Electrode System Notes - Final
Electrode System Notes - Final
Electrode System Notes - Final
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Electrochemistry is a branch of physical chemistry which delas with (a) properties and
behaviors of electrolytes (b) interconversion of electrical and chemical energy. A system
which converts chemical energy into electrical energy is known as electrochemical cell. An
electrochemical cell is an arrangement consisting of two electrodes, and an electrolyte
connected externally by means of electric wire. Eg : Daniel Cell. Here zinc is anode where
oxidation reaction take place and copper is cathode where reduction take place.
A potential difference exists between anode and cathode. This is known as EMF of the
cell. It is given by Ecell = Ecathode − Eanode ( Considering reduction potentials)
Single electrode and Single electrode potential
One half of a galvanic cell where either oxidation half- reaction or the reduction-
reaction of the cell occurs is called a single electrode. A single electrode is an arrangement
where a metal electrode is immersed in a solution containing its own ions. A single electrode
develops a definite electric potential due to the spontaneous oxidation or reduction half-
reaction occurring at it. It is called single electrode potential
Potential developed at the interface of an electrode and electrolyte, when metal is
immersed in a solution containing its own ions (ions reversible to itself) is known as single
electrode potential. If measurements are done under standard condition of temperature (298 K)
pressure ( 1atm) and unit concentration of electrolyte is known as standard electrode potential.
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Single electrode potential can be calculated using Nernst equation. Nernst equation is
an expression of a quantitative relationship between electrode potential/cell potential and
concentration of the electrolyte species in an electro-chemical reaction.
E= Electrode potential
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1) Primary reference electrode: Standard hydrogen electrode (SHE) is the only primary
reference electrode. The potential of all other electrodes is measured with respect to the
hydrogen electrode. For all practical purpose, electrode potential of standard hydrogen
electrode is fixed as zero.
2) Secondary reference electrode: Due to the difficulties involved in the use of SHE as a
reference electrode, some other electrodes of constant electrode potential are referred
to as secondary reference electrodes. E.g., calomel electrode.
Calomel electrode :
Calomel electrode is an example for metal-metal insoluble salt electrode. Here mercury
is in contact with its sparingly soluble salt mercurous chloride. It is immersed in a solution
containing chloride ions ( KCl solution). The concentration of KCl solution used is either
decinormal, normal or saturated. Correspondingly, the electrode is known as decinormal,
normal or saturated calomel electrode respectively.
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The calomel electrode can act as anode or cathode depending on the nature of another
electrode with which it is contact with
When it acts anode, the electrode reaction is
2Hg (l) → Hg22+ (aq) + 2e-
Hg22+(aq) + 2Cl- (aq)→Hg2Cl2
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2Hg(l) + 2Cl- (aq) → Hg2Cl2 (s)+ 2e-
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When it acts as cathode, the electrode reaction is,
Hg22+(aq) + 2e-→ 2Hg(l)
Hg2Cl2 (s) → Hg22+ (aq) + 2Cl-
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Hg2Cl2(s) + 2e- → 2Hg (l) + 2 Cl-(aq)
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Applications:
(i) To determine the electrode potential of the unknown electrode
The test electrode, Zn(s) /Zn2+(aq) is coupled with a saturated calomel electrode.
Zn(s) / Zn2+ (aq) ││ Cl- (saturated soln) /Hg2Cl2(s) / Hg (l)
The EMF of the so formed cell is determined experimentally by potentiometric method.
Then
E cell = E cathode – Eanode = 0.2444 – Ezn
Ezn = 0.2444 – E cell
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The galvanic cell in which t h e electrode and electrolytes present in both half cells
are the same but only the concentration of the electrolyte is different iscalled the electrolyte
concentration cell.
At Cathode:𝑍𝑛2+ (𝐶2 ) + 2𝑒 − → 𝑍𝑛
0.0591 0.0591
= [𝐸 0 + 𝑙𝑜𝑔(𝐶2 )] − [𝐸 0 + 𝑙𝑜𝑔(𝐶1 )]
𝑛 𝑛
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Where E0 is the standard electrode potential of the species in the concentration cell.
