Electrochemistry slides from Thapar University, Punjab, India.

1. Electrochemistry
Migration of ions, Transference number, Determination
of Transference number by Hittorf`s method,
Conductometric titrations, Types of electrode: Calomel
and glass electrode, Liquid junction potential,
Potentiometric Titrations.

2. Electrochemistry
 In ionic or electrolytic conduction ionic motion transports the electrons.
 Positively charged ions, cations, move toward the negative electrode, cathode
 reduction
 Negatively charged ions, anions, move toward the positive electrode, anode
 oxidation
Electrical Conduction
Electrolyte & Electrolysis
 Electrolytes are electrovalent substances that form ions (Cation & Anion) in
solution which conduct an electric current.
 The Phenomenon of decomposition of an electrolyte by passing electric current
through its solution is known as Electrolysis.
 Ex. Electrolysis of Hydrochloric acid (HCL)
Electrolysis of Sodium Chloride (NaCl)
 Electrolytic Cell

3.  Electrolytic Cell
The reciprocal of solution resistance (1/R) is called Conductance, G.
It is expressed in the unit called reciprocal ohm (ohm-1 or Ω-1)
In SI system, the unit of conductance is Siemen, S
Where, A = surface area of each electrode
l = distance btwn electrode
 = conductivity
l
A
R
G


1
Conductance (G):

4. The resistance of any conductor varies directly as its length (l) and inversely as its
cross-sectional area (a), i.e.
R  l/a
or R =  (l/a)
• Where  is a constant depending upon the nature of the material and is called
specific resistance of the material.
• If, l =1cm and a =1cm2, then  = R
• Specific resistance is defined as the resistance in ohms of a specimen 1cm in
length and 1cm2 cross-section (1cm3 of the material).
• The reciprocal of specific resistance, i.e., 1/ is called specific conductance.
Specific Conductance

5. Specific Conductance (ĸ ):
From equation of specific resistance
 = (a/l)
ĸ = 1/  = (l/a) (1/ R)
= (l/a) x (conductance)
Since, conductance is measured in Ω-1, length in cm and area in cm2,
hence,
ĸ = Ω-1 x (cm/cm2) = Ω-1 cm-1 - units of specific conductance
In SI system, the units of specific conductance are Sm-1 where S
stands for Siemen.
Equivalent Conductance (Λ):
It is defined as the conducting power of all the ions produced by one
gram equivalent of an electrolyte in a given solution.

6. Imagine 1cc of a solution of an electrolyte placed between two large
electrodes 1cm apart. The cross-sectional area of the solution will be
1cm2.
The conductance of the solution will evidently be its specific
conductance because we are having one cm cube of solution.
Further, If 1cc of the solution contains 1gm of equivalent of
electrolyte dissolved in it.
Then according to the definition, the conductance of the solution
will be equal to the equivalent conductance (Λ)
i.e., Conductance (G) = Specific conductance (k)
= Equivalent conductance (Λ)
Relation Between Specific Conductance and Equivalent Conductance

7. Molar Conductance (Λm ):
Conducting power of all the ions produced by one mole of the electrolyte in a
given solution
Molar conductance is related to specific conductance by the relation
Λm = k / c
Where c = concentration of the solution in moles/m3
Since units of k is Sm-1 and those of c is mol.m3, the units of Λm are Sm2 mol-1

8. Dilution   Molar conductance  degree of
dissociation of the electrolyte .
Degree of dissociation is defined as the fraction of
the total electrolyte in solution which exists in the
form of its ions.
On dilution, the same amount of electrolyte is
capable of furnishing a larger number of ions
However, the increase in number of ions on
dilution is much lesser than increase in the
volume of the solution
Variation of Molar Conductance with dilution
Therefore, the number of ions per unit volume (per cc) actually decreases.
Hence, the specific conductance decreases although with molar conductance
increases on progressive dilution.

