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90
Viva Voce in
Pharmaceutical Chemistry
1 ml of 0.1 M AgNO
3
58.44
10 × 1000
g of NaCl
= 0.005844 g of NaCl
Q 464 How the exact molarity of silver nitrate solution (0.1 M) is
calculated?
Ans: The molarity of AgNO
3
solution =
M × W
V × F
where, M = Standard/expected molarity of AgNO
3
solution
W = Weight of sodium chloride
V = Volume of AgNO
3
solution
F = Factor
Molarity of AgNO
3
solution =
0.1 × W
V × 0.005844
Q 465 What are the uses of silver nitrate?
Ans: Silver nitrate is used as an astringent and germicide.
PREPARATION AND STANDARDISATION OF AMMONIUM
THIOCYANATE SOLUTION
Q 466 How would you prepare 1000 ml of 0.1M ammonium thio-
cyanate solution?
Ans: Molecular weight of ammonium thiocyanate is 76.12 g. We
know that when we dissolve 1 mole, i.e. 76.12 g up to 1000 ml
of water we get 1 M solution. Therefore to prepare 1000 ml of
0.1 M ammonium thiosulphate solution we should dissolve
76.12
10
g, i.e. 7.612 g of ammonium thiocyanate in water and
make up the volume to 1000 ml.
Q 467 How would you standardise ammonium thiocyanate solution?
Ans: We standardise ammonium thiocyanate solution by argenti-
metric type of precipitation titration against the standardised
silver nitrate solution.
Q 468 Describe the procedure for standardisation of ammonium
thiocyanate solution as per IP 2007.
Ans:
a. Take 30 ml of 0.1 M silver nitrate into a glass stoppered flask.
b. Add 50 ml of water, 2 ml of nitric acid and 2 ml of ferric
ammonium sulphate solution.
Fundamentals of Volumetric Analysis 91
c. Titrate with ammonium thiocyanate solution to the first
appearance of red brown colour.
Q 469 Write the chemical reactions occurring in standardisation
of ammonium thiocyanate solution.
Ans:
Q 470 Which indicator is used in ammonium thiocyanate stan-
dardisation?
Ans: Ferric ammonium sulphate is used as the indicator.
Q 471 What chemical changes occur at the end point in standar-
disation of ammonium thiocyanate solution?
Ans:
Q 472 How the factor is calculated in standardisation of ammonium
thiocyanate solution?
Ans: From the reaction, it is clear that 1 mole of silver nitrate is
reacting with 1 mole of ammonium thiocyanate.
1000 ml of 1 M AgNO
3
1000 ml of 1 M NH
4
SCN
1000 ml of 1M AgNO
3
76.12 g of NH
4
SCN
1 ml of 0.1 M AgNO
3
76.12
10 1000
0.007612 g of NH
4
SCN
Q 473 How the molarity of ammonium thiocyanate solution is
calculated?
Ans: At the equivalence point, the gram moles of two solutions are
same if one mole of both the reactants react with each other.
Thus, if the molarity and volume of standard solution is
known (i.e. silver nitrate solution), equivalent volume of
ammonium thiocyanate can be found by titration
M
1
V
1
(NH
4
SCN) = M
2
V
2
(AgNO
3
)
Molarity of NH
4
SCN =
33
4
Molarit
y
of A
g
NO × Volume of A
g
NO
Volume of NH SCN solution
92
Viva Voce in
Pharmaceutical Chemistry
Q 474 Is there any other method to calculate molarity of ammonium
thiocyanate solution?
Ans: The other method for calculating molarity is,
The factor is :
1 ml of 0.1 M AgNO
3
0.007612 g of NH
4
SCN ...(i)
1 ml of xM AgNO
3
'x' × 0.007612
0.1
g of NH
4
SCN ...(ii)
Say, we have used 30 ml of AgNO
3
, then
30 ml of xM AgNO
3
30 × 'x' × 0.007612
0.1
g of NH
4
SCN ...(iii)
At equivalence point
30 ml of x M AgNO
3
Volume of NH
4
SCN × its molarity
From equation (iii),
30 × 'x' × 0.007612
0.1
g of NH
4
SCN = BR × Molarity of NH
4
SCN
Molarity of NH
4
SCN
30 'x' 0.007612
0.1 BR
COMPLEXOMETRIC TITRATIONS
GENERAL QUESTIONS
Q 475 What are complexometric titrations?
