chapter 7 electrochemistry §7.10 application of emf and electrode potential
TRANSCRIPT
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Chapter 7 Electrochemistry
§7.10 Application of EMF and electrode potential
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I. N. Levine
pp. 431--443
14.7 Standard electrode potentials
14.8 Concentration cell
14.9 Liquid-junction potential
14.10 Applications of EMF measurements
14.12 ion-selective membrane electrodes
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7.10.1 Computation of emf
For cell with single solution:
Cd(s)|CdSO4(a±) |Hg2SO4(s)|Hg(l)
2 24SO Cd
2
ln ln
( ) ln
E
RT RTa a
nF nF
RTa
nF
y y
y y
Because a is a measurable quantity, E of the cell with
single electrolyte can be calculated exactly.
E
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For cell with two electrolytic solutions:
Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s)
2
2
Cu
Zn
lnaRT
E EnF a
y
1 ,1
2 ,2
lnmRT
E EnF m
y
we have to use mean activity coefficient () which is
measurable in stead of the activity coefficient of individual
ion (+ or -) which is unmeasurable.
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7.10.2. Judge the strength of the oxidizing and reducing agents
⊖ (Fe3+/Fe2+) = 0.771 V
⊖ (I2/I) = 0.5362 V
Oxidative form: Fe3+, I2
Reductive form: Fe2+, I-
The oxidative form with higher (standard) electrode
potential is stronger oxidizing species, while the reductive
form with lower (standard) electrode potential is stronger
reducing agent. Why? E > 0 criterion
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(Ox)1 + (Red)2 = (Red)1+ (Ox)2
7.10.3 Determination of the reaction direction
When concentration differs far from the standard concentration, should be used in stead of ⊖.
Stronger oxidizing species oxidizes stronger reducing species to produce weaker reducing and weaker oxidizing species.
⊖ (Fe3+/Fe2+) = 0.771 V; ⊖ (I2/I) = 0.5362 V
Fe3+ + I = Fe2+ + 1/2I2
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Application of Pourbaix diagram
Cu2+ Cu(OH)2
Cu
pH
/ V
2 4 6 8 10 12 140
CuO22
Cu2O0.0
0.5
1.0
1.5
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+Au 1e Au =1.7 Vy
2 2O +2H O+4e 4OH =0.401y
Example
In order to make Au in mine dissolve in alkaline solution
with the aid of oxygen, people usually add some
coordinating agent into the solution. Which coordination
agent is favorable? Please answer this question based on
simple calculation.
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Divergent /Disproportionation reaction
Cl2 + 2NaCl = NaCl + NaClO + H2O
Divergent reaction occur when R > L
HIO IO3 + I2
R 2
L 3
(HIO/I ) 1.45V
(IO /HIO) 1.13V
y
y
-1+7 +5 +1 01.7 1.13 1.45 0.534- -25 6 3H I O I O H I O I I
which species can undergo divergent reaction?
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Exercise
Can what species undergo divergent reaction?
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7.10.4. Advance of reaction (equilibrium constants)
1 mol dm-3 iodine solution + Fe2+ (2 mol dm-3)
32
2
22
3
I3 2 Fe2
I Fe
IFe
Fe I
(Fe / Fe ) (I / I ) ln ln
ln ln a
a aRT RT
nF a nF a
a aRT RTE K
nF a a nF
y y
y y
32
2
22
3
I3 2 Fe2
I Fe
IFe
Fe I
(Fe / Fe ) (I / I ) ln ln
ln ln a
a aRT RT
nF a nF a
a aRT RTE K
nF a a nF
y y
y y
3
2
3 2 3 2 Fe
Fe
(Fe / Fe ) (Fe / Fe ) lnaRT
nF a
y
2I2 2
I
(I / I ) (I / I ) lnaRT
nF a
y
3 22(Fe / Fe ) (I / I )
Fe3+ + I¯ Fe2+ + ½ I2
At equilibrium
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Standard emf and standard equilibrium constant
lnr mG nFE RT K y y y
lnRT
E KnF
y ylnRT
E KnF
y y
For any reaction that can be designed to take place in an electrochemical cell, its equilibrium constant can be measured electrochemically.
Four equilibria in solution
1) Dissolution equilibrium
2) Reaction equilibrium
3) Dissociation equilibrium
4) Coordination equilibrium
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Example
Determine the solubility products of AgCl(s).
