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100
Viva Voce in
Pharmaceutical Chemistry
PREPARATION AND STANDARDISATION OF EDETATE SOLUTION
Q 520 What is the molecular weight of disodium edetate?
Ans: Molecular weight of disodium edetate is 372.2 g.
Q 521 How would you prepare 100 ml of 1M solution of disodium
edetate?
Ans: Dissolve
372.2
37.22
10
g of disodium edetate in water and
make up the volume to 100 ml with water.
Q 522 How would you prepare 1000 ml 0.5 M solution of disodium
edetate?
Ans: Dissolve
372.2 0.5
186.1 g
1
of disodium edetate in water
and make up the volume to 1000 ml with water.
Q 523 How would you prepare 500 ml of 0.05 M disodium edetate
solution?
Ans: Dissolve
372.2 0.05
9.305 g
12
of disodium edetate in water
and make up the volume to 500 ml with water.
Q 524 Can disodium edetate be taken as primary standard?
Ans: Disodium edetate cannot be used as primary standard
because even AR grade disodium edetate may contain
moisture and it is not 100% pure.
Q 525 How would you standardise the prepared 0.05 M disodium
edetate solution?
Ans: The prepared 0.05 M disodium edetate solution can be
standardised by one of the following methods.
1. By titrating with nearly neutralised zinc chloride or zinc
sulphate solution, prepared from known weight of zinc pellets.
2. By titration with a solution made from specially dried lead
nitrate.
Q 526 Give detail procedure for standardisation of 0.05M disodium
edetate solution.
Ans:
1. Weigh accurately about 0.8 g of granulated zinc.
Fundamentals of Volumetric Analysis 101
2. Dissolve by gentle warming in 12 ml of dilute hydrochloric
acid.
3. Add 0.1 ml of bromine water.
4. Boil to remove excess bromine, cool and add water to produce
200 ml.
5. Pipette 20 ml of the resulting solution into another flask.
6. Neutralise with 2 M sodium hydroxide.
7. Dilute to about 150 ml, with water.
8. Add sufficient ammonia buffer pH 10.00 to dissolve the
precipitate.
9. Add 5 ml in excess.
10. Add 50 mg of mordant black II mixture, solution becomes pink.
11. Titrate with disodium edetate solution until the solution
turns green.
12. Note the burette reading.
Q 527 Why bromine water is added in the standardisation of dis-
odium edetate solution?
Ans: Bromine water ensures the oxidation of trace iron (Ferrous)
impurity to ferric ions. Ferric ions form a much less stable
edetate complex than ferrous ions.
Q 528 Why 2M sodium hydroxide is added?
Ans: 2 M sodium hydroxide is added to nearly neutralise the titra-
tion mixture, so that added buffer solution can easily maintain
the desired pH.
Q 529 Why do we add ammonia buffer pH 10 to the titration mixture?
Ans: At pH 10, the zinc EDTA complex is stable and mordant black
II gives sharp colour change. We add ammonia buffer pH 10
to maintain the pH of titration mixture at 10.
Q 530 How ammonia buffer pH 10 is produced?
Ans: Ammonia buffer pH 10 can be prepared by dissolving 5.4 g
of ammonium chloride in 70 ml of 5 M ammonia and diluting
with water to 100 ml.
Q 531 What is mordant black II mixture?
Ans: Mordant black II mixture is 1 part of mordant black II mixed
with 99 parts of sodium chloride. We should add 50 mg of
this mixture as indicator.
102
Viva Voce in
Pharmaceutical Chemistry
Q 532 Why mordant black II is not used as solution?
Ans: The solutions are unstable and thus it is stored as solid
dispersion with sodium chloride.
Q 533 Write the chemistry of standardisation of EDTA solution.
Ans:
1.
2.
3. Indicator reaction
Mordant Black II
Fundamentals of Volumetric Analysis 103
Q 534 How the factor is calculated?
