Chemistry Reference
In-Depth Information
Convert the number of moles of sample to a weight, and the number of moles of
titrant to a volume.
3.
Calculate the weight of sample that will react with 1 mL of the titrant. This figure
is called the
equivalent relationship
or sometimes
the equivalent
and is the most
important part of the calculation.
4.
Carry out the assay, at least in duplicate. If agreement is not achieved with two
results, the assay should be repeated until concordant results are obtained.
5.
Calculate the weight of active drug in the sample, and express the answer as
percentage weight in weight (% w/w) of sample weighed. This answer represents
the percentage purity of the drug and should be compared with the
British
Pharmacopoeia
(BP) limits to see whether the sample complies with the require-
ments of the BP. The
British Pharmacopoeia
lays down purity criteria and limits
within which a sample must lie to be of BP quality. Both determinations must fall
within the BP limits to be acceptable. If one result falls within the BP limits and
the duplicate result does not, then the sample does not comply with the BP limits,
and should not be used.
6.
In addition to the limits of purity, the
British Pharmacopoeia
contains
a wealth of information about the substance in question. The
British
Pharmacopoeia
is a legally enforceable document produced every four or
five years by the Pharmacopoeia Commission and lists the criteria for the
purity of drugs and medicines used in the UK and Commonwealth. Each
substance in the
British Pharmacopoeia
is given a specific
monograph
,
which lists the chemical structure of the compound (if known), the defini-
tion and statement of BP limits (quoted to one decimal place), a description
of its characteristics (colour, solubility, etc.), some tests for identification of
a sample of the material and limit tests for impurities (usually a colour test
that compares the levels of an impurity with the maximum permitted limit
allowed by the BP for that impurity). Limit tests are often used when the BP
assay is not
stability indicating
, i.e. does not differentiate between the drug
and its major decomposition product. The monograph ends with the offi-
cial BP assay for determination of purity. Formulated medicines may have,
in addition to a specific monograph, a
general monograph
, which applies to
that class of medicine. For example Aspirin Tablets BP will have to comply
with all of the monograph for Aspirin BP as well as the general monograph
for
tablets
. Similarly, Chloramphenicol Eye Drops BP must comply with
the general monograph on
eye drops
for sterility, etc. in addition to the
requirements for the purity of chloramphenicol.
To illustrate these points, we can consider the assay of citric acid.
Citric acid is a natural product found in citrus fruits (lemons, oranges,
limes, etc.) and is used in pharmaceutical formulations as a buffer and a
preservative. Its structure is shown in Figure 6.2.
