Welc
ome
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Dr. Bassam
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Pharmaceutical
Analytical Chemistry II
(PA 202)
Level 2 PharmD
Dr. Bassam Shaaban
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CONTENTS
LECTURE (2)
➢
Factors Affecting Oxidation Potential
➢
Potentiometry principle
➢
Electrochemical Cells and Electrode Potential.
➢
Calculation of electrode potential
➢
Classification of electrodes
➢
Applications of Potentiometry
➢
Direct Potentiometry
➢
Indirect
potentiometry
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Standard Oxidation Potential (Eo)
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Measurement of the Electrode Potential
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Standard Oxidation Potential (Eo)
It is the e.m.f. produced when a half cell consisting of an
inert electrode (as platinum) dipped in a solution of equal
concentration of both the oxidized and reduced forms
(such as Fe3+ / Fe2+) is connected with a NHE
The higher E°, the stronger the oxidizing power of its oxidant
and the weaker the reducing power of its reduced form.
The most powerful oxidizing agents are those at the top
(higher +ve) and the most powerful reducing agents are at
the bottom (higher – ve).
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Standard Oxidation Potential (Eo)
If any two redox systems are combined, the stronger oxidizing
agent gains electrons from the stronger reducing agent with the
formation of weaker reducing and oxidizing agents.
Cl2/Cl- (E° = + 1.36 V) & Fe3+/Fe2+ (E°= + 0.77 V)
Cl2
+
2Fe2+
→
strong oxid. + strong red. →
agent
agent
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2Fe3+
+ 2Cl-
weak oxid. + weak red.
agent
agent
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Standard Oxidation Potential (Eo)
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Common
ion
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Complexing
agents
pH
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Precipiting
agents
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Factors Affecting Oxidation Potential
0.0591
E25 °C = Eo +
log [Oxid] /[Red]
n
➢The potential of MnO4− /Mn2+ varies with the ratio
[MnO4− ]/[Mn2+].
➢If ferrous is titrated with MnO4− in presence of Cl− ,
chloride will interfere by reaction with MnO4− and gives
higher results.
1. Common Ion
Zimmermann’s Reagent (MnSO4, H3PO4 and H2SO4)
➢ MnSO4 has a common ion (Mn2+) with the reductant that
lowers the potential of MnO4− /Mn2+ system:
➢ Phosphoric acid lowers the potential of Fe3+/Fe2+ system
by complexation with Fe3+ as [Fe(PO4)2]3− .
➢ Sulphuric acid is used for acidification.
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Factors Affecting Oxidation Potential
2. Effect of pH
The oxidation potential of an oxidizing agent containing
oxygen increases by increasing acidity and vice versa.
➢ Potassium permenganate:
MnO4− + 8H+ + 5e− → Mn2+ + 4H2O
E MnO4−/Mn2+ = E o +
0.0591
5
log
[MnO4−][H+]8
[Mn2+]
➢ Potassium dichromate:
Cr2O72− + 14H+ +6e− → 2Cr3+ + 7H2O
2−][H+]14
[Cr
O
2
7
0.0591
o
E Cr2O72−/Cr3+ = E +
log
[Cr3+]
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Factors Affecting Oxidation Potential
3. Effect of Complexing Agents
[I2]
0.0591
E I2/I− =Eo +
log − 2
Iodine: I2 + 2e− → 2I−
2
[I ]
E (I2/2I−) system increases by the addition of HgCl2 since it
complexes with iodide ions.
Hg2+ + 4I− → [HgI4]2−
(low dissociation complex)
Ferric: Fe3++e− → Fe2+
3+]
[Fe
0.0591
E Fe3+/Fe2+ =Eo +
log
1
[Fe2+]
• E (Fe3+/Fe2+) is reduced by the addition of F − or PO43− due to
the formation of the stable complexes [FeF 6]3− and
[Fe(PO4)2]3− respectively. Thus, ferric ions, in presence of F− or
PO43− cannot oxidize iodide although Eo(Fe3+/Fe2+) = 0.77
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o
−
while E (I2/2I ) = 0.54.