9. • Ionic Mobility
• Although, at infinite dilution, all electrolytes are completely dissociated,
their molar conductances differ vastly from one another
• This is because of differences in speeds of the ions.
• Ex. The molar conductance at infinite dilution of HCl is more than three
times as high as that of NaCl. Since chloride ion is common, it follows
that the speed of hydrogen ion is more than three times of the speed of
sodium ion.
• Speed of an ion varies with the potential applied.
• Ionic mobility is defined as the distance travelled by an ion per second
under potential gradient of 1 volt per meter
• Potential gradient is given by the potential difference applied at the
electrodes divided by the distance between the electrodes

10. • Ionic Mobility
• The ionic mobility is extremely small as compared to the speed of
gaseous molecules which is about 102m s-1. The low mobility of ions is
due to the fact that there are frequent collisions between the ions and
the solvent molecules since the mean free path of molecules in the
liquid is very small.
• The ionic mobility of H+ ion is found to be five to ten times that of
other ions, except OH- ion
• The hydrogen ion, because of its small size and high charge density, is
heavily hydrated
• H+ ion in aqueous solutions is hydrated to form H2O4
+ ion, i.e., a
trihydrate of hydronium ion, viz., H3O+. 3H2O, having the following
structure
O H
H
H
O
H
H
O H
H
O
H
H
+

11. • Ionic Mobility
• The structure because of its large size and shape, should predict the
mobility of H+ ion to be low rather than high
• The high mobility in hydroxylic solvents such as water can be explained
by Grotthus type mechanism in which the proton moves rapidly from
H3O+ to a hydrogen bonded water molecule and is transferred further
along a series of hydrogen bonded water molecules by a
rearrangement of hydrogen bonds.
• This accounts to high mobility of hydrogen ions in water.
H
O
H
H
O
H
H
H
O
H
H
O
H O H
H
O
H
H
+
+
Grotthus-type
Mechanism for
High mobility of
H+ ions

12. • Ionic Mobility
• Grotthus model also explains as to why H+ ions move about 50 times
more rapidly through ice than through liquid water
• Ice has tetrahedral structure with each oxygen atom surrounded by
four oxygen atoms
H
O
H
O
H
HH
O
H H
O
H
H
O
H
1
A
2
3
4
Covalent bond
Hydrogen bond
The central oxygen atom A is
surrounded tetrahedrally by the
atoms marked 1,2,3 and 4

13. • Each hydrogen atom lies on the line
joining the centres of the oxygen
atoms.
• When water molecules are oriented
properly, as in the case of ice, the
hydrogen ions can move rapidly
through its tetrahedral structure
H
O
H
O
H
HH
O
H H
O
H
H
O
H
1
A
2
3
4
 Lithium and sodium ions have comparatively lower ionic mobilities
 This is due to the higher charge density around these ions because of
their small radii
 The higher density causes these ions to be more highly hydrated by ion-
dipole interactions than the larger ions
 Since hydrated ions has to drag along a shell of water as it moves
through the solution, its mobility is naturally less than that of an
unhydrated ion

15. First Law: Amount of any substance that is deposited or liberated at an electrode
is directly proportional to the quantity of electricity passed through
the electrolyte
m = ZQ
Where m = mass in grams of the substance deposited
Q = quantity of electricity (in coulombs) that flows
Z = constant known as electrochemical equivalent of the
substance deposited
if Q = 1 coulomb, then Z = m
Thus, electrochemical equivalent of a substance is defined as the mass of
the substance deposited or liberated by the passage of 1 coulomb of
electricity
The quantity of electricity (Q) that flows in an given time (t) -
Q = it
Where ‘i’ is the current strength in amperes
Therefore, m=ZQ can be m=Zit
Faraday’s law of electrolysis

16. Second Law:
When the same quantity of electricity flows through different
electrolytes, the amount of different substances produced at the
electrodes are directly proportional to their equivalent masses, i.e.
m1/m2 = E1/E2
Where E1 and E2 are equivalent masses
Ohm’s Law
Metallic as well as electrolytic conductors obey Ohm’s law which
states that:
The strength of current (i) flowing through a conductor is directly
proportional to the potential difference (V) applied across the
conductor and is inversely proportional to the resistance (R ) of the
conductor
i = V / R
The current strength is measured in amperes; electrical resistance
is measured in ohms (Ω) and potential difference in volts.