Ans: These are the titrations, in which complex is formed in a
quantitative manner by reaction between a ligand or
complexing agent. Titrimetric methods based on complex
formation are called complexometric titrations.
Q 476 What are complexes?
Ans: Coordination compounds formed by reaction between metal
ions and electron pair donors (ligands) are called complexes.
Q 477 What are chelates?
Ans: Chelates are the coordination compounds formed when a
metal ion coordinates with two or more donor groups of a
single ligand to form a five or six membered heterocyclic ring.
Q 478 What is the difference between a complex and a chelate?
Ans: Complexes are the coordination compounds formed by
reaction between ligands which can be unidentate, bidentate
or polydentate. While chelates are a type of complexes which
Fundamentals of Volumetric Analysis 93
are formed by reaction between metal ions with multidentate
ligands to form heterocyclic ring.
Q 479 What is a ligand or complexing agent?
Ans: A ligand or complexing agent is an electron donating ion or
molecule which has ability to form one or more covalent or
coordinate bonds with the metal ion to produce a complex.
Q 480 What are chelating agents?
Ans: Ligands having more than one electron donating group are
called chelating agents.
Q 481 Name the factors influencing stability of complexes.
Ans: Stability of the complexes depends upon complexing ability
of metals, characteristics of the ligand, pH of the solution,
concentration of the metal ion.
Q 482 What is coordination number of a metal?
Ans: The maximum number of groups that can be bound to the
metal ions, is the coordination number of that metal or the
number of covalent bonds that a cation tends to form with
electron donors is its coordination number.
Q 483 What is the basis for classification of the ligands?
Ans: Ligands are classified on the basis of the number of points of
attachment to the metal ion.
Q 484 Classify ligands.
Ans:
1. Unidentate ligands/monodentate ligands.
2. Bidentate ligands.
3. Multidentate ligands.
Q 485 What are monodentate or unidentate ligands?
Ans: The ligands which can bind to the metal ion at only one point, by
donating lone pair of electron to the metal, are called unidentate
ligands, e.g. molecule of ammonia, H
2
O or halide ions.
Q 486 What are bidentate ligands?
Ans: When the molecule has two donor atoms to form two co-
ordinate bonds with the same metal ion, it is called a bidentate
ligand.
For example, ethylenediamine (1, 2-diaminoethane) molecule
is bound to the metal ions through the lone pair of electrons of
the two nitrogen atoms and thus it is a bidentate ligand.
94
Viva Voce in
Pharmaceutical Chemistry
Q 487 What are multidentate ligand?
Ans: A multidentate ligand is a ligand which has more than two
coordinating atoms/electron donors per molecule, e.g.
ethylenediamine tetra-acetic acid has two electron donor
nitrogen atoms and four electron donor oxygen atoms in the
molecule, so it can be hexadentate.
Q 488 Classify the complexes.
Ans: Complexes can be classified on the basis of number of metal
ions used for formation of the complex.
1. Mononuclear complexes.
2. Binuclear complexes.
3. Polynuclear complexes.
Q 489 What are mononuclear complexes?
Ans: Mononuclear complexes use only one metal ion for the
formation of complex.
Q 490 What are binuclear complexes?
Ans: Binuclear complexes use two metal ions for the formation of
the complex.
Q 491 What are polynuclear complexes?
Ans: Polynuclear complexes are formed when more than two metal
ions are used for formation of the complex.
Q 492 What is the effect of concentration of metal ions on the type
of complex formed?
Ans: If the concentration of metal ions is low, only mononuclear
complex is formed. But as concentration of metal ions increase
probability of formation of binuclear and polynuclear
complexes increase.
Q 493 What is the type of bonding in complexes?
Ans: The bonds are:
1. Ordinary covalent bonds in which both the metal and the
ligand contribute one electron each.
2. Coordinate bonds in which both electrons are contributed by
the ligand.
Q 494 What is the possible charge on the metal ion complex?
