Chemistry Reference
In-Depth Information
sometimes required. What precautions should be observed
for assays of this type.
(b)
Describe, in detail, how the perchloric acid used in this assay
may be standardised (no calculation required).
(c)
The above assay was carried out and the following results
were obtained. Derive the equivalent relationship for this
assay and hence determine the purity of
the sample of
methyldopa.
Weight of sample taken
0.2016 g
Volume of 0.1
M
HClO
4
(
f
0.986) required
9.64 mL
Vitamin C (ascorbic acid) is used in pharmaceutical formulation
as an antioxidant and also has a medical use as a vitamin. Tablets
of vitamin C may be assayed by titration with complex salts of
cerium. The reactions occurring are as follows and are shown in
Figure 6.7.
Q6.3
vitamin C
2Ce
4
1
vitamin C (oxidised)
2Ce
3
CH
2
OH
CH
2
OH
H
COH
H
COH
O
O
O + 2Ce
4+
O + 2Ce
3+
OO
HO
OH
Figure 6.7
Reactions of ascorbic acid with cerium.
(a)
What name is given to this type of titration?
(b)
Ten 50 mg vitamin C tablets were weighed and powdered
and an amount of powder equivalent to 0.15 g of ascorbic
acid was dissolved as completely as possible in a mixture of
30 mL of water and 20 mL of 1
M
sulfuric acid. This sample
was then titrated with 0.1
M
ammonium cerium sulfate
(ACS) using ferroin sulfate solution as indicator. Given that
the relative molecular mass of ascorbic acid is 176.12, derive
the equivalent relationship for this assay and hence calculate
the percentage of the stated amount of ascorbic acid in the
tablets from the following data.