17. Imagine 1cc of a solution of an electrolyte placed between two large
electrodes 1cm apart. The cross-sectional area of the solution will be
1cm2.
The conductance of the solution will evidently be its specific
conductance because we are having one cm cube of solution.
Further, If 1cc of the solution contains 1gm of equivalent of
electrolyte dissolved in it.
Then according to the definition, the conductance of the solution
will be equal to the equivalent conductance (Λ)
i.e., Conductance (G) = Specific conductance (k)
= Equivalent conductance (Λ)
Relation Between Specific Conductance and Equivalent Conductance

18. • Molar Conductance
• Conducting power of all the ions produced by one mole of the
electrolyte in a given solution
• It is denoted as Λ m
• Molar conductance is related to specific conductance by the relation = Λ
m = ĸ / c
• Where c is the concentration of the solution in moles/m3
• Since units of ĸ is Sm-1 and those of c is mol.m3, the units of Λ m are
Sm2mol-1

19. • Variation of Molar Conductance with dilution
• Molar conductance of an electrolyte increases with increse in dilution
• This may be attributed to increase in the degree of dissociation of the
electrolyte.
• Degree of dissociation is defined as the fraction of the total electrolyte
in solution which exists in the form of its ions
• On dilution, the same amount of electrolyte is capable of furnishing a
larger number of ions
• However, the increase in number of ions on dilution is much lesser than
increase in the volume of the solution
• Therefore, the number of ions per unit volume (per cc) actually
decreases
• Hence, the specific conductance decreases although with molar
conductance increases on progressive dilution.

20. • Ionic Mobility
• Although, at infinite dilution, all electrolytes are completely dissociated,
their molar conductances differ vastly from one another
• This is because of differences in speeds of the ions.
• Ex. The molar conductance at infinite dilution of HCl is more than three
times as high as that of NaCl. Since chloride ion is common, it follows
that the speed of hydrogen ion is more than three times of the speed of
sodium ion.
• Speed of an ion varies with the potential applied.
• Ionic mobility is defined as the distance travelled by an ion per second
under potential gradient of 1 volt per meter
• Potential gradient is given by the potential difference applied at the
electrodes divided by the distance between the electrodes

21. • Ionic Mobility
• The ionic mobility is extremely small as compared to the speed of
gaseous molecules which is about 102m s-1. The low mobility of ions is
due to the fact that there are frequent collisions between the ions and
the solvent molecules since the mean free path of molecules in the
liquid is very small.
• The ionic mobility of H+ ion is found to be five to ten times that of
other ions, except OH- ion
• The hydrogen ion, because of its small size and high charge density, is
heavily hydrated
• H+ ion in aqueous solutions is hydrated to form H2O4
+ ion, i.e., a
trihydrate of hydronium ion, viz., H3O+. 3H2O, having the following
structure
O H
H
H
O
H
H
O H
H
O
H
H
+

22. • Ionic Mobility
• The structure because of its large size and shape, should predict the
mobility of H+ ion to be low rather than high
• The high mobility in hydroxylic solvents such as water can be explained
by Grotthus type mechanism in which the proton moves rapidly from
H3O+ to a hydrogen bonded water molecule and is transferred further
along a series of hydrogen bonded water molecules by a
rearrangement of hydrogen bonds.
• This accounts to high mobility of hydrogen ions in water.
H
O
H
H
O
H
H
H
O
H
H
O
H O H
H
O
H
H
+
+
Grotthus-type
Mechanism for
High mobility of
H+ ions

23. • Ionic Mobility
• Grotthus model also explains as to why H+ ions move about 50 times
more rapidly through ice than through liquid water
• Ice has tetrahedral structure with each oxygen atom surrounded by
four oxygen atoms
H
O
H
O
H
HH
O
H H
O
H
H
O
H
1
A
2
3
4
Covalent bond
Hydrogen bond
The central oxygen atom A is
surrounded tetrahedrally by the
atoms marked 1,2,3 and 4

24. • Each hydrogen atom lies on the line
joining the centres of the oxygen
atoms.
• When water molecules are oriented
properly, as in the case of ice, the
hydrogen ions can move rapidly
through its tetrahedral structure
H
O
H
O
H
HH
O
H H
O
H
H
O
H
1
A
2
3
4
 Lithium and sodium ions have comparatively lower ionic mobilities
 This is due to the higher charge density around these ions because of
their small radii
 The higher density causes these ions to be more highly hydrated by ion-
dipole interactions than the larger ions
 Since hydrated ions has to drag along a shell of water as it moves
through the solution, its mobility is naturally less than that of an
unhydrated ion

25. • Ionic mobilities increase with temperature, the temperature
coefficients being very nearly the same for all the ions in a given
solvent.
• Thus, ionic mobilities increase by about 2% per degree in the vicinity
of 250C

26. • Migration of ions
• The migration of ions with different speeds can be demonstrated by a
simple experiment devised by Noyes
 The U tube is filled with 3/4th with
dilute solution of agar-agar containing
KCl and a little phenolphthalein.
 A drop of dilute solution of NaOH is
added to the right limb and drop of
HCl to the left limb.
 As a result, the soln. of agar-agar is
coloured pink in the right limb and
remains colourless in the left limb
Solutioncolourless
SolutionPink
Agar-Agar + KCl soln.
Phenolphthalein
HCl NaOH
c c
A