Ans: From the above reactions we know that
1 mole of disodium edetate 1 mole of ZnCl
2
1000 ml of 1 M disodium edetate 65.38 g of Zn
1 ml of 0.05 M disodium edetate
65.38 × 0.05
1000
g of Zn
= 0.003269 g of Zn
Q 535 How exact molarity of prepared disodium edetate solution
is calculated?
Ans:
1. Say we have prepared ‘m’ molar sodium edetate solution.
2. We have titrated ‘W’ g of zinc
3. Burette reading = V ml
4. Factor is
1 ml of 0.05 M of disodium edetate 0.003269 g of Zn
5. 1 ml of ‘m’ M disodium edetate
0.003268 × m
0.05
g of Zn
6. V ml of ‘m’ M disodium edetate =
0.003269
0.05
× m × V g of Zn
7. At equivalence point
V ml correspond to W g of zinc
0.003269 m V
W
0.05
g of Zn
W0.05
m
V 0.003269
8. Molarity of prepared disodium edetate solution is
=
W × 0.05
V × 0.003269
where, W= weight of zinc
V = Volume of disodium edetate corresponding to
W g of Zn.
Q 536 What is the use of disodium edetate?
Ans: Disodium edetate is a chelating agent and is used in metal
poisoning.
104
Viva Voce in
Pharmaceutical Chemistry
Q 537 What is the official dosage form of disodium edetate?
Ans: The official dosage form of disodium edetate is injection.
Q 538 What are the official standards for disodium edetate?
Ans: As per IP 2010, disodium edetate should contain not less than
98.5% and not more than 101% of C
10
H
14
N
2
Na
2
O
8
, 2H
2
O.
Fundamentals of Volumetric Analysis 105
ASSAY OF FERROUS SULPHATE
Q 539 What are the standards for ferrous sulphate as per IP 2010?
Ans: Ferrous sulphate should contain not less than 98.0 percent
and not more than 105 % of FeSO
4
.
7H
2
O.
Q 540 Give the procedure for assay of ferrous sulphate.
Ans:
1. Weigh accurately 1 g of sample.
2. Dissolve in 20 ml distilled water.
3. Add 10 ml of dilute sulphuric acid.
4. Titrate aganist standardised solution of potassium perman-
ganate.
5. Potassium permanganate is self indicator.
6. The end point is from colourless to pink.
Q 541 Write the principle involved in the assay of ferrous sulphate.
Ans: It is oxidation-reduction type of titration, where ferrous ions
are readily oxidised by potassium permanganate, in acidic
medium, to ferric ions. It is a permanganate titration in an
acidic medium. Ferrous sulphate is the reducing agent. The
permanganate ion gets converted to manganese ions in acidic
medium with gain of 5 electrons.
These electrons gained are released by ferrous ions.
Q 542 Write the reactions involved in the assay of ferrous sulphate.
Ans:
Assays
4
105
106
Viva Voce in
Pharmaceutical Chemistry
The net reaction is
Oxidation
Reduction
Q 543 What is the factor in the assay of ferrous sulphate?
Ans: Each ml of 0.1 N KMnO
4
solution corresponds to 0.0278 g of
FeSO
4
.
7H
2
O.
Q 544 How the factor is calculated in the assay of ferrous sulphate
solution?
Ans: 2KMnO
4
= 10 FeSO
4
.
7H
2
O = 5 [O]
2 × 158 = 10 (278) = 5(16)
316 = 2780 = 80
31.6 = 278 = 8
1000 ml of 1 N KMnO
4
solution 278 g of FeSO
4
.
7H
2
O.
Each ml of 0.1 N KMnO
4
solution 0.0278 g of FeSO
4
.
7H
2
O.
Q 545 Which indicator is used in ferrous sulphate assay?
Ans: Potassium permanganate acts as a self indicator in this
titration.
Q 546 What are the uses of ferrous sulphate?
