q6.1
