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Chapter 7 Electrochemistry
§7.8 Electrode potential
Daniell cell
How does electrode potential establish?
Zinc metal
Copper metal
ZnSO4 solution
CuSO4 solution
Zn2+
Zn2+
Zn2+
Zn2+
Cu2+
Cu2+
Cu2+
Cu2+
Porous partition
Diagram
electromotive forces
potential difference
1) Metal-metal interface: contact potential + + + + + +
Cu
Zn
7.8.1. Interfacial charge and electrode potential
KCl solution HCl solution
2) Liquid-liquid interface: Liquid junction is the interface between two miscible electrolyte solutions.
liquid junction potential, liquid potential, diffusion potential
3) Liquid-metal:
Cu2+ + 2e Cu
+
+
+
+
+
+
+
+
+
+
++
+
+
+
+ +
+
exchange current, electrode potential
7.8.2. Models of electric double layer
1) Holmholtz double layer (1853)
Compact double layer
++++++++
0
d
E
2) Gouy-Chappman layer (1910, 1913)
Diffuse double layer
++++++++
0d
E
3) Stern double layer (1924)
++++++++
0
d
E
7.8.3 Electromotive forces and relative electrode potential
Cu(s)Zn(s)ZnSO4(m1)CuSO4(m2)Cu (s)
E = c + + j + +
anode cathode
E = c + (l,1- ) + ( +- l,2)
= +- + (c+ l,1- l,2) When emf of a cell was measured, we , in fact, measured t
he potential difference between the two electrodes.
E
Can the absolute potential of electrode be unmeasured?
Absolute potential
lm
Only the difference between two electrodes, i.e.,
electromotive of the cell E = + can be measured.
potentiometerarbitrary reference
(2) Normal/Standard Hydrogen Electrode (NHE/SHE)
In 1953, IUPAC defined normal hydrogen electrode (NHE) as the reference for measurement of electrode potential. IUPAC conventions
acidic solution with activity of H+ equals to 1.
platinized platinum foil electrode
pure hydrogen gas at standard pressure
1
H H H1.0mol kg , 1, 1m a
definition
H+/H2 = 0.000000 V.
(3) standard electrode potential
The potential of other electrode can be obtained by
combination of NHE and any other unknown electrode
into an electrochemical cell with NHE serving as
negative electrode and the unknown electrode as
positive electrode:
- NHE || unknown electrode +
The sign and the value of the emf of the cell is thus the sign and value of the potential of the unknown electrode.
All standard electrode potentials are reduction potentials.
Cf. Levine, p. 431-435
NHE Cu2+ (a=0.1)Cu
E = 0.342 V
the electrode potential of the Cu|Cu2+ (a=0.1) electrode at
pressure p and temperature T is thus
2Cu / Cu0.342V
Example
Because the unknown electrode is always arranged as positive electrode, the electrode reaction is, therefore, written in reduction form.
(reduction)(standard) electrode potential
Cu2+ + 2e Cu
SHE||Cu2+ (a=0.1)|Cu
Cell reaction: H2(g, p) + Cu2+(a) = 2H+ (a=1) + Cu
22
2CuH
H Cu
lna aRT
E EnF a a
y
(4) Nernst equation for electrode
2+ + 2+2+ +2 Cu /Cu H /H2
Cu /Cu H /H Culn
RTa
nF y y
2+ 2+2+Cu /CuCu /Cu Cu
lnRT
anF
y
ox
red
lnaRT
nF a y red
ox
lnaRT
nF a y
7.8.4 Reference electrode
Problems with NHE (primary standard):
1)The platinized platinum electrode is easily poisoned by adsorption of impurities from the solution and the gas.
2) An elaborate purification is required to purify the hydrogen before it is passed through the cell.
3) Changes in barometric pressure or in the depth of immersion of the electrode in the solution produce a small variation in the potential of the electrode.
4) The preparation and the maintenance of the unit activity solution are both much complicated.
The calomel electrode:
Hg(l)Hg2Cl2(s)KCl (m)
Some electrodes with stable potential usually used as the secondary standard, named as reference electrode
saturated calomel electrode (SCE)
(T)/V = 0.2412 - 6.61 10-4 (T/ -25) - 1.7 ℃
10-6 (T/ -25)℃ 2 - 9 10-10 (T/ -25)℃ 3
HgHg2Cl2
past
mercury-mercurous sulfate electrode:
Hg(l)Hg2SO4(s)SO42(m) ⊖ = +0.640
Vmercury-mercuric oxide electrode: Hg(l)HgO(s)OH(m) ⊖ = +0.098 V
silver-silver chloride electrode: Ag(s)AgCl(s)Cl(m) ⊖ = 0.197 V
⊖(Ag+/Ag) = +0.799 V vs NHE
⊖(Ag+/Ag) = __?___ V vs SCE
Other common reference electrodes
Ag+/Ag0.799 V
0.2412 V
0.000 V
SCE
NHE
7.8.5. liquid junction potential and salt bridge
1) liquid junction potential
The diffusion of ions is irreversible, which destroys the reversibility of the cell.
The value of Ej can reach ca. 30 mV, which is too large fo
r the measurement of emf.
2) influential factor of Ej
Pt(s), H2(g,p)HCl(m)HCl(m′)H2(g, p), Pt(s)
Ej
On passage of 1 mole of electrons through the cell, t+ mol H+ and t mol Cl pass the boundary
-+
+ -
ClH
H Cl
''ln lnj j
aaG t RT t RT nFE
a a
For uni-univalence electrolytes
'ln)12(
'ln)(
m
m
F
RTt
m
m
F
RTttE j
3) Effects of salt bridge
Every measurement of emf of a cell whose two electrodes require different electrolyte raises the problem of the liquid junction potential.
The problem can be solved either by measuring the junction potential or eliminating it. The salt bridge is often used to connect the two electrode compartments to reduce the junction potential.
4) Salt bridge electrolyte1) does not react with either solution2) transference number of cation and anion is close3) of high concentration.
ions K+ NH4+ Cl NO3
102 /S·m2·mol-1 0.7352 0.734 0.7634 0.7144
t+ of some common salt bridge electrolytes
c/mol·dm-3 0.01 0.05 0.10 0.20
KCl 0.490 0.490 0.490 0.489
NH4Cl 0.491 0.491 0.491 0.491
KNO3 0.508 0.509 0.510 0.512
m
c/mol·dm-3 0 0.2 1.0 2.5 3.5
Ej 28.2 19.95 8.4 3.4 1.1
Concentration-dependence of Ej
Why does salt bridge reduce the junction potential.
+ + + + +- - - - -
5) Effects of salt bridge:
6) Elimination of junction potential