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Volumetric and Gravimetric Analysis 47
Principle of Adsorption:
In the Titration of Cl
–
with Ag
+
This method is associated with the addition of standard solution of silver nitrate (AgNO
3
) ito
unknown concentration of halide ion. Therefore, a white ppt of silver chloride is formed.
AgNO
3
+ NaCl → AgCl (ppt) + NaNO
3
➢ Before the equivalence point: Before the equivalent point, the adsorbed Cl
–
form the
primary adsorbed layer over the ppt of AgCl, and attract the positively charged Na
+
ions
from the solution to form a secondary layer.
➢ After the equivalent point: Silver ions (Ag
+
) in excess displace the Cl
–
ions and form
the primary layer at the surface of the ppt, which attract the negatively charged nitrate
ions NO
3
–
and form the secondary layer.
➢ At the end point: Anion of indicator in (fluorescein) replaces the NO
3
–
over the surface
of AgCl, form the second layer and produce the intense pink color.
.
Fig. 2.5: (a) AgCl precipitated in the presence of excess of Cl
‒
(b) AgCl precipitated in the presence of excess of Ag
+
(c) Excess of Ag
+
reacts with indicator to give color change
Procedure:
Preparation of 0.1 M Silver Nitrate: Weigh accurately 3.4 g of finely powdered silver nitrate
and dissolve in 200 ml of distilled water for the preparation of 0.1 M silver nitrate (AgNO
3
).
Assay Procedure:
Reaction medium: Reaction medium is maintained between the pH 6.5 to 10.3.
Indicator: Adsorption indicator Eosin is used as an indicator.
Sample solution: Sample solution is prepared by dissolving 0.1 g of sodium chloride (NaCl) in
5 ml of distilled water. Then 5 ml of acetic acid and 50 ml of ethanol is added to the solution.
Standard solution: Burette is filled with 0.1 M of AgNO
3
.
End-point detection: Appearance of pink color.
48 Text Book of Pharmaceutical Chemistry
3. Volhard’s Method
Volhard’s method was given by Volhard in 1874. It is an indirect method for chloride
determination in which colored complex is formed. In this method, an excess amount of standard
Ag
+
is added to the chloride solution containing Fe
3+
as an indicator. Thereafter the excess of Ag
+
is titrated with standard solution of thiocyanate ions (SCN
–
). The end-point is detected by the
appearance of a red complex. Volhard’s method of titration must be performed in acidic medium
(HNO
3
).
First, Cl
–
is precipitated by excess AgNO
3
.
Ag
+
(aq) + Cl
–
(aq) → AgCl (s)
AgCl(s) is removed by filtration/washing and excess of Ag
+
is titrated with KSCN in the
presence of Fe
3+
.
Ag
+
(aq) + SCN
–
(aq) → AgSCN (s)
End-point is detected by appearance of a red complex. As Ag
+
has been consumed, a red
complex forms as a result of:
SCN
–
+ Fe
3+
→ [FeSCN]
3
Red complex
Procedure:
Preparation of 0.1 M silver nitrate: Weigh accurately 3.4 g of finely powdered silver nitrate
and dissolve in 200 ml of distilled water for the preparation of 0.1 M silver nitrate (AgNO
3
).
Preparation of 0.1 M Ammonium Thiocyanate: For the preparation of 0.1 M Ammonium
thiocyanate (NH
4
SCN), weigh accurately 1.522 g of NH
4
SCN and dissolve in 200 ml of distilled
water.
Assay Procedure of Sodium Chloride:
Reaction medium: Acidic (HNO
3
), 5 ml of 2 M nitric acid is added in the flask.
Indicator: 1 ml of ferric ammonium sulphate [FeNH
4
(SO
4
)
2
] is used as an indicator.
Sample solution: Sample solution is prepared by dissolving 0.1 g of sodium chloride (NaCl) in
50 ml of water. Previously standardized 50 ml of 0.1 M AgNO
3
is added and 2 ml of dibutyl
phthalate is added.
Standard solution: Burette is filled with 0.1 M Ammonium thiocyanate (NH
4
SCN).
End-point detection: Appearance of blood red complex of ferric thiocyanate [FeSCN]
3
.