Ans:
Ferrous sulphate is used as
1. Iron supplement to treat iron deficiency anaemia.
Assays 107
2. Manufacture of ink.
3. Wood dying.
4. Lawn conditioner.
5. Purification of water and waste water treatment.
6. Identification of mushrooms.
ASSAY OF CALCIUM GLUCONATE
Q 547 What are the standards for calcium gluconate as per IP 2010?
Ans: As per IP 2010, calcium gluconate should contain not less than
98.5% and not more than 102.0% of C
12
H
22
CaO
14
.
H
2
O.
Q 548 Write the structure of calcium gluconate.
Ans:
Q 549 What are the uses of calcium gluconate?
Ans: Calcium gluconate is used as calcium replenisher. It is used
for treating hypocalcemic tetany or in calcium deficiency.
Q 550 What are the official dosage forms of calcium gluconate?
Ans: As per IP calcium gluconate can be given as injection or tablets.
Q 551 Describe IP 2010 method for assay of calcium gluconate.
Ans:
1. Weigh accurately 0.5 g of calcium gluconate.
2. Dissolve in 50 ml of warm water.
3. Cool , add 5.0 ml of 0.5 M magnesium sulphate and 10 ml of
strong ammonia solution.
4. Add mordant black II mixture as indicator.
108
Viva Voce in
Pharmaceutical Chemistry
5. Titrate with 0.05M disodium edetate solution till the colour
changes from reddish violet to blue.
6. Take the burette reading.
7. From the burette reading, substract the volume of the
magnesium sulphate solution added.
8. Note the reading.
Q 552 What is the principle behind calcium gluconate assay?
Ans: Calcium gluconate assay is based on complex formation
between calcium ions and the ligand, i.e. EDTA solution.
Q 553 Write the chemical reactions occurring in calcium gluconate
assay.
Ans:
Q 554 How the indicator, Mordant black II produces the colour
change at equivalence point?
Ans:
1. Following reaction occurs when indicator is added to the
titration mixture.
Magnesium forms more stable complex with indicator than
the calcium, so formation of Mg–In complex is favoured.
2. During titration, first all the calcium ions are consumed by
EDTA. Then the free magnesium ions are consumed followed
Assays 109
by reaction of EDTA with Mg–In complex. As Mg-EDTA
complex is more stable than Mg–In complex, following
reaction occurs.
Q 555 Why magnesium sulphate solution is added in calcium
gluconate assay?
Ans: With only calcium ions, sharp end point cannot be obtained
with mordant black II and the transition from red to pure blue
is not observed. Compared to calcium, magnesium forms less
stable complex with EDTA. The magnesium–indicator
complex is more stable than calcium–indicator complex but
less stable than magnesium-EDTA complex. Consequently,
during the titration of a solution containing magnesium and
calcium ions with EDTA in the presence of mordant black II,
the EDTA reacts first with free calcium ions and finally with
magnesium–indicator complex. Since the Mg–In complex is
wine red and free indicator is blue between pH 7 and 11, the
sharp colour change from wine red to blue is observed at
equivalence point.
Q 556 Why strong ammonia solution is added?
Ans: Strong ammonia solution is added to maintain the alkaline
pH at around 10.
Q 557 Why the volume of magnesium sulphate should be substracted
from burette reading?
Ans: Substraction of the volume of magnesium sulphate solution
will give the exact amount of EDTA utilised by calcium ions
in formation of complex.
Q 558 How the factor is calculated?
Ans: We know from the reaction that
1 mole of disodium edetate 1 mole of calcium gluconate
1000 ml of 1 M disodium edetate 448.4 g of C
12
H
22
CaO
4
, H
2
O
1 ml of 0.05 M disodium edetate
448.4 × 0.05
1000
g of C
12
H
22
CaO
4
.
H
2
O
1 ml of 0.05 M disodium edetate 0.02242 g of C
12
H
22
CaO
4
.
H
2
O