Instrumental Methods of Analysis
Unit Instrumental Methods of Analysis
2
1. Reference electrode :
(a) Definition : An electrode which has stable and reproducible electrode potential, is known as
reference electrode.
OR
An electrode which has stable and reproducible potential and complete the cell, acting as
half-cell, is called as reference electrode.
(b) The purpose of reference electrode is to complete the cell and provide stable potential. The
reference electrode helps in the measuring potential of any electrode connected to it.
(c) Examples: Standard hydrogen electrode (SHE), Calomel electrode, Silver –silver chloride
electrode, Mercury – mercury sulphate electrode, Quinhydrone electrode etc.
2. Types of reference electrode: There are two types of reference electrodes.
(a) Primary reference electrode: e.g. Standard hydrogen electrode (SHE).
(b) Secondary reference electrode: Calomel electrode, Silver –silver chloride electrode,
Mercury – mercury sulphate electrode, Quinhydrone electrode etc.
3. Calomel electrode :
Calomel electrode is an example of secondary reference electrode.
Mercurous chloride (Hg2Cl2) is called as Calomel.
Q. Explain the construction of calomel electrode with labelled diagram and its representation.
Ans.: Calomel electrode is a secondary reference electrode which has stable and reproducible
electrode potential.
(a) Construction of calomel electrode:
1) It consists of an outer glass tube fitted with a frit at
the bottom to permit electrical contact with solution.
2) Inside this glass tube there is another tube having
mercury at the bottom over which a paste of
mercury-mercurous chloride (Hg-Hg2Cl2) is
placed.
3) A solution of potassium chloride (KCl) is then
placed over the paste.
4) A platinum wire sealed in glass tube helps in making the electrical contact.
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, Instrumental Methods of Analysis
5) The electrode is connected with the help of the side tube on the left through a salt bridge
with the other electrode to make a complete cell.
(b) Representation of Calomel electrode: Calomel electrode is represented as,
Hg | Hg2Cl2(s)| KCl (xM)
OR
Hg | Hg2Cl2(s)| Clsat .
(c) Reaction at calomel electrode:
If the calomel electrode act as cathode, then reaction is:
Hg 2Cl2 2e 2 Hg 2Cl
And if it act as anode, then the reaction is reverse.
(d) Potential of Calomel electrode: The potential of the calomel electrode depend upon the
concentration of potassium chloride (KCl) solution. The reduction potentials of the calomel
electrodes on hydrogen scale at 298 K are as follows.
KCl concentration 0.1 N (Decinormal) 1 N (Normal) Saturated
E0 calomel 0.334 volts 0.2800 volts 0.2422 volts
(e) Advantages of Calomel electrode:
1) It is handy, compact and easy to transport.
2) It has constant reproducible potential.
3) It is easy to construct and maintain.
4) It has very small temperature coefficient of potential.
(f) Demerits of Calomel electrode:
1) It should not be used above 800 C, as mercurous chloride (Hg2Cl2) starts decomposing into
mercury and mercuric chloride.
2) It involves handling of poisonous mercury (Hg) and mercurous chloride (Hg2Cl2).
Q. What are reference electrodes? Give the electrode potentials of saturated, normal and
decinormal calomel electrodes as 298 K.
Ans.:
(a) Definition : An electrode which has stable and reproducible electrode potential, is known
as reference electrode.
OR
An electrode which has stable and reproducible potential and complete the cell, acting as
half cell, is called as reference electrode.
(b) The electrode potentials of saturated, normal and decinormal calomel electrodes as 298 K are
as follows:
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, Instrumental Methods of Analysis
KCl concentration 0.1 N (Decinormal) 1 N (Normal) Saturated
E0 calomel 0.334 volts 0.2800 volts 0.2422 volts
Q. Give representation of calomel electrode. Give the electrode reactions, when the calomel
electrode acts as cathode.
Ans.:
(a) Representation of Calomel electrode: Calomel electrode is represented as,
Hg | Hg2Cl2(s)| KCl (xM)
OR
Hg | Hg2Cl2(s)| Clsat .
(b) Reaction at calomel electrode:
If the calomel electrode act as cathode, then reaction is:
Hg 2Cl2 2e 2 Hg 2Cl
Q. Give reasons, why a calomel electrode cannot be used above 80 °C.
Ans.: Calomel electrode cannot be used above 80 °C because at such a high temperature mercurous
chloride (Hg2Cl2) disproportionate into mercury and mercuric chloride (HgCl2).
4) Indicator Electrode:
a) Definition : “The electrode of a cell in which the potential depends on the concentration of a
particular ion is called as an indicator electrode.”
b) Examples: Glass electrode and ion selective electrodes (ISE) are an examples of indicator
electrode.
Q. Give principle and construction of glass electrode with labelled diagram and its
representation.
Ans. : The glass electrode is an example of indicator electrode whose potential changes depending
on the concentration of the analyte.
a) Principle of Glass electrode: When two solutions of different [H+] are separated by a thin glass
membrane, a potential difference (P.D.) is developed at the two surfaces of membrane. The
potential difference (P.D.) developed is proportional to the difference in [H +] of the two
solutions.
b) The glass membrane acts as ion exchanger i.e. exchanger of Na+ of glass with H+ of solution.
c) Electrode representation of glass electrode:
Ag, AgCl|0.1M HCl | H+ (Test solution)
OR
Pt, 0.1 M HCl | Glass
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Unit Instrumental Methods of Analysis
2
1. Reference electrode :
(a) Definition : An electrode which has stable and reproducible electrode potential, is known as
reference electrode.