27. • The agar-agar solution is then
allowed to set into a jelly by placing U
tube in an ice-bath for few minutes A
small amount of powdered charcoal
marked CC is scattered on the surface
of the solution on each side to mark
the position of the boundary.
 A solution of HCl and CuCl2 is poured
gently over the charcoal boundary on
the right side and a solution of NaOH
is poured over the charcoal boundary
on the left side.
Solutioncolourless
SolutionPink
HCl +
CuCl2
c c
NaOH
B

28. • Current is allowed to flow by inserting a
metal electrode in each limb, the anode
on the right limb and cathode on the
left.
• As a result, the hydrogen and cupric ions
move towards left limb while hydroxyl
ions move towards the right limb
• The movement of hydrogen ions is
indicated by the disappearance of the
pink colour and that of the cupric ions by
the development of blue colour in the
solution on the right side.
• The movement of the hydroxyl ions is
indicated by the appearance of pink
colour of soln. on the left
colourless
Pink
c c
- +
OH-
H+ Cu2+
Pink
Blue
colourless
C
Experimental results show that if in a
given time, the hydrogen ions move
through a distance of 5 cm, the Cu2+
ions move through a distance of
0.91cm while OH- ions move through
a distance of 2.83cm

29. • Discharge of ions on electrolysis. Hittorf’s Theoretical Device
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
II
I
III
IV
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
ACDB Cathodic
compartment
central
compartment
Anodic
compartment
cathode anode
E

30. • Discharge of ions on electrolysis. Hittorf’s Theoretical Device
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
II
I
III
IV
+ + + + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + + + +
_ _ _ _ _ _
+ + + + + +
_ _ _ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + +
_ _ _ _ _ _ _ _ _ _ _
+ + + + + + + + + +
_ _ _ _ _
ACDB Cathodic
compartment
central
compartment
Anodic
compartment
cathode anode
E
• Although most of the ions differ largely in their mobility, their equal
numbers are discharged, on electrolysis at appropriate electrodes
• The anomaly is explained by Hittorf’s theoritical device (E) containing
equal number of +ve and –ve ions of same valency

31. • The two metal electrodes A & B
represent anode and cathode
• The vessel is divided into three
compartments by putting imaginary
partitions C and D which are permeable
to ions
• The three imaginary compartments AC, CD, DB are termed anodic,
central and cathodic compartments
• Before electrolysis, the position of the solution is represented as at I in
the figure
• On applying potential, only 2 cations are capable of moving and in a
given time, two of the cations move towards the cathode. The condition
is represented in the figure at II. There will be two unpaired cations in
the cathodic compartment. There will also be two unpaired anions left in
anodic compartment. As unpaired ions always gets discharged at the
representative electrodes (by gain or loss), two cations will be discharged
at the cathode and 2 anions at anode, even through only cations were
able to move.

32. • In case both cations and anions move
and both have same speed, if 2 cations
move towards cathode, 2 anions will
move towards anode at the same time.
• i.e., 4 cations and 4 anions will be discharged at the respective electrodes
as shown in III.
• In case, cations and anions move with different speeds so that if in a
given time 2 cations move towards the cathode, 3 anions move towards
the anode, again the same number i.e., 5 unpaired cations and 5
unpaired anions will be discharged at the respective electrodes as shown
in IV.
• The concentration of the central compartment remains constant and this
can be realised in practice when the current strength is small and there
are no temperature variations

33. • Transport Number
• From Hittorf’s theoretical device, it is also evident that the number of
ions discharged at each electrode depends upon the sum of the speeds
or mobilities of two ions
• In first case when only two cations could move in a given time, the
number of ions discharged at each electrode was also two
• In last case, when two cations and 3 anions moved in a given time the
no. of ions discharged at each electrode was equal to five
• According to Faraday’s first law of electrolysis, the number of ions
discharged at a electrode is proportional to the total quantity of
electricity passed through the solution, hence it follows that:
• Total quantity of electricity that passes through the solution is
proportional to sum of the mobilities of the ions
• The quantity of electricity carried by a particular ions is
proportional to the mobility of that particular ion