Advantages of Volhard’s Method:
➢ This method is applicable for the direct determination of silver and for the indirect
determination of halide ions.
➢ In this titration, the concentration of the indicator is not important.
Volumetric and Gravimetric Analysis 49
Disadvantages of Volhard’s Method:
➢ Silver nitrate is required in excess amount which can be economically expensive.
➢ This method is time-consuming due to the additional filtration process required to
remove the silver chloride precipitate.
Modified Volhard’s Method
For the analysis of chloride, a slight modification to the Volhard’s method is required. At the
time of titration, the solution comes in contact with two precipitations simultaneously: silver
chloride and ammonium thiocyanate, which has different solubility. Therefore, after the titration,
excess of thiocyanate ions react with AgCl and the red color of iron thiocyanate complex
gradually disappear. Upon addition of more SCN‾ cause restoration of red color which induced
huge error in analysis. Hence, to avoid this error before titration with thiocyanate, the solution of
few ml of organic liquid which does not mix with water is added.
2.17.2 Mercurometric Titration
Complexometric Titration
Complexometric titration is based upon the formation of a co-ordination complex. It is also
called as chelatometry. This type of titrations is particularly useful for the determination of a
mixture of different metal ions in solution. In complexometric analysis, the end-points are
detected by the formation of colored complex, called as complexometric titrations.
For example, the determination of water hardness using EDTA with Eriochrome black-T as
the indicator is one example of complexometric titration.
Theory and Principle of Complexometric Titration:
(a) Complex or Chelate is a compound that is formed by combining the metal ion with a
molecule that is capable of donating electrons.
(b) A chelate is a compound that is formed by combining a polyvalent metal ion with a
molecule which actually contains two or more groups that can donate electrons.
Ethylene diamine tetraacetic acid (EDTA) reacts with polyvalent metal ions to form a
chelate compound.
(c) A ligand is a molecule, which affords groups for attachment to metal ions such as
EDTA. Ligands are classified on the basis of the number of points of attachment to the
central ion into:
➢ Monodentate ligand: These ligands bound to the central ion at only one point
(share one lone pair of electron), e.g., H
2
O, NH
3
, etc.
➢ Bidentate ligand: These ligands bound to the central ion at two points (share two
lone pairs of electrons), e.g., ethylenediamine
➢ Polydentate ligand or Multidentate ligand: These ligands has multi-point
attachment (share more than one lone pair of electrons), e.g., EDTA; hexadentate
ligand (six points of attachment).
50 Text Book of Pharmaceutical Chemistry
EDTA (Ethylene Diamine Tetraacetic Acid):
EDTA was synthesized in 1935 by Ferdinand Munz. It is an aminopolycarboxylic acid
containing basic ethane group that is attached with the two amine (–NH
2
) groups at the terminal
positions forming an ethylene diamine. It contains four acetic acid groups attached to the ethylene
diamine.
A unique property of EDTA is to donate its six lone pairs of electrons for the formation of
coordinate covalent bonds to metal ion; form EDTA as a hexadentate ligand. Structure of
EDTA is given below:
Fig. 2.6: Structure of EDTA
2.18 TYPES OF COMPLEXOMETRIC TITRATION
Ethylene diamine tetraacetic acid (EDTA) acts as a chelating titrant and can be used in
various ways. Therefore, the titrations with EDTA performed in different ways:
Direct Titration Method
In the direct titration method, the metal ion is titrated with a standard solution of EDTA. The
solution containing the metal ion is buffered to suitable pH and directly titrated with a standard
EDTA solution. The standard solution of EDTA is slowly added using a burette to the metal ion
solution until the desired end-point is reached.
This method is used to determine the barium, chromium, copper, mercury, zinc, aluminum,
and lead, etc., e.g., BaSO
4
, Ca(C
2
O
4
)
2
, PbSO
4
, Mg
3
(PO
4
)
2
, etc.
Indirect Titration Method
Indirect titration is used to determine certain anions that form precipitate with metal cations
and do not react with EDTA. Ex. CO
3
2‒
, CrO
4
2‒
, SO
4
2‒
can be determined by this method. SO
4
2‒
can be determined by adding excess of Ba
+2
to precipitate as BaSO
4
.
