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70
Viva Voce in
Pharmaceutical Chemistry
Q 3 52 Which chemical reactions occur in the standardisation of sodium
hydroxide solution by using paotassium hydrogen phthalate?
Ans:
Q 353 How the exact molarity of sodium hydroxide solution is calculated?
Ans: Same as described in answer 304.
Q 354 How the factor is derived?
Ans: We know from the chemical reactions
1 mole of potassium hydrogen phthalate reacts with 1 mole
of sodium hydroxide
C
8
H
5
KO
4
NaOH
i.e.
204.2 g of C
8
H
5
KO
4
1000 ml of 1M NaOH
0.2042 g of C
8
H
5
KO
4
1 ml of 1M NaOH.
Q 355 How 1N or 0.5 M solution of succinic acid is prepared?
Ans: Molecular weight of succinic acid is 118 g. And the basicity of
succinic acid is 2. So, to prepare 1N or 0.5 M solution of succinic
acid, we must dissolve
118
2
= 59 g of succinic acid in water
and make up the volume to 1000 ml.
Fundamentals of Volumetric Analysis 71
Q 356 How the exact molarity of prepared succinic acid is calculated?
Ans: We know that 11.8 g succinic acid 100 ml of 1M solution of
succinic acid
‘w’ g of succinic acid 100 ml of ‘x’ M solution of succinic acid.
Molarity =
w × 1
11.8
Q 357 Enumerate the procedure of standardisation of sodium
hydroxide using succinic acid.
Ans:
1. Take 10 ml of prepared 1N succinic acid.
2. Add two drops of phenolphthalein.
3. Titrate with prepared sodium hydroxide solution till the
solution becomes just pink.
4. Take burette reading.
Q 358 How would you calculate the exact normality of sodium
hydroxide?
Ans:
N
1
V
1
= N
2
V
2
where,
N
1
= Normality of NaOH
V
1
= Volume of NaOH (burette reading)
N
2
= Normality of succinic acid solution
V
2
= Volume of succinic acid solution
N
1
=
22
1
NV
V
Q 359 How would you calculate the molarity of sodium hydroxide?
Ans: Molarity of sodium hydroxide is same as normality as its
acidity is one.
REDOX TITRATIONS
Q 360 What are redox titrations?
Ans: These are the titrations in which oxidation and reduction
process is taking place simultaneously. One reactant
undergoes oxidation while the other reactant undergoes
reduction.
72
Viva Voce in
Pharmaceutical Chemistry
Q 361 What is oxidation?
Ans: Oxidation is loss of one or more electrons by atoms or ions,
e.g. Fe
2+
Fe
3+
+ e
–
Q 362 What is reduction?
Ans: Reduction is gain of one or more electrons by atoms or ions,
e.g. Ce
4+
+ e
–
Ce
3+
Q 363 What is an oxidising agent?
Ans: Oxidising agent is the reagent which gains electrons and is
reduced.
Q 364 What is a reducing agent?
Ans: Reducing agent is a reagent which loses electrons and is
oxidised.
Q 365 What is an oxidant?
Ans: An oxidant is an oxidising agent, it undergoes reduction.
Q 366 What is a reductant?
Ans: A reductant is a reducing agent, it undergoes oxidation.
Q 367 Name four oxidising agents.
Ans: Potassium permanganate, potassium dichromate, iodine,
potassium bromate, and hydrogen peroxide are oxidising
agents.
Q 368 Name four reducing agents.
Ans: Zinc, ferrous sulphate, sodium thiosulphate, and hydrogen
peroxide are reducing agents.
Q 369 What happens in a redox reaction?
Ans: Oxidation-reduction reactions involve transfer of electrons from
reductant (it releases electrons) to oxidant (it accepts electrons).
EQUIVALENT WEIGHT OF OXIDISING AND REDUCING AGENTS
Q 370 What is the equivalent weight of oxidising or reducing agent?
Ans: In oxidation–reduction titration, equivalent weight is that
weight which combines with 1.008 g of available hydrogen
or 8.008 g of available oxygen for use in the redox reaction
under consideration.
Q 371 How equivalent weight of oxidising agent is calculated?
Ans: Equivalent weight of oxidising agent can be calculated by
dividing the gram molecular weight by the number of
electrons gained by one molecule of oxidising agent.
Fundamentals of Volumetric Analysis 73
In the above reaction permanganate ion is converted to
manganous ion in the redox process by the gain of 5
electrons, so the equivalent weight is one-fifth of potassium
permanganates molecular weight (158).
=
158
31.6
5
Q 372 What is the equivalent weight of potassium permanganate
in acidic medium?
Ans: Partial ionic equation of potassium permanganate in acidic
medium is
The reaction of KMnO
4
with acid is as follows:
Thus, potassium permanganate gives 5 atoms of oxygen which
are taken up by the reducing agent or it gains 5 electrons.