Ans: Metal ion complex can be
1. Negatively charged: When the number of negative groups is
more than the valency of the metal ion, the complex has
negative charge. The negative charge on the complex ion is
Fundamentals of Volumetric Analysis 95
equal to the total number of negative groups minus the
valency of metal [Fe(CN)
6
]
3–
.
2. Positively charged: When only neutral groups are involved in
the formation of complex, the charge on the complex is positive
and is equal to the valency of the metal ion, e.g. [Cu (NH
3
)
4
]
2+
.
3. Neutral: When the sum of valency of the metal ions is equal
to the number of negatively charged groups, then the complex
formed is neutral K
3
[Fe(CN)
6
].
Q 495 How the equivalence point is determined in complexometric
titrations?
Ans: The equivalence point is determined visually either with the
help of specific metal ion indicators or with the help of
potentiometric or photometric technique.
Q 496 Which metal ions can form complexes?
Ans: Any metal ion, formed from the metals given in periodic table,
can form complexes with electron donors.
Q 497 State the functional groups responsible for the formation of
complexes.
Ans: The groups with an easily replaceable proton (like –COOH,
Phenolic and enolic –OH) or neutral groups offering a lone
pair of electrons (NH
2
, CO and alcoholic OH) can easily form
complexes with metal ions.
Q 498 What is a sequestering agent?
Ans: Sequestering agents are the chelating agents which form water
soluble complexes with bi or polyvalent metal ions, e.g. EDTA.
Q 499 What is the difference between a chelating agent and
sequestering agent?
Ans: A chelating agent forms complexes, which are insoluble in
water but soluble in organic solvent while sequestering agents
form water soluble complexes, which are not soluble in
organic solvents.
Q 500 Give examples of chelating agents and sequestering
agents.
Ans:
1. Chelating agents: EDTA [ethylene diamine tetra-acetic acid],
EGTA [ethylene glycobis (2-amino ethyl ether) N,N,N’,N’
tetra acetic acid.
2. Sequestering agents: Dimethylglyoxime salicylaldoxine.
96
Viva Voce in
Pharmaceutical Chemistry
Q 501 Name the complexing agent most widely employed in
complexometric titrations.
Ans: EDTA
Q 502 What are the uses of EDTA?
Ans:
1. In treating mercury and lead poisoning.
2. To remove excess iron from body.
3. To remove the inorganic debris, in dentistry.
4. Useful in blood analysis.
5. Analysis of water hardness.
6. Metal scavenger in biochemistry and molecular biology.
Q 503 Why EDTA is preferred ligand?
Ans: EDTA has the widest application in analysis because of
its powerful complexing action and commercial
availability. The spatial structures of its anion, which
has six donor atoms, enables it to satisfy the coordination
number of six, frequently encountered among metal
ions and to form strainless five membered rings on
chelation.
Q 504 How EDTA complexes with metal ions?
Ans: EDTA forms complexes with most cations in a 1:1 ratio,
irrespective of the valency of the ion.
EDTA is assigned the formula H
4
Y. The disodium salt of
EDTA will have the formula Na
2
H
2
Y and the complex
forming ion is H
2
Y
2–
in aqueous solution. The reaction of
H
2
Y
2–
with cations can be written as
The structures of these complexes with di, tri and tetravalent
metals contain three, four and five rings respectively.
Fundamentals of Volumetric Analysis 97
Q 505 Write the structures of EDTA and disodium edetate.
Ans: EDTA
Disodium edetate
Q 506 Why the disodium salt of EDTA is preferred over EDTA?
Ans: EDTA (edetic acid) is only sparingly soluble in water (about
0.2 %) while the solubility of disodium salt of EDTA is about
10% . Thus it is preferred.
Q 507 Why is it essential to buffer the titration mixture in
complexometric titrations?
or
Why is it necessary to maintain a particular pH in
complexometric titration?
Ans: It is necessary to maintain a particular pH in complexometric
titration because only at specific pH the specific complex is
stable and the colour change of the indicator is most distinct,
e.g. EDTA complexes with metal ions of the charge number
2, are stable in alkaline or slightly acidic solution (pH 6–10)
whereas complexes with charge numbers 3 or 4, may exist
in solutions of much higher acidity (pH 3).
Q 508 What is pM indicator?
