每日一句 · pdf filesolutions. graphical representation of the role of ph and...
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每日一句
伤害我们的并非事情本身,而是我们对事情的看法。
任何事情,不管看起来多么可怕,也不能阻止我们探
寻其中隐藏的机遇。但在各种情形中寻找机遇,这需要很
大的勇气,因为你周围的大多数人总是用最粗俗的词语去
解释事情:成功或失败,好或坏,对或错。这些简单化、
极端化的分类,遮蔽了哪些对事情更有创造性,也更有用
的解释,他们恰恰比那些粗俗的解释要有益和有趣得多。
Epictetus:《The meditations》中央编译出版社,北京,2015
Chapter 7 Electrochemistry
§7.9 Electrode potential and electromotive forces
7.9.4 pH-dependence of : Pourbaix diagram
For electrode reaction with H+ or OH- participating in, the
electrode potential will depend on pH.
O2 + 4H+ + 4e- 2H2O
= ⊖ + 0.05916 lgaH+
= 1.229 - 0.05916 pH
= ⊖ + 0.05916 lgaH+
= 0.000 - 0.05916 pH
2H+ + 2e- H2
pH-potential diagram/Pourbaix diagram
§7.9 Electrode potential and electromotive forces
Marcel Pourbaix, (1904–1998), a
Russian-born, Belgian chemist,
“Atlas of electrochemical equilibria
in aqueous solutions”, National
Association of Corrosion Engineers,
1974.
7.9.4 pH-dependence of : Pourbaix diagram
§7.9 Electrode potential and electromotive forces
By 1938, he had devised the potential-
pH diagrams for which he became
famous. In 1939, he presented to the
Faculty his doctoral dissertation,
accompanied by a thesis entitled
"Thermodynamics of Dilute Aqueous
Solutions. Graphical Representation of
the Role of pH and Potential."
Pourbaix (left) and Evans (right)
图形化方法(Graphic method, visualization)
7.9.4 pH-dependence of : Pourbaix diagram
§7.9 Electrode potential and electromotive forces
Construction of Pourbaix diagram
Cu2+Cu(OH)2
Cu
pH
/ V
2 4 6 8 10 12 140
CuO22
Cu2O
7.9.4 pH-dependence of : Pourbaix diagram
§7.9 Electrode potential and electromotive forces
(1) When we plate Cu,
why do we use acidic
copper sulfate solution?
(2) When we anodize
copper in neural solution,
what phenomenon can we
observed?
§7.10 Application of EMF and electrode potential
Levine
pp. 431--443
14.8 Concentration cell
14.9 Liquid-junction potential
14.10 Applications of EMF measurements
14.12 ion-selective membrane electrodes
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
Because a is a measurable quantity, E of the cell with
single electrolyte can be calculated exactly.
E
§7.10 Application of EMF and electrode potential
For cell with two electrolytic solutions:
Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s)
2
2
Cu
Zn
lnaRT
E EnF a
1 ,1
2 ,2
lnmRT
E EnF m
we have to use mean activity coefficient () which is
measurable in stead of the activity coefficient of individual
ion (+ or -) which is unmeasurable.
7.10.1 Computation of emf
§7.10 Application of EMF and electrode potential
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
§7.10 Application of EMF and electrode potential
(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
§7.10 Application of EMF and electrode potential
Application of Pourbaix diagram
Cu2+Cu(OH)2
Cu
pH
/ V
2 4 6 8 10 12 140
CuO22
Cu2O
0.0
0.5
1.0
1.5
7.10.3 Determination of the reaction direction
§7.10 Application of EMF and electrode potential
Corrosion with hydrogen
evolution
Corrosion with oxygen
absorption
+Au 1e Au =1.7 V
2 2O +2H O+4e 4OH =0.401
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.
7.10.3 Determination of the reaction direction
§7.10 Application of EMF and electrode potential
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
-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?
7.10.3 Determination of the reaction direction
§7.10 Application of EMF and electrode potential
Exercise
Can what species undergo divergent reaction?
7.10.3 Determination of the reaction direction
§7.10 Application of EMF and electrode potential
元素电势图(Latimer diagram)
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
3
2
3 2 3 2 Fe
Fe
(Fe / Fe ) (Fe / Fe ) lnaRT
nF a
2I
2 2
I
(I / I ) (I / I ) lnaRT
nF a
3 2
2(Fe / Fe ) (I / I )
Fe3+ + I¯ Fe2+ + ½ I2
At equilibrium
§7.10 Application of EMF and electrode potential
Standard emf and standard equilibrium constant
lnr mG nFE RT K lnRT
E KnF
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
7.10.4. Advance of reaction (equilibrium constants)
§7.10 Application of EMF and electrode potential
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
7.10.4. Advance of reaction (equilibrium constants)
§7.10 Application of EMF and electrode potential
7.10.5 Potentiometric titrations
GEH+(mx)SCE
automatic potential titrator
§7.10 Application of EMF and electrode potential
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
For combined concentration cell
1,1
2,2ln
2
m
m
F
RTE
Using one electrolytic solution with known mean activity
coefficient, the mean activity coefficient of another unknown
solution can be determined.
§7.10 Application of EMF and electrode potential
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
7.10.6 Determination of mean ion activity coefficients
§7.10 Application of EMF and electrode potential
7.10.7 Determination of transference number
Zn|ZnSO4(a,1) |ZnSO4(a,2) |Zn
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.
§7.10 Application of EMF and electrode potential
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
poison of platinized platinum
The way to determine pH
§7.10 Application of EMF and electrode potential
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 2
Q H
SCE
ln2
0.6995 0.05916pH
E
aRT
F a a
O
O H
H O
O
O
O
2e-2H+
OH
OH
+ +
7.10.8 Measurement of pH
§7.10 Application of EMF and electrode potential
3) Glass electrode
0.1 molkg-1 HCl 内充液 离子选择性膜
7.10.8 Measurement of pH
§7.10 Application of EMF and electrode potential
membrane potential
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:
Ag(s) AgCl(s) HCl(as) GM H+(ax)(SCE)
Reference-1Reference-2
7.10.8 Measurement of pH
§7.10 Application of EMF and electrode potential
4) Operational definition of pH
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
7.10.8 Measurement of pH
§7.10 Application of EMF and electrode potential
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.
7.10.8 Measurement of pH
§7.10 Application of EMF and electrode potential
7.10.9. Determination of ion concentration
Ion-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.
§7.10 Application of EMF and electrode potential
7.10.9. Determination of ion concentration
§7.10 Application of EMF and electrode potential
antigen antibody
electrochemical sensor
of potential type
7.10.10 Electrochemical sensor
electrochemical sensor of current type
Electrochemical nose
Electroanalytical chip
§7.10 Application of EMF and electrode potential
7.10.10 Electrochemical sensor
§7.10 Application of EMF and electrode potential
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