Thus equivalent weight of KMnO
4
is
Eq. wt =
Mol. wt
No. of electrons gained
=
158
5
= 31.6
Q 373 What is the equivalent weight of potassium permanganate
in neutral medium?
Ans: The partial ionic equation of potassium permanganate in
neutral solution is
Here, the permanganate ion is converted to manganese
dioxide by gain of 3 electrons. Thus, the equivalent weight of
potassium permanganate is
158
3
= 52.66
Q 374 What is the equivalent weight of potassium permanganate
in strongly alkaline solution?
Ans: The partial ionic equation of potassium permanganate in
strongly alkaline medium is
74
Viva Voce in
Pharmaceutical Chemistry
Here, permanganate ion is gaining one electron and thus the
equivalent weight of potassium permanganate is
158
1
= 158
Q 375 What is the equivalent weight of potassium dichromate in
acidic medium?
Ans: The partial ionic equation for potassium dichromate as
oxidising agent is
During the oxidation, the dichromate ion accepts 6 electrons
and thus
equivalent weight of K
2
Cr
2
O
7
=
(mol. wt)
6
=
294.18
6
= 49.03
Q 376 What is the equivalent weight of iodine?
Ans: The partial ionic equation of iodine is
Thus, iodine gains two electrons and reduces to iodide ions.
The equivalent weight is
253.8
126.9
2
Q 377 What is the equivalent weight of potassium bromate?
Ans: Potassium bromate is an oxidising agent, with partial ionic
equation as follows.
As KBrO
3
accepts 6 electrons its equivalent weight is
=
167
6
= 27.8
Q 378 What is the equivalent weight of sodium thiosulphate?
Ans: Sodium thiosulphate is a reducing agent and thus loses
electrons. The partial ionic equation for Na
2
S
2
O
3
is
Only one electron is lost by one molecule. Thus equivalent
weight of sodium thiosulphate is
=
mol wt.
1
=
248.17
1
= 248.17
Fundamentals of Volumetric Analysis 75
Q 379 What is the equivalent weight of oxalic acid as a reducing agent?
Ans: The partial ionic equation is
Thus, the equivalent weight is
Mol. wt.
2
126.07
2
= 63.03
PERMANGANOMETRY
Q 380 What is permanganometry?
Ans: When potassium permanganate is used as an oxidising agent
in the titration, it is called permanganometry.
Q 381 State the names of inorganic compounds assayed by
permanganometry.
Ans: Ferrous sulphate, hydrogen peroxide are assayed by
permanganometry.
Q 382 Give the properties of potassium permanganate.
Ans:
1. Potassium permanganate appears as dark purple or brownish
black granular powder or as prismatic cyrstals with metallic lustre.
2. It is soluble in cold water, freely soluble in boiling water.
3. It decomposes on contact with certain organic substances.
Q 383 What are the uses of potassium permanganate?
Ans: Potassium permanganate is used as oxidising agent. It is used
as antiseptic in mouth wash and in cleaning ulcers and
abscesses, antidote in case of poisoning by barbiturates,
alkaloids, etc. It is also used in urethritis.
Q 384 What precautions should be taken while handling potassium
permanganate?
Ans: With potassium permanganate dangerous explosions are liable
to occur if it is brought into contact with organic or other readily
oxidisable substances, either in solutions or in the dry condition.
Q 385 How potassium permanganate should be stored?
Ans: Potassium permanganate should be stored in tightly closed
umber coloured container protected from light. The bright
sunlight slowly decomposes even pure solutions.
76
Viva Voce in
Pharmaceutical Chemistry
Q 386 Why potassium permanganate solution is boiled for
15–30 minutes and then allowed to stand for cooling?
Ans: Potassium permanganate reacts with organic matter present
in the distilled water and decomposes it, with formation of
manganese dioxide (MnO
2
). The manganese dioxide formed
may further catalyse the formation of more MnO
2
from
permanganate ions. To prevent this the solution is boiled
and cooled and then filtered for removing traces of MnO
2
.
Q 387 Should KMnO
4
solution be filtered through ordinary filter
paper?
Ans: KMnO
4
solution should be filtered through a plug of glass wool
or asbestos. Filter paper is of organic nature and thus should
be avoided. Organic matter can act as a reducing agent.
Q 388 Why permanganometric titrations are preferably conducted
in acidic solution?
Ans: In the acidic medium, the permanganate ion (MnO
4
–
) is
converted to manganese ion by accepting five electrons. In
acidic medium permanganate ion acts as strong oxidising
agent and thus acidic medium is preferred.
Q 389 Why permanganate solution is decolourised when added
to a solution of a reducing agent?
Ans: Permanganate ions are reduced to manganese ions, when
potassium permanganate acts as oxidising agent.
Solutions containing permanganate ions are purple in colour
while solutions containing manganese ions are colourless. So,
when the purple permanganate solution is added to reducing
agent manganese ions are formed and solution is decolorised.
The moment, all reducing substance is consumed and excess
of KMnO
4
is added, the solution becomes pink to purple.