Ans: The indicators, which are sensitive to metal ion concentration,
are called pM indicators, i.e. log M
+
. These indicators are used
in complexometric titrations. The pM indicator is a dye, which
is capable of acting as a chelating agent.
Q 509 How does the pM indicator produce the colour change?
Ans: The pM indicator, i.e. a dye reacts with metal ion to form dye–
metal complex. The colour of this dye–metal complex is
different from the colour of the dye. The dye–metal complex
has a lower stability constant than the chelate metal complex.
Thus, the colour of the titrant solution is the colour of the
dye–metal ion complex until the equivalence point is reached.
98
Viva Voce in
Pharmaceutical Chemistry
As soon as, there is the slightest excess of the edetate, the
metal–dye complex decomposes to produce free dye, which
causes the change in colour. The colour of the dye and complex
is pH dependent.
Q 510 Name commonly used pM indicators.
Ans: Solochrome black T (eriochrome black T or mordant black II)
and calcon are most commonly used pM indicators. These
are azo dyes. Other pM indicators are xylenol orange, Zincon,
methylthymol blue, etc.
Q 511 In which titrations solochrome black T is used as indicator?
Ans: Solochrome black T is used as an indicator in complexometric
titrations involving Mg, Mn, Zn, Cd, Hg, Pb, Ca ions. The pH
of the titration mixture should be 10 and the colour change is
from red to blue. The solochrome black T and metal complex
has red colour and when slight excess EDTA is added,
solochrome black T is released, which is blue.
Q 512 Name different types of EDTA titrations.
Ans:
1. Direct titration.
2. Back titration.
3. Replacement or substitution titration.
Q 513 What is direct complexometric titration?
Ans: In direct complexometric titration, the metal ion to be
determined is first buffered to the desired pH. In the titration
mixture some auxillary complexing agent, such as tartarate or
citrate or triethanolamine is added. Then, it is directly titrated
with standard EDTA solution. Equivalence point is determined
by the change in colour of metal indicator or by amperometric,
spectrophotometric or potentiometric methods.
Q 514 Why auxillary complexing agents are added in direct
complexometric titrations?
Ans: Auxillary complexing agents like tartarate, citrate or
triethanolamine are added to prevent precipitation of the
hydroxide of the metal or a basic salt.
Q 515 What is back complexometric titration?
Ans: In back complexometric titration, excess of standard EDTA
solution is added, then the solution is buffered to the desired
pH. The solution is heated to effect complex formation. Then
excess of EDTA is back titrated with a standard metal ion solution
like a solution of zinc chloride or sulphate or magnesium chloride
Fundamentals of Volumetric Analysis 99
or sulphate. The end point is detected using pM indicator which
responds to zinc or magnesium ions introduced in back titration.
Q 516 When back titration needs to be performed?
Ans: Back titration needs to be performed when:
1. The metal ions precipitate from the solution in the pH range
necessary for titration.
2. The metal ions form inert complexes.
3. Suitable metal indicator is not available
4. For insoluble metal salts, e.g. lead as sulphate, calcium as oxalate.
Q 517 What is substitution complexometric titration?
Ans: Substitution titration is also known as replacement titration.
In substitution titration, metal ion to be determined may be
treated with the magnesium or zinc complex of EDTA.
The metal ions displace the equivalent amount of magnesium
or zinc from a less stable Mg
–
edetate complex, as shown in
the equation.
In displacement titrations, an unmeasured excess of a solution
containing magnesium or zinc complex of EDTA is introduced
into the analyte solution. If the analyte forms a more stable
complex than that of magnesium or zinc, displacement as
shown in equation occurs. The liberated cation is then titrated
with standard EDTA.
For example: Calcium, lead, mercury can be determined
satisfactorily using mordant Black II as indicator. The
magnesium or zinc ions give sharper change in colour as
compared to direct titration of these metal ions.
Q 518 When substitution titrations are employed?
Ans: Substitution titrations are employed when:
1. Metal ions do not react or react unsatisfactorily with metal
indicator.
2. For those metal ions, which form EDTA complexes that are
more stable than those of other metals like Mg and Ca.
3. When direct titration or back titration do not give sharp end point.
Q 519 What is the use of complexometric titration, in day-to-day life?
Ans: Complexometric titrations are used in determination of
hardness of water.