OR
An electrode which has stable and reproducible potential and complete the cell, acting as
half-cell, is called as reference electrode.
(b) The purpose of reference electrode is to complete the cell and provide stable potential. The
reference electrode helps in the measuring potential of any electrode connected to it.
(c) Examples: Standard hydrogen electrode (SHE), Calomel electrode, Silver –silver chloride
electrode, Mercury – mercury sulphate electrode, Quinhydrone electrode etc.
2. Types of reference electrode: There are two types of reference electrodes.
(a) Primary reference electrode: e.g. Standard hydrogen electrode (SHE).
(b) Secondary reference electrode: Calomel electrode, Silver –silver chloride electrode,
Mercury – mercury sulphate electrode, Quinhydrone electrode etc.
3. Calomel electrode :
Calomel electrode is an example of secondary reference electrode.
Mercurous chloride (Hg2Cl2) is called as Calomel.
Q. Explain the construction of calomel electrode with labelled diagram and its representation.
Ans.: Calomel electrode is a secondary reference electrode which has stable and reproducible
electrode potential.
(a) Construction of calomel electrode:
1) It consists of an outer glass tube fitted with a frit at
the bottom to permit electrical contact with solution.
2) Inside this glass tube there is another tube having
mercury at the bottom over which a paste of
mercury-mercurous chloride (Hg-Hg2Cl2) is
placed.
3) A solution of potassium chloride (KCl) is then
placed over the paste.
4) A platinum wire sealed in glass tube helps in making the electrical contact.
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, Instrumental Methods of Analysis
5) The electrode is connected with the help of the side tube on the left through a salt bridge
with the other electrode to make a complete cell.
(b) Representation of Calomel electrode: Calomel electrode is represented as,
Hg | Hg2Cl2(s)| KCl (xM)
OR
Hg | Hg2Cl2(s)| Clsat .
(c) Reaction at calomel electrode:
If the calomel electrode act as cathode, then reaction is:
Hg 2Cl2 2e 2 Hg 2Cl
And if it act as anode, then the reaction is reverse.
(d) Potential of Calomel electrode: The potential of the calomel electrode depend upon the
concentration of potassium chloride (KCl) solution. The reduction potentials of the calomel
electrodes on hydrogen scale at 298 K are as follows.
KCl concentration 0.1 N (Decinormal) 1 N (Normal) Saturated
E0 calomel 0.334 volts 0.2800 volts 0.2422 volts
(e) Advantages of Calomel electrode:
1) It is handy, compact and easy to transport.
2) It has constant reproducible potential.
3) It is easy to construct and maintain.
4) It has very small temperature coefficient of potential.
(f) Demerits of Calomel electrode:
1) It should not be used above 800 C, as mercurous chloride (Hg2Cl2) starts decomposing into
mercury and mercuric chloride.
2) It involves handling of poisonous mercury (Hg) and mercurous chloride (Hg2Cl2).
Q. What are reference electrodes? Give the electrode potentials of saturated, normal and
decinormal calomel electrodes as 298 K.
Ans.:
(a) Definition : An electrode which has stable and reproducible electrode potential, is known
as reference electrode.
OR
An electrode which has stable and reproducible potential and complete the cell, acting as
half cell, is called as reference electrode.
(b) The electrode potentials of saturated, normal and decinormal calomel electrodes as 298 K are
as follows:
2
, Instrumental Methods of Analysis
KCl concentration 0.1 N (Decinormal) 1 N (Normal) Saturated
E0 calomel 0.334 volts 0.2800 volts 0.2422 volts
Q. Give representation of calomel electrode. Give the electrode reactions, when the calomel
electrode acts as cathode.
Ans.:
(a) Representation of Calomel electrode: Calomel electrode is represented as,
Hg | Hg2Cl2(s)| KCl (xM)
OR
Hg | Hg2Cl2(s)| Clsat .
(b) Reaction at calomel electrode:
If the calomel electrode act as cathode, then reaction is:
Hg 2Cl2 2e 2 Hg 2Cl
Q. Give reasons, why a calomel electrode cannot be used above 80 °C.
Ans.: Calomel electrode cannot be used above 80 °C because at such a high temperature mercurous
chloride (Hg2Cl2) disproportionate into mercury and mercuric chloride (HgCl2).
4) Indicator Electrode:
a) Definition : “The electrode of a cell in which the potential depends on the concentration of a
particular ion is called as an indicator electrode.”
b) Examples: Glass electrode and ion selective electrodes (ISE) are an examples of indicator
electrode.
Q. Give principle and construction of glass electrode with labelled diagram and its
representation.
Ans. : The glass electrode is an example of indicator electrode whose potential changes depending
on the concentration of the analyte.
a) Principle of Glass electrode: When two solutions of different [H+] are separated by a thin glass
membrane, a potential difference (P.D.) is developed at the two surfaces of membrane. The
potential difference (P.D.) developed is proportional to the difference in [H +] of the two
solutions.
b) The glass membrane acts as ion exchanger i.e. exchanger of Na+ of glass with H+ of solution.
c) Electrode representation of glass electrode:
Ag, AgCl|0.1M HCl | H+ (Test solution)
OR
Pt, 0.1 M HCl | Glass
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