0.0591 𝐶2
𝐸𝑐𝑒𝑙𝑙 = 𝑙𝑜𝑔 ( )
𝑛 𝐶1
Note: Concentration cell will have positive EMF, only when C2 > C1
Numerical problems
1. A concentration cell was constructed by immersing two silver electrodes in 0.02M and
2M AgNO3 solutions. Write the cell representation, cell reactions, and calculate the emf
of the cell at 250C.
(i)Cell representation:
0.0591 𝐶2
𝐸𝑐𝑒𝑙𝑙 = 𝑙𝑜𝑔
𝑛 𝐶1
0.0591 2
𝐸𝑐𝑒𝑙𝑙 = 𝑙𝑜𝑔
1 0.02
𝐸𝑐𝑒𝑙𝑙 = 0.0591 × 2
𝐸𝑐𝑒𝑙𝑙 = 0.1182𝑉
2. Represent the cell formed by the coupling of two Cu electrodes immersed in CuSO4
solutions. The concentration of cupric ions in one electrode system is 100 times more
concentrated than other. Write the cell reaction and calculate the potential at 300K.
(i)Cell representation:
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2.303𝑅𝑇 𝐶2
𝐸𝑐𝑒𝑙𝑙 = 𝑙𝑜𝑔
𝑛𝐹 𝐶1
𝐸𝑐𝑒𝑙𝑙 = 0.0297 × 2
𝐸𝑐𝑒𝑙𝑙 = 0.0595𝑉
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Glass electrode consists of a thin glass membrane, typically about 0.03 to 0.1 mm thick,
sealed onto one end of a heavy–walled glass tube. A special variety of glass (corning 0l5 glass
with approximate composition 20 % Na2O, 6 % CaO & 72 % SiO2) is used which has a low
melting point and high electrical conductance.
The glass bulb is filled with a solution of constant pH (0.1 M HCl). A small volume of
saturated silver chloride is contained in the tube. A silver wire in this solution forms a
silver/silver chloride reference electrode which is connected to one of the terminals of a
potential measuring device. The internal reference electrode is a part of the glass electrode and
it is not the pH sensing element. Only the potential that occurs between the outer surface of the
glass bulb and the test solution responds to pH changes.
Glass electrode can be represented as follows:
Ag/AgCl, HCl ( 0.1M) // Glass (x=?)
Working:
A silicate glass used for membranes consists of an infinite 3D- network of SiO44- groups
in which each silicon is bonded to four oxygen and each oxygen is shared by two silicons.
There are sufficient cations to balance the negative charge of the silicate groups within the
interstices of this structure. Singly charged cations such as sodium and lithium are mobile in
the lattice and are responsible for electrical conduction within the membrane. The glass is a
partially hydrated aluminosilicate containing sodium or calcium ions.
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(1) The boundary potential Eb, : It varies with the pH of the analyte solution. It is made up
of two potentials, E1 & E2 which develop at the two surface of the glass membrane i.e. the
potential developed at the inner glass surface and the potential developed at the outer glass
surface.
Eb = E1−E2 (1)
Where Eb is the boundary potential
E1 = potential developed at the interface between the exterior of the glass and the
analyte solution
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E2 = Potential developed at the interface between the internal solution and the interior
of the glass. The boundary potential is related to the concentration of hydrogen ion in
each of the solutions by the Nernst-like equation.
Eb = E1 – E2 = 0.0592 log Cl / C2 (2)
Where C1 = concentration of the unknown solution
C2 = concentration of the internal / known solution
For a glass pH electrode the hydrogen ion concentration of the internal solution is held
constant.
To measure the hydrogen ion concentration of the test solution, the glass electrode
(indicator electrode) must be combined with an external reference electrode, which is
required for all kinds of ion-selective electrode determinations.
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Set up of indicator electrode and reference electrode for the measurement of pH.
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The standard glass electrode (EoG) potential can be determined by dipping the glass
electrode in a solution of known pH.
(i) Glass electrode must be immersed in distilled water, when it is not used
(ii) Standard glass electrode potential need to be determined frequently
(iii) Can not used in highly acidic and alkaline solution
(iv) Commercial versions are moderately expensive