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Factors Affecting Oxidation Potential
4. Effect of Precipitating Agents
Ferricyanide:
[Fe(CN)6]3− + e− → [Fe(CN)6]4−
0.0591
o
E Ferri/Ferro = E +
1
log
[[Fe(CN)6]3−]
[[Fe(CN)6]4−]
Addition of Zn2+ salts which precipitates ferrocyanide:
[Fe(CN)6]4- + Zn2+ → Zn2 [Fe(CN)6]
The oxidation potential of ferri/ferrocyanide system in
presence of Zn2+ ions can oxidize iodide to iodine,
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although the oxidation potential
of I2/2I- system is higher.
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Potentiometry
• Potentiometry is a measured of analysis used in
the
determination
of
ions
or
substances
concentration by measuring the voltage (potential
difference) developed between two electrodes,
where electron transfer occurs at electrode surface
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• Electrons transfer from reductant to
oxidant, in the same solution (i.e electrons
transfer takes place in one phase) will
produce electromotive force (E.M.F) which
is known as oxidation potential.
• In some other cases transfer of electrons
takes place between element and its ions (i.e
electron transfer takes place between two
phases. In this case E.M.F. produced is
known as electrode potential.
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• Transfer of electrons between element and its ions depends
on the nature of element; this can be represented by the
following equilibrium.
Solution pressure
------------------------------------
M0
-----------------------Ionic pressure
Mn+ + ne
• According to the nature of metal, it may has:
The element has high solution pressure i.e. tendency to
loose electrons and converted to its ions e.g. Zno, Feo,
Coo, Nio.
The element has high ionic pressure i.e. tendency to accept
electrons and converted to elements e.g. Cuo, Hgo, Ago
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• Anodic half reaction (oxidation process) High
solution pressure E.M.F produced (electrode
potential) has a negative (-ve) signs
Zno
Zn 2+
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ZnSO4
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Cathodic half reaction (reduction process)
E.M.F produced (electrode potential) CuSO4
has a positive (+ve) sign
Cuo
Zn2+
CuSO4
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Electrochemical Cell
A) Galvanic cell
B) Electrolytic cell
The chemical energy is
converted into electrical
energy.
The electrical energy is
converted to chemical
energy.
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A) Galvanic cell
•
Where the chemical energy is converted to electrical
energy which can be supplied to an external circuit i.e. it
produce electrical energy
•
i) Voltaic cell (Danielle cell) Zno/Zn2+ // Cu2+ /cu0
•
•
The cupper electrode is the cathode.
The cathodic half reaction . the cathodic half reaction is :
Gain
Cu2+ + 2e
↔
Cuo(s)
electron
The zinc electrode is the anode
Loss
The anodic half reaction is Zno ↔ Zn2+ + 2e
electron
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N.B.
1. Cathode is electrode at which reduction
takes place while
2. anode is electrode at which oxidation
takes place.
In this case E cell = E Cuo – E Zno
3. In galvanic cell electrons flow from anode
to cathode.
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Salt bridge
• It is a liquid junction connect between the
two half cell without mixing.
• Salt bridge may be in the form of bond tube
or inverted U shape tube filled with agar gel
prepared in saturated KCl or KNO3
solution.
• Ions in salt bridge must not pass to the two
half cell, this can be achieved by blocking
the two ends of salt bridge with cotton wool
or gelatin or agar.
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Liquid junction potential
• Sometimes potential is developed between
the two boundaries of junction at the two
ends of salt bridge, this potential is known
as liquid junction potential.
• Liquid junction potential is produced due
to the difference in the rates of migration of
both cations and anions of the salt bridge
which leads to unequal distribution of
charges at the end of salt bridge thus
producing a potential.
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To reduce liquid junction potential we have
to:
1. Choose the electrolyte of salt bridge, that
its cations and anions have nearly the same
mobility so that they move by the same
rate, leading to equal distribution of charge
e.g KCl or KNO3 (K+ = 73.5, Cl- = 76.3.
NO3- = 71.5).
2.Use of high concentration of electrolyte in
salt bridge to reduce difference in rates of
migration of ions.
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B) Electrolytic cell
• In this type external EMF is applied which
is transformed to chemical energy.
• Daniel cell can be converted to electrolytic
cell if we apply potential from external
source apposing that of galvanic cell.
• Zn2+ + 2 e =
Zno
• Cuo
=
Cu2+ + 2e
i.e. Copper electrode become the anode and
zinc electrode become the cathode.
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See Lecture
Part (ii)