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80
Viva Voce in
Pharmaceutical Chemistry
PREPARATION AND STANDARDISATION OF
SODIUM THIOSULPHATE SOLUTION
Q 409 Can sodium thiosulphate be used as primary standard?
Ans: Sodium thiosulphate (Na
2
S
2
O
3
.
5H
2
O) is readily available in a
state of high purity, but there is uncertainly about the exact
water content, because of the efflorescent nature of the salt.
Thus, it is unsuitable as primary standard.
Q 410 How would you prepare xM Na
2
S
2
O
3
?
Ans: Dissolve 248x g of sodium thiosulphate and 2x g of sodium
carbonate in sufficient carbon dioxide free water to produce
1000 ml.
Q 411 How would you prepare 0.1M Na
2
S
2
O
3
solution?
Ans: Dissolve 24.8 g of sodium thiosulphate and 0.2 g of sodium
carbonate in sufficient carbon dioxide free water to produce
1000 ml.
Q 412 Is sodium thiosulphate an oxidising agent?
Ans: Sodium thiosulphate is not an oxidising agent. It is a reducing
agent as it undergoes oxidation, as follows:
Q 413 How would you prepare 200 ml of 0.1 M sodium thiosulphate
solution?
Ans: The molecular weight of sodium thiosulphate (Na
2
S
2
O
3
.
5H
2
O)
is 248. If we dissolve 248 g of sodium thiosulphate in water
and make up the volume to 1000 ml, we get 1M solution.
248 g of sodium thiosulphate 1000 ml of 1M solution
24.8 g of sodium thiosulphate 1000 ml of 0.1M solution
24.8 200
g
1000
of sodium thiosulphate 200 ml of 0.1 M solution
Dissolve 5 g of sodium thiosulphate and 40 mg of sodium
carbonate in water and make up the volume to 200 ml with
carbon dioxide free water.
Q 414 Why sodium carbonate is added in preparation of sodium
thiosulphate solution?
Ans: Sodium thiosulphate solution is decomposed by bacterial
action, e.g. thiobacillus thioparus, if the solution has been
stored for some time. Bacterial activity is least when the pH
Fundamentals of Volumetric Analysis 81
lies between 9 and 10. Adding small amount of sodium
carbonate (0.1 g /L) is advantageous to ensure the correct pH.
Q 415 Is the prepared sodium thiosulphate solution stable?
Ans: Solutions prepared by using distilled water (which contains
excess of carbon dioxide), may undergo slow decomposition
with the formation of sulphur.
Further, bacterial decompostion may also take place as
explained in the previous answer.
Q 416 What precautions should be taken to prevent decomposition
of sodium thiosulphate solution?
Ans:
1. Prepare the solution with recently boiled distilled water.
2. Add chloroform (3 drops) or mercury iodide (10 mg/L).
3. Add sodium carbonate (0.1 g/L) to ensure pH between 9–10,
as at this pH bacterial activity is least.
4. Avoid exposure to light, as it hastens decomposition.
Q 417 Name the primary standard, which can be employed in the
standardisation of sodium thiosulphate solution.
Ans: Potassium bromate, potassium iodate, potassium dichromate,
copper and iodine can act as primary standards in stan-
dardisation of sodium thiosulphate solution.
Q 418 Which secondary standard is used for standardisation of
sodium thiosulphate solution?
Ans: Standardised potassium permanganate solution or iodine
solution can be used as secondary standard in stan-
dardisation of sodium thiosulphate solution.
Q 419 Describe the procedure for standardisation of sodium
thiosulphate solution.
Ans:
1. Weigh 0.2 g of potassium bromate.
2. Take 50 ml of this solution.
3. Add 2 g of potassium iodide and 3 ml of 2 M hydrochloric acid.
4. Titrate with sodium thiosulphate solution with constant
shaking.
5. When the colour of the mixture becomes pale yellow, dilute
with water and add 2 ml of starch solution.
82
Viva Voce in
Pharmaceutical Chemistry
6. The mixture becomes blue.
7. Continue the titration until colour changes from blue to
colourless or pale yellow.
8. Take the burette reading.
Q 420 Enumerate the chemical reactions occurring in standardi-
sation of sodium thiosulphate solution.