Step 1: Anion is precipitated from solution by addition of excess metal ion.
Ex. SO
4
2‒
+ excess Ba
+2
→ BaSO
4
(s) (pH 1)
Precipitate BaSO
4
(s) is filtered & washed.
Volumetric and Gravimetric Analysis 51
Step 2: Precipitate (BaSO
4
) is then reacted with excess EDTA to bring the metal ion back into
solution (boiled at pH 10) → BaEDTA
2‒
+ EDTA.
Step 3: The excess EDTA is titrated with Mg
2+
standard solution.
Back Titration Method
A back titration is a type of complexometric titration in which the analyte concentration is
determined. It is performed for the determination of several metal ions that cannot be titrated
directly. In this method, a known excess of EDTA is added to the metal ion (buffered to an
appropriate pH). Then, the excess EDTA left after the complex formation with the metal is back
titrated with a standard solution of a different metal ion. The choice of a second metal ion is
important as it must not displace the analyte metal ion from its EDTA complex.
Replacement Titration Method
This method of complexometric titration is used when direct or back titrations fail to produce
sharp end-points. It is a metal ion that forms EDTA complexes, which are more stable than metals
like magnesium and calcium. The metal ion is determined by the replacement of zinc or
magnesium ions from an EDTA complex with an equivalent number of metal ions, then titrating
the liberated Zn or Mg ion with a standard EDTA solution.
For example, Determination of Calcium Salt: Standard volume of Mg-EDTA solution adds
to Ca salt in the presence of buffer. Calcium displaces Mg ion and gives a stable complex with
EDTA as Ca-EDTA complex. The displaced and liberated Mg ions are then titrated with standard
EDTA solution using Mordant black as indicator.
Ca
2+
+ Mg-EDTA → Ca-EDTA + Mg
2+
Mg
2+
+ EDTA
2–
→ Mg-EDTA
2.19 METAL ION INDICATORS
The indicators used in complexometric titration are called metal ion indicators. These
indicators are always necessary to be used in a complexometric titration. They change color when
they bind to metal ions in solution. Common indicators are:
➢ Organic dyes – capable of acting as a chelating agent to give a dye-metal complex.
➢ Other indicators are different in color from the dye and have a low stability constant than
the chelate-metal complex.
Properties of Metal Ion Indicators
➢ Metal indicator complex should be less stable than the metal-EDTA complex.
➢ Metal and indicator binding should not be too weak.
➢ The metal-indicator complex should be 10 to 100 times less stable than the metal-titrant
complex.
➢ Indicator and the metal complexed indicator color must be different.
52 Text Book of Pharmaceutical Chemistry
Indicators Used in Complexometric Titrations
Sr. No.
Name of Indicator
Color Change
Metal Detected
1
Eriochrome black T
Solochrome black T
Mordant black II
Red to blue
Zn, Mg, Mn, Ba, Cd, Ca,
Pb, Hg
2
Muroxide or Ammonium
purpurate
Violet to blue
Ca,Cu, Co
3
Thymol blue
Blue to grey
Zn, Pb, Cd, Hg
4
Xylenol orange
Lemon to yellow
Zn, Pb, Cd, Hg, Bi,
Thorium
5
Methyl Blue
Blue to yellow
Zn, Pb, Cd, Hg
6
Alizarin
Red to yellow
Zn, Pb, Co, Mg, Cu
7
Catechol-violet
Violet to red
Mg, Mn, Co, Fe, Pb
8
Sodium Alizarin sulphonate
Blue to red
Al, Thorium
2.20 MASKING AND DEMASKING AGENTS
Masking
Masking is the process in which a substance is transformed into a new form without physical
separation of it or its reaction products. By the process of masking, a substance is so transformed
that it does not enter into a particular reaction. Masking is used to prevent one element from
interfering in the analysis of another element.
Masking agents are the reagents that prevent some components of the analyte from reaction
with EDTA. By using masking agents, certain cations in a mixture can often be masked.
Therefore, they can no longer react with EDTA or with the indicator.