Q 390 Why permanganate ion or potassium permanganate can act
as self indicator in acidic conditions?
Ans: Same as previous answer.
Q 391 Which primary standard is used for standardisation of
potassium permanganate?
Ans: Oxalic acid/Sodium oxalate is available in pure and
anhydrous state and thus it is used as primary standard for
standardisation of potassium permanganate. Potassium
Fundamentals of Volumetric Analysis 77
permanganate can be standardised using standard sodium
thiosulphate solution.
Q 392 Name the assays carried out using permanganometry?
Ans: Assay of ferrous sulphate, hydrogen peroxide solution can
be carried out by permanganometry.
PREPARATION AND STANDARDISATION OF POTASSIUM
PERMANGANATE
Q 393 Give the procedures for standardisation of potassium
permanganate as per IP 2010.
Ans:
1. Weigh accurately about 0.3 g of KMnO
4
.
2. Dissolve in water to produce 100 ml.
3. Take 20 ml of this solution and add 20 ml of water, 1g of KI
and 10 ml of 2 M HCl.
4. Titrate the liberated iodine with 0.1 M sodium thiosulphate
solution using starch solution as indicator, added towards the
end of titration indicated by pale yellow colour of the mixture.
5. The solution becomes blue.
6. Continue titrating till the solution becomes once again pale
yellow.
7. Take the burette reading.
Q 394 How the exact molarity or normality of the potassium
permanganate is calculated?
Ans: The factor is
1 ml of 0.1 M sodium thiosulphate is equivalent to 0.003160 g
of KMnO
4
.
Exact molarity of KMnO
4
solution =
M × W
V × F
where, M = Expected/standard molarity.
W = Weight of KMnO
4
.
V = Burette reading corresponding to W g of KMnO
4
.
F = Factor.
Once the molarity is found, it should be divided by oxidation
number to find out the exact normality.
78
Viva Voce in
Pharmaceutical Chemistry
Q 395 What are the reactions involved in standardisation of
KMnO
4
?
Ans:
IODIMETRIC AND IODOMETRIC TITRATIONS
Q 396 What do you understand by iodimetric titration?
Ans: Iodimetric titrations use standard solution of iodine as an
oxidising agent in redox titration.
Q 397 What is iodometry?
Ans: In iodometric type of redox titration, liberated iodine is used
for titration. It is an indirect titration.
Q 398 Give the properties of iodine.
Ans: Iodine appears as greyish, violet brittle plates or small crystals
with a metallic sheen. It has irritable odour. It volatilises
slowly at room temperature.
Q 399 Differentiate between iodimetry and iodometry.
Ans: In iodimetry, solution of iodine is used in the titration while
in iodometry the iodine liberated after reaction between two
other reagents is used as an oxidising agent.
Q 400 State the names of compounds which are assayed by
iodimetry.
Ans: Assay of iodine, ascorbic acid, and analgin is carried out by
iodimetry.
Q 401 State the names of compounds assayed by iodometry.
Ans: Copper sulphate, chlorinated lime, and potassium
permanganate are assayed by iodometry.
Q 402 List out different indicators used in iodine titration.
Ans: Starch solution, and sodium starch glycolate can be used in
iodine titration.
Q 403 Why starch shows blue colouration with iodine?
Ans: Starch consists of two major components amylose and
amylopectin. Amylose, which is a straight chain compound,
Fundamentals of Volumetric Analysis 79
gives blue colour with iodine and assumes a spiral form.
Amylopectin, which is a branched chain structure forms a
red purple product, probably by adsorption.
Q 404 When starch solution is added as indicator in the titration?
Ans: When iodine reacts with reducing agent like sodium
thiosulphate, it is converted into iodide. When most of the
iodine is consumed and only a small amount is present, the
solution becomes yellow, towards the end of titration. When
the solution becomes just pale yellow, starch solution is
added.
Q 405 Why starch solution is added near to the end point of the
titration?
Ans: Starch solution is added near to the end point of the titration
rather than at the initial stages of titration because amylopectin
from starch would form water insoluble blue complex with
iodine which would interface with the detection of the end
point of the titration.
Q 406 What precautions should be taken to get correct end point
using starch indicator?
Ans:
1. The starch solution should be added near the end point.
2. The temperature should not be higher because colour
sensitivity decreases with increase in temperature. The
temperature should be around 25°C.
3. Ethanol should not be present in the titration.
4. Starch cannot be used in strong acid medium because
hydrolysis of starch may occur.
Q 407 How starch solution is prepared?
Ans: Starch solution is prepared by mixing 0.1g of soluble starch
with little water and then pouring this paste with constant
stirring in 100 ml of boiling water, then boil the contents
for 1 minute. Allow the solution to cool and add 2–3 g of
potassium iodide.
Q 408 How much starch solution should be added in a titration?
Ans: Add 2 ml of 0.1% starch solution per 100 ml of the solution to
be titrated.