Ans:
Q 421 Which type of titration occurs in standardisation of sodium
thiosulphate?
Ans: It is an iodometric type of oxidation–reduction titration. Iodine
is liberated by reaction between potassium iodide and
potassium bromate. This liberated iodine is reduced by
sodium thiosulphate solution.
Q 422 Differentiate between 1 N sodium thiosulphate solution and
1M sodium thiosulphate solution.
Ans: 1 N and 1 M solution of sodium thiosulphate are not different
because equivalent weight and molecular weight of sodium
thiosulphate is same. This is because, in redox reaction only
one electron is lost by one molecule of sodium thiosulphate.
Q 423 What is the factor in standardisation of sodium thiosulphate?
Ans: Each ml of 0.1 M sodium thiosulphate is equivalent to
0.002784 g of KBrO
3
.
Q 424 How the factor is calculated in the standardisation of sodium
thiosulphate?
Ans: From the chemical reactions, we know that
KBrO
3
3I
2
6Na
2
S
2
O
3
6 Na
2
S
2
O
3
KBrO
3
1000 ml of 6 M Na
2
S
2
O
3
1000 ml of 1 M KBrO
3
1000 ml of 6 M Na
2
S
2
O
3
167 g of KBrO
3
1 ml of 0.1 M Na
2
S
2
O
3
167
6 1000 10
g of KBrO
3
.
1 ml of 0.1M Na
2
S
2
O
3
= 0.002784 g of KBrO
3
.
Fundamentals of Volumetric Analysis 83
Q 425 How do you calculate the exact molarity of prepared sodium
thiosulphate solution?
Ans:
1. Say burette reading for 50 ml KBrO
3
solution is = V
1
ml
2. Burette reading for 250 ml KBrO
3
solution is = V
1
× 5 = V
2
ml
3. Weight of KBrO
3
= W
4. W g of KBrO
3
V
2
ml of Na
2
S
2
O
3
solution
5. F is the factor
6. Molarity of Na
2
S
2
O
3
solution
=
2
M × W
V F
=
2
0.1 × W
V 0.002784
Q 426 What are the uses of sodium thiosulphate?
Ans:
1. Sodium thiosulphate is an antidote to cyanide poisoning and
arsenic poisoning.
2. Topical antifungal agent for tinea versicolor.
3. In foot baths for prophylaxis of ring worm.
4. In measuring volume of extracellular body fluid and renal
glomerular filtration rate.
5. In treatment of calciphylaxis in hemodialysis patients with
end-stage renal disease.
6. Useful in gold extraction.
7. Photographic fixer.
PRECIPITATION TITRATIONS
Q 427 What is precipitation?
Ans: Precipitation is the combination of two ionic species to form a
very insoluble product.
Q 428 What is precipitation titration?
Ans: These are the titrations based on principle of quantitative
combination of two ionic species to form an insoluble salt
which is precipitated. The point at which precipitation is
complete can be determined, using suitable indicator.
Q 429 What are argentimetric titrations?
Ans: The titrations which involve the use of standard solution of
silver nitrate are known as argentimetric titrations.
84
Viva Voce in
Pharmaceutical Chemistry
Q 430 Give the classification of argentimetric titrations.
Ans: Argentimetric titrations are of two types:
1. Mohr’s method —Direct titration
2. Volhard’s method —Back titration
MOHR’S METHOD
Q 431 When Mohr’s method is used?
Ans: Mohr’s method can be used only if the solution to be titrated
is either neutral or basic.
Q 432 What should be the pH range for performing Mohr’s method?
Ans: The pH of the solution should be between 7 and 10.
Q 433 How the pH is maintained between 7 and 10?
Ans: pH is maintained between 7 and 10 by saturating the analyte
solution with sodium hydrogen carbonate.
Q 434 Describe Mohr’s method.
Ans: In Mohr’s method, the halide is titrated directly with standard
silver nitrate solution. Potassium chromate is used as an
indicator. The silver nitrate reacts with halide (iodide, bromide
or chloride) and precipitates as silver halide. When the
reaction is complete the next drop of silver nitrate reacts with
potassium chromate forming red precipitate of silver
chromate, indicating end point.
Q 435 Why Mohr’s method is not useful when the solution to be
titrated is acidic?