An example of masking agent is the cyanide ion. This forms stable cyanide complexes with
the cations of Cd, Zn, Hg(II), Cu, Co, Ni, Ag, and the platinum metals, but not with the alkaline
earths, manganese, and lead.
Examples of masking agent:
KCN: It is used as masking agent for Ag
+
, Cu
2+
, Cd
2+
, Co
2+
, Ni
2+
, Zn
2+
, …, etc.
M
+
+ 2CN
–
→ [M(CN)
2
]
–
M
+
+ 4CN
–
→ [M(CN)
4
]
2–
Demasking
Demasking may be defined as the process in which the masked substance regains its ability to
enter into a particular reaction.
Volumetric and Gravimetric Analysis 53
Demasking agents are the reagents that are useful in introducing the mask ion which was
masked before. By this process, the masked substance regains its ability to enter into the desired
chemical reaction.
Examples:
➢ Selective demasking of aluminum makes use of triethanolamine as a demasking agent.
We can use this demasking agent in a reaction where we used ascorbic acid as a masking
agent in order to avoid any interference from iron.
➢ The masking by CN– can be removed by mixture of formaldehyde acetic acid. On
addition of demasking agent to [Zn(CN)
4
]
2–
, Zn is liberated and titrated.
2.21 PREPARATION AND STANDARDIZATION OF 0.05 M DISODIUM EDETATE SOLUTION
Preparation:
Weigh 18.6 g of disodium edetate and dissolve in sufficient distilled water in a volumetric
flask, properly mixing it. Once it has completely dissolved, make up the volume to 1000 ml.
Standardization:
➢ Weight accurately about 0.8 g of granulated zinc and dissolve by gentle warming in 12
ml of diluted Hydrochloric acid.
➢ Add about 0.1 ml of bromine solution and boil to remove the excess bromine.
➢ After cooling, add sufficient water to produce 200 ml.
➢ Pipette out 20 ml of this solution in to a conical flask and neutralize with 2 M sodium
hydroxide.
➢ Add 125 ml water and sufficient ammonia buffer pH 10 to dissolve the precipitate and
add 5 ml in excess.
➢ Add 50 mg of mordant black II mixture and titrate with the prepared disodium edetate
solution until the solution turns to green color.
2.22 ESTIMATION OF MAGNESIUM SULPHATE
➢ Weigh accurately 0.3 g of anhydrous magnesium sulphate.
➢ Dissolve in 50 ml of distill water.
➢ Add about 10 ml of (pH 10) ammonia buffer and 50 mg mixture of mordant black II and
NaCl as indicator.
➢ Titrate the solution with 0.05 M disodium edetate until the solution becomes blue.
54 Text Book of Pharmaceutical Chemistry
2.23 OXIDATION-REDUCTION REACTION (REDOX TITRATION)
Oxidation Reaction
Oxidation is the process in which an atom loses an electron and thus increases its oxidation
number. In other words, oxidation is defined as the addition of oxygen or the more
electronegative element or removal of hydrogen or the more electropositive element from a
substance is called an oxidation reaction.
Reduction Reaction
Reduction is the process in which an atom gains an electron and therefore decreases (or
reduces its oxidation number). In other words, reduction is defined as the addition of hydrogen or
more electropositive element or removal of a more electronegative element or oxygen from a
substance.
Oxidation and Reduction
Oxidation
Reduction
➢ Oxidation is the loss of electrons.
➢ Increase in oxidation number.
➢ Gain of oxygen.
➢ Loss of hydrogen by covalent compound
➢ Reduction is the gaining of electrons
➢ Decrease in oxidation number
➢ Loss of oxygen
➢ Gaining of hydrogen by covalent compound
2.24 REDOX REACTION
The oxidation-reduction reactions or redox reactions always occur simultaneously. A redox
reaction is defined as a chemical reaction in which electrons are transferred between two reactants
species. In the chemical reaction illustrated, an electron is removed from reactant A and this
reactant is oxidized. Similarly, reactant B was handed an electron and was therefore reduced.
In the given reaction, an electron-accepting species that tend to undergo a reduction in redox
reactions are called oxidizing agents while an electron-donating species that transfer electrons are
called as reducing agents.