AgCl(s) Ag+ + Cl¯
The designed cell is
Ag(s)|AgNO3(a1)||KCl(a2)|AgCl(s)|Ag(s)
lnRT
E KnF
y y
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7.10.5 Potentiometric titrations
GEH+(mx)SCE
automatic potential titrator
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0.00 10.00 20.00 30.00 40.00 50.00
0.300
0.100
0.500
0.700
3NaOH / cmV
E /
V
HAc-NaOH
HCl-NaOH
30.0020.00 40.003
NaOH / cmV
Δ
Δ
E
V 0.4
0.2
inflexion point
Differential plot
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7.10.6 Determination of mean ion activity coefficients
Pt(s), H2 (g, p⊖)|HCl(m)|AgCl(s)-Ag(s)
1/2 H2 (g, p⊖) + AgCl(s) = Ag(s) + H+(m) + Cl(m)
H Cl
2 2ln ln ln
RT RT RTE E a a E m
nF nF nF y y
For combined concentration cell
1,1
2,2ln2
m
m
F
RTE
Using one electrolytic solution with known mean activity coefficient, the mean activity coefficient of another unknown solution can be determined.
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Answer: = 0.9946
Example:
Pt(s), H2 (g, p) |HBr(m) | AgBr(s)-Ag(s)
Given E = 0.0714 V, m = 1.262 10-4 mol·kg-1, E = 0.5330 V,
calculate .
2lnRT
E E mF
y
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7.10.7 Determination of transference number
Zn|ZnSO4(a,1) |ZnSO4(a,2) |ZnZn|ZnSO4(a,1) |ZnSO4(a,2) |Zn
Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s) Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s)
2,
1,
2,
1, ln)12(ln)(
a
a
F
RTt
a
a
F
RTttE j
The relationship between transference number and liquid junction potential can be made use of to determine the transference number of ions.
Electromotive forces of cell with and without liquid junction potential gives liquid junction potential.
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7.10.8 Measurement of pH
1909, Sorensen defined: pH = log [H+]
present definition: H
H logp a Non-operational definition
1) Hydrogen electrode
Pt(s), H2 (g, p⊖)|H +(x) |SCE
+ +2
SCE H /H H
SCE
lg
0.05916pH
RTE a
nF
y
poison of platinized platinum
The way to determine pH
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2) Quinhydrone electrode
supramolecule :
1:1 quinone: hydroquinone
1) Equal concentrations of both
species in the solution.
2) Being nonelectrolytes, activity
coefficients of dilute Q and
H2Q is unity.
Q + 2H + + 2e- H2Q
Pt(s)|Q, H2Q, H+(mx) |SCE
2
2
2
SCE Q/H Q
H Q
SCE Q/H Q 2Q H
SCE
ln2
0.6995 0.05916pH
E
aRT
F a a
y
2
2
2
SCE Q/H Q
H Q
SCE Q/H Q 2Q H
SCE
ln2
0.6995 0.05916pH
E
aRT
F a a
y
O
O H
H O
O
O
O
2e-2H+
OH
OH
+ +
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3) Glass electrode
0.1 molkg-1 HCl内充液 离子选择性膜
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membrane potential
2 4 6 8 10 12 14 160-2
GE
/ m
V
pH
GE = ⊖ GE - 0.05915 pH
Linear relation of GE and pH exists within pH range from 0 to 14.
GE H+(mx)(SCE)Test cell:
Inner
fixed
Outer
Variable
Ag(s) AgCl(s) HCl(as) GM H+(ax)(SCE)
Reference-1 Reference-2
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4) Operational definition of pH
s x( )pH(x) pH(s)
2.303
E E F
RT
s x( )pH(x) pH(s)
2.303
E E F
RT
Buffer A B C D E
pH 3.557 4.008 6.865 7.413 9.180
pH meter with standard buffer solution
pH of standard buffer solutions at 25 oC
Es = ⊖SCE –(⊖GE - 0.05915 pHs )
Ex = ⊖SCE –(⊖GE - 0.05915 pHx )
Calibration
Measurement
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What is the concentration of hydrogen ion in this solution?
Composite electrode:
with reference electrode, usually AgCl/Ag electrode embedded on the side of glass electrode.
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7.10.9. Determination of ion concentrationIon-selective electrode
Cutaway view of an ion selective electrode
For F- electrode, thin film of LaF3 single crystal is used as ion selective membrane.
For S2- electrode, compressed thin film of AgCl-Ag2S mixture is used as ion-selective membrane.
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antigen antibody
electrochemical sensor
of potential type
7.10.10 Electrochemical sensor
electrochemical sensor of current type
Electrochemical nose
Electroanalytical chip
PbO2
Ion-exchange membrane
amplifierannunciator
Pt electrode
Gas-permeable membrane