Ans: Mohr’s method is not useful in acidic medium because the silver
chromate formed at the end point is soluble in acidic medium.
Q 436 Which compounds can be determined using Mohr’s method?
Ans: Sodium chloride and its formulation, potassium iodide,
sodium nitroprusside [Na
2
Fe(CN)
5
NO] can be determined
using Mohr’s method.
Q 437 Give the chemical reaction occurring in Mohr’s method.
Ans:
For example
For sodium chloride:
Fundamentals of Volumetric Analysis 85
For potassium iodide:
For sodium nitroprusside:
Q 4 38 What chemical reaction occurs at the end point in Mohr’s method?
Ans: In Mohr’s method potassium chromate is used as indicator.
At the end-point potassium chromate reacts with silver nitrate
to form red coloured precipitate of silver chromate.
Q 439 How Mohr’s method can be used for acidic solutions?
Ans: As we know, Mohr’s method is applicable only when the
solution to be titrated is neutral. Acid solutions can be
determined by Mohr’s method, if they are first neutralised
with chloride free calcium carbonate or sodium bicarbonate.
Q 440 Why Mohr’s method is a direct titration method?
Ans: Mohr’s method is a direct titration method because the salt to
be determined is directly titrated with standard solution of
silver nitrate.
VOLHARD’S METHOD
Q 441 When Volhard’s method is used?
Ans: When the assay needs to be performed in acidic medium,
Volhard’s method is used.
Q 442 Which compounds are assayed by Volhard’s method?
Ans: Ammonium chloride, which on dissolution in water forms
slightly acid solution is assayed by Volhard’s method. Sodium
chloride, and chlorbutanol can also be determined by
Volhard’s method.
86
Viva Voce in
Pharmaceutical Chemistry
Q 443 What is back titration?
Ans: Back titration is a method in which excess of the standard
solution is added in the solution to be titrated (sample). The
reaction is allowed to proceed and the excess of standard
solution not required by sample, is determined by titration
with another standard solution.
Q 444 Why is Volhard’s method a back titration method?
Ans: In Volhard’s method excess of standard silver nitrate solution
is added to the solution to be titrated and then excess of silver
nitrate is back titrated with ammonium thiocyanate solution.
Q 445 Describe Volhard’s method?
Ans: Volhard’s method is an argentimetric type of precipitation
titration carried out in acidic medium.
In this method, a measured volume of standard silver nitrate
is added to solution of the halide acidified with concentrated
nitric acid. The unreacted excess of silver nitrate is titrated with
standard ammonium thiocyanate (NH
4
SCN) solution. Ferric
ammonium sulphate, FeNH
4
(SO
4
)
2
is used as an indicator. At
the end point a permanent red colour is developed due to
formation of ferric ferrithiocyanate, Fe[Fe(SCN)
6
].
In case of chloride salt determination, either the silver chloride
is filtered off or the precipitate is coagulated using nitro-
benzene or dibutyl phthalate, when ferric ammonium
sulphate is used as an indicator.
Q 446 Give the reactions involved in determination of halides
using Volhard’s method.
Ans: Let us take example of ammonium chloride assay.
Fundamentals of Volumetric Analysis 87
Q 447 Why nitric acid is added in Volhard’s method?
Ans: Volhard’s method is used when the assay needs to be
performed in acidic medium. Addition of nitric acid provides
the desired acidic medium/mixture for titration. The titration
is carried out in acidic medium to prevent precipitation of
ferric ions (from ferric ammonium thiocyanate indicator) as
hydrated oxide.
Q 448 Why silver chloride precipitate is filtered before back
titration?
Ans: Silver chloride (formed by reaction between silver nitrate
and halide) is more soluble than silver thiocyanate (which
is formed by reaction between silver nitrate and ammonium
thiocyanate). Thus silver chloride would react with
thiocyanate releasing chloride ions.
Thus, there would be over consumption of thiocyanate ions,
which in turn leads to low values for chloride analysis.
Q 449 Why nitrobenzene is added in estimation of chloride using
Volhard’s method?
Ans: The silver chloride formed reacts with thiocyanate ions
leading to error in detection of end point. Nitrobenzene
prevents the reaction of silver chloride with thiocyanate
ions by forming film around silver chloride particles.