34. • Transport Number
• The fraction of the total current carried by each ion is called its
Transport Number
• Thus, if u+ is the mobility of cation and u- that of the anion then
Transport number of the cation, t+ = current carried by the cation /
total current
= u+ / [ u+ + u- ]
Transport number of the anion, t- = u- / [ u+ + u- ]
Further since t+ + t- = 1
By Hittorf’s theoretical device, it is possible to show that
[mobility of cation / mobility of anion] =
fall of concentration round anode
fall of concentration round cathode

35. • Transport Number
• Ex. In the last case of IV in Hittorf’s device, 2 cations moved towards the
cathode while 3 anions moved towards the anode in the same time.
• Thus mobility of cations/mobility of anions is 2/3.
• As given in Hittrof’s device, the concentration round anode has fallen by
2 (from 8 to 6 ions) while that around cathode has fallen by three (8 to 5
ions)
• Thus, the fall of concentration round anode/fall of concentration round
cathode is also 2/3

36. • Transport Number
• Transport numbers of the 3 alkali ions, viz., Li+, Na+ and K+ ions in their
chlorides increase in the order of their mention.
• Since chloride ion is common in each case, it means that the speed of
the cations increases in the order of Li+, Na+ and K+
• Lithium, the smallest of these ions should have the highest speed and
potassium, the largest should have the lowest speed
• Bare Li ion has high charge density because of its small size and
therefore it attracts a large number of water dipoles
• Degree of hydration is maximum in the case of Li+ ion and minimum in
the case of K+ ion, The degree of hydration of Na+ ion lies in between.
• As a result of this the highly hydrated Li ion in solution is bigger in size
than the hydrated Na ion and the hydrated K ion.
• Speed of K ions in aqueous solution is the maximum and that of lithium
ion is the minimum

37. • Conductometric titrations
• Conductance measurements are frequently employed to find the end
points of acid-alkali and other titrations
• The principle involved i.e., electrical conductance depends upon the
number and mobility of ions
• Ex. Titration of strong acid (HCl) with a strong base (NaOH) with acid in
conducting vessel and alkali in burette – the conductance is HCl is due to
the presence of hydrogen and chloride ions
• H+ (aq) + Cl- (aq) + [Na+ (aq) + OH- (aq)]  Na+ (aq) + Cl- (aq) + H2O (liq)
X
Volume of alkali added
Conductance
End point
• On plotting conductance against
volume of alkali added, the point
of intersection X of these two
lines gives the volume of alkali
required for the neutralization

38. • Conductometric titrations
• If a weak acid (such as acetic acid) is titrated against a strong alkali (such
as NaOH), the conductance of acid will be low on account of poor
dissociation.
• On adding alkali, highly ionized sodium acetate is formed and hence the
conductance begins to increase
• CH3COOH (aq) + [Na+ (aq) + OH- (aq)]  Na+ (aq) + CH3COO- (aq) + H2O (l)
• When the acid is completely neutralized, further addition of alkali
introduces excess of fast moving OH ions
X
Volume of alkali added
Conductance
End point
• The conductance of solution
begins to increase even more
sharply than earlier case.
• On plotting conductance against
volume of alkali added, the point
of intersection X of these two
lines gives the volume of alkali
required for the neutralization

39. • Conductometric titrations
• When a mixture of a strong and a weak acid is to be titrated against a
strong alkali a combination of earlier curves is obtained.
• If a mixture of HCl and CH3COOH is to be titrated against NaOH, HCl will
get titrated first, and the titration of acetic acid will commence only
after HCl has been completely neutralized
Conductance
• Point B corresponds to the
neutralization of HCl, the point C
corresponds to the
neutralization of CH3COOH
Volume of NaOH added
B C

40. • Conductometric titrations
• If a strong acid like HCl is titrated against a weak base, like NH4OH, the
conductance will fall at first due to replacement of fast moving H+ ions
by slow moving NH4
+ ions
• H+ (aq) + Cl- (aq) + [NH4OH (aq)]  NH4
+ (aq) + Cl- (aq) + H2O (l)
• After neutralization of the acid, further addition of weakly ionised
ammonium hydroxide will not cause any appreciable change in the
conductance
X
Volume of NH4OH added
Conductance
End point

41. • Precipitation titrations
• Titration of silver nitrate against potassium chloride
• Ag+ (aq) + NO3
- (aq) + [K+ + Cl-]  K+ (aq) + NO3
-(aq) + AgCl (s)
• Since the mobility of K ion in nearly the same as that of silver ion which
it replaces, the conductance will remain more or less constant and will
begin to increase only after the end point.
X
Volume of NH4OH added
Conductance
End point