Volumetric and Gravimetric Analysis 55
Types of Redox Reactions: Redox reactions are of different types. They are:
➢ Decomposition Reaction
➢ Combination Reaction
➢ Displacement Reaction
➢ Disproportionation Reaction
2.25 OXIDATION NUMBER (OXIDATION STATE)
Oxidation number (oxidation state) is a number that is assigned to an atom in a substance, and
it indicates if electrons are lost or gained charge, gained or lost electrons of an atom, represented
in a positive, negative, or neutral expression.
For example, H
3
PO
4
Hydrogen has an oxidation number of +1 in most compounds. Oxygen has an oxidation
number of –2 in most compounds. The sum of these oxidation numbers is 3(+1) + 4(−2) = −5.
Since H
3
PO
4
has no net charge, the oxidation number of P must be +5.
2.26 REDOX TITRATION
A redox titration is same as an acid-base titration except it involves a redox reaction. Redox
titration is based on an oxidation-reduction reaction between the titrant and the analyte. It is one
of the most common laboratory methods to identify the concentration of unknown analytes.
These types of titrations sometimes require the use of a potentiometer or a redox indicator.
A chemical reaction is where both oxidation and reduction reaction occurs simultaneously.
The substance being oxidized will donate electrons to the substance being reduced. A reductant
(reducing agent) will undergo oxidation and an oxidant (oxidizing agent) will undergo reduction.
Example 1:
Mg + ZnC1
2
→ MgC1
2
+ Zn
In the reaction, Mg is the reducing agent which is being oxidized and Zn
+2
is the oxidizing
agent which is being reduced.
Mg → Mg
+2
+ 2e
–
Zn
2+
+ 2e → Zn
Example 2:
2Fe
2
O
3
+ 3C → 4Fe + 3CO
2
Hence, in the reaction, Fe
+2
is the oxidant which is being reduced whereas C is the reductant
which is being oxidized.
Fe
2
+3
+ 3e
–
→ Fe
C → C
+4
+ 4e
–
56 Text Book of Pharmaceutical Chemistry
Standard Electrode Potential
In an electrochemical shell, the standard electrode potential is a measurement of the potential
for equilibrium. Between the electrode and the electrolyte, there is a potential difference known as
the potential of the electrode. When the concentrations of all the species in a semi-cell are equal,
the electrode potential is referred to as the standard electrode potential.
The standard electrode potential occurs in an electrochemical cell under standard conditions,
e.g., temperature = 298 K, pressure = 1 atm and concentration = 1 M.
The symbol ‘E
o
cell
’ represents the standard electrode potential of a cell.
2.27 REDOX INDICATORS
A redox indicator or oxidation-reduction indicator is an indicator which undergoes a definite
color change at a specific electrode potential. Redox indicators changes color at specific potential
differences.
Type of Redox Indicators
➢ Specific indicators: The substances that react specifically with one of the reagents in a
titration to produce a color, e.g., starch gives blue color with iodine SCN
–
ion gives red
color with Fe
3
+
.
➢ Self-indicators: The oxidizing agent or reducing agent will behave as the indicator. The
color of the titrating agent undergoes a sharp enough change in color at the equivalence
point, e.g., acidified potassium manganate (VII) (KMnO
4
).
When KMnO
4
is used as a titrant for reducing agents during titration, the purple color of
permanganate disappears due to formation of colorless Mn
2+
. When all the reducing
agents have been oxidized, a slight excess of KMnO
4
colors the solution pink.
➢ External indicator: External indicators are the indicators which are added from outside
means indicator are kept separately. They do not exist in the reaction, e.g.,
Phenolphthalein, Methyl orange, etc. Solution being titrated will be removed every few
minutes and tested with the external indicator.
Example: For the determination of Fe
2+
with K
2
Cr
2
O
7
near the equivalence point, drops
of ferrous sample are removed and brought in contact with freshly prepared solution of
potassium ferricyanide on a spot plate.
➢ Internal indicator: Internal indicators are the substances that will be added together
with the solutions involved in titration, e.g., Methyl orange, starch, phenolphthalein,
diphenylamine, etc.