Q 450 Is it necessary to filter the silver bromide or silver iodide
precipitate before back titrations? Justify.
or
Do we need to add nitrobenzene or dibutyl phthalate
in analyte solution in bromide or iodide detection?
Justify.
Ans: It is not necessary to filter the silver bromide or silver iodide
before back titration. The silver bromide or silver iodide
precipitate formed is less soluble than silver thiocyanate and
thus does not interfere in the end point, as it does not
consume thiocyanate ions. For the same reason the
precipitate need not be coated using nitrobenzene or dibutyl
phthalate.
88
Viva Voce in
Pharmaceutical Chemistry
Q 451 Which other agent is preferred over nitrobenzene? Why?
Ans: For avoiding reaction between silver chloride and thiocyanate
ions dibutyl phthalate is preferred over nitrobenzene, as it is
non-toxic.
Q 452 Which indicator other than ferric ammonium sulphate can
be employed in Volhard’s method?
Ans: Tartrazine (0.5% aqueous solution) can be used in place of
ferric ammonium sulphate. The end point is colourless to
bright lemon yellow green colour (of supernatant liquid).
Q 453 What precautions should be taken while performing
titration by Volhard’s method?
Ans:
1. The nitric acid should be free from nitrous acid because nitrous
acid gives red colour with thiocyanate ion.
2. The temperature of the solution must be kept below 25°C
because at higher temperature, the colour of the ferric
thiocyanate complex fades.
Q 454 Can adsorption indicators be used in precipitation titrations?
Ans: Adsorption indicators can be used in precipitation titrations.
Q 455 How adsorption indicators work?
Ans: At equivalence point, adsorption indicators are adsorbed by
the precipitate and gets changed, producing a new colour,
e.g. fluorescein used in chloride detection by Fajan’s method.
Fluorescein changes its colour from yellow-green in solution
to red colour of the silver fluoresceinate at the surface of
silver chloride precipitate.
Q 456 Name some adsorption indicators.
Ans: Fluorescein, dichlorofluorescein, tetrabromofluorescein
(eosin), tartrazine are some adsorption indicators.
PREPARATION AND STANDARDISATION OF 0.1M
SILVER NITRATE SOLUTION
Q 457 How would you prepare 1000 ml of 0.1 M silver nitrate solution?
Ans: The molecular weight of silver nitrate is 169.99. So, when we
want to prepare 1000 ml of 0.1 M solution, we should dissolve
169.9/10 g, i.e. 16.99 g of silver nitrate in water and make up
the volume to 1000 ml.
Fundamentals of Volumetric Analysis 89
Q 458 How would you standardise silver nitrate solution?
Ans: We standardise silver nitrate solution using Fajan’s method,
i.e. direct titration with pure and dry sodium chloride (which
is a primary standard), using adsorption indicator, eosin.
Q 459 Describe the procedure of standardisation of 0.1 M silver
nitrate solution, as per IP 2007.
Ans:
1. Dry sodium chloride at 110°C for 2 hrs.
2. Weigh accurately 0.1 g of sodium chloride. Transfer to conical flask.
3. Dissolve it in 5 ml of water.
4. Add 5 ml of acetic acid, 50 ml of methanol and 0.15 ml of
eosin solution.
5. Stir vigorously to dissolve the contents.
6. Titrate with prepared silver nitrate solution.
7. Continue adding silver nitrate until the precipitate suddenly
assumes red violet colour.
8. Note the burette reading.
Q 460 Which indicator is used in standardisation of silver nitrate
solution by IP 2007 procedure?
Ans: As per IP 2007 procedure, eosin, an adsorption indicator is
used with colour change of precipitate from white to red violet.
Potassium chromate 5% can also be used as an indicator with
permanent faint reddish brown colour as end point.
Q 461 What is the other name of eosin?
Ans: Eosin is nothing but tetrabromo fluorescein.
Q 462 Give the chemical reaction in standardisation of silver nitrate
solution.
Ans:
Q 463 How the factor is calculated in standardisation of silver
nitrate using sodium chloride?
Ans: From the chemical reaction, we know that 1 mole of NaCl
reacts with 1 mole of AgNO
3
.
1000 ml of 1 M AgNO
3
58.44 g of NaCl