Chemistry Reference
In-Depth Information
tions may occur on the parent drug or on the product of a Phase
1 metabolic conversion.
(a)
Back titrations are used where the forward reaction is slow
or does not proceed 100% to the right-hand side.
A6.1
(b)
Li
2
CO
3
2HCl
1
2LiCl
H
2
O
CO
2
2H
2
O
Therefore, since the relative molecular mass of Li
2
CO
3
is
73.9,
2HCl
2NaOH
1
2NaCl
73.9 g Li
2
CO
3
2000 mL 1
M
NaOH
0.03695 g Li
2
CO
3
1 mL 1
M
NaOH
(c)
(i)
100.1% w/w.
(ii)
100% suggests a basic impurity, which is
using up titrant in the same way as the sample; probably
another metal carbonate.
(iii) A suitable indicator would be methyl orange. Dissolved
carbon dioxide in the sample may ionise to produce an
acidic solution; therefore, an indicator that changes on
the acid side of neutrality is required.
An answer
(a)
This technique is a non-aqueous titration, which is used for
the assay of compounds that are insufficiently acidic or basic
to provide a sharp end point in aqueous solution.
Precautions to be observed are to carry out the assay in
totally anhydrous conditions. This means that all glassware,
apparatus and solutions used should be dry.
A6.2
(b)
The solution of perchloric acid should be standardised using
a primary standard, i.e. a compound that can be obtained in
a very high level of purity. In the case of perchloric acid,
either benzoic acid or potassium hydrogenphthalate would
be suitable.
(c)
The equivalent relationship is that 1 mL of 0.1
M
HClO
4
is
equivalent to 0.02112 g of methyldopa, which gives a purity
of the sample of 99.6%.
(a)
This is a REDOX (oxidation-reduction) titration. The
cerium
gains an electron
and is
reduced
, while the ascorbic
acid
loses two electrons
and is
oxidised
.
A6.3