42. • The change in the volume during the titration should be as small as
possible
• For this purpose, the titration solution in the burette should be five to
ten times stronger than solution taken in the conducting vessel
Advantages of Conductometric titrations
• Coloured solutions which cannot be titrated by ordinary volumetric
methods with help of indicators, can be successfully titrated
conductometrically
• The method can also employed in the case of very dilute solutions and
also for weak acids and bases
• No special care is necessary near the end point as it is determined
graphically

43. Types of Electrode
1. Metal-metal Ion Electrode:
2. Gas Electrodes:
a. H2 Electrodes
b. Cl2 Electrodes
c. O2 Electrodes
3. Metal-Insoluble Metal Salt Electrode (Calomel Electrode).
4. Oxidation-reduction Electrodes

44. It consists of mercury, solid mercurous chloride and a solution of
KCl.
Schematic representation:-
Hg, Hg2Cl2(s); KCl (solution)
3. Metal-Insoluble Metal Salt Electrode (Calomel Electrode).

45. If electrode reaction is reduction: Hg++ ions given by sparingly
soluble Hg2Cl2 get discharged at electrode. So more Hg2Cl2 would
pass into solution and the conc. of Cl- ion increases.
Hg++(aq) +2e- 2Hg (l)
Hg2Cl2 (s) Hg2
++ (aq) + 2Cl– (aq)
If electrode reaction is oxidation: then it liberate electrons and
send Hg++ ions in solution. The Hg++ ions combine with Cl- ions
(furnished by KCl) forming sparingly soluble Hg2Cl2. So conc. Of
Cl- ions decreases in solution.
2Hg(l) Hg2
++ (aq)+ 2e-
Hg2
++ (aq) + 2Cl- (aq) Hg2Cl2(s)
Electrode reaction is---
Hg2Cl2(s) + 2e- 2Hg (l) + 2Cl- (aq)
So, electrode is reversible with respect to Cl- ion

46. • The term oxidation-reduction electrode is used for those
electrodes in which the potential is developed due to the
presence of ions of same substance in two different oxidation
states.
• It is set up by inserting an unattackable metal (Platinum or
gold) in an appropriate solution.
When platinum wire is inserted in a solution containing Fe++ and Fe+++
ions or Sn++ and Sn++++ ions or Ce+++ and Ce++++ ions, the wire acquires
potential. The potential at electrode arises due to tendency of ions in one
oxidation state to change in to more stable oxidation state.
Electrode reactions:
Fe+++ (aq) + e- Fe++ (aq)
Sn++++(aq) + 2e- Sn++ (aq)
Ce++++ (aq) +e- Ce+++ (aq)
Oxidation Reduction electrode

47. • The function of Pt wire is to ‘pick up’ the electrons and to provide electrical
Contact to the electrode.
Quinhydrone electrode – Oxidation reduction electrode
•It consists of a platinum wire placed in a solution containing hydroquinone (QH2)
and quinone(Q) in equimolar amounts.
Electrode reaction:
Electrode representation:
Pt, Q, QH2; H+ (aq)
It is reversible with respect to H+ ions.
OO + 2e-
+ 2H+
Quinone
HO OH
Hydroquinone

48. • When two salt solutions of different concentration are placed in contact with one
another, the ions from the concentrated solution will tend to diffuse in to dilute
one.
• The rate of diffusion of each ion is approximately proportional to the speed of
the ion in electric field.
• Suppose positive ion moves with a greater speed than the negative ion. It means
that the positive ion from the conc. solution will diffuse ahead of the –ve ion in the
dilute solution.
• Thus dilute solution becomes positively changed with respect to concentrated
solution.
Liquid Junction Potential

49. • Suppose the negative ion moves faster. It means that negative ion will diffuse
rapidly in dilute solution than the positive ion and the dilute solution gets a
negative change.
• In both cases, an electrical double layer is set up at the junction of the two
solutions and thus a potential difference is setup at this junction.
• This potential difference developed at the junction of two solutions of
different concentration is termed as liquid junction potential (LJP).
(1) If two liquid one moving with the same speed, there will not be any liquid –liquid
junction potential.
(2) The liquid-liquid junction potential is due to the different migration velocities of
the two ions.
(3) Magnitude of liquid-liquid junction potential depends on the relative speed of the
ions.