1. §7.10 Application of EMF and electrode potential
Levine pp
14.8 Concentration cell
14.9 Liquid-junction potential
14.10 Applications of EMF measurements
14.12 ion-selective membrane electrodes

2. Our Goals Ability: Compare the oxidation ability of species;
Ability: determine the reaction direction through potential;
Ability: determine activity coefficient of electrolyte;
Knowledge: principle of electroanalysis.
Knowledge: principle of ion-selective electrode.

3. 7.10.1 Computation of emf For cell with single solution:
Cd(s)|CdSO4(a±) |Hg2SO4(s)|Hg(l)
Because a is a measurable quantity, E of the cell with single electrolyte can be calculated exactly.

4. Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s)
For cell with two electrolytic solutions:
Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s)
we have to use mean activity coefficient () which is measurable in stead of the activity coefficient of individual ion (+ or -) which is unmeasurable.

5. 7.10.2. Judge the strength of the oxidizing and reducing agents
Oxidative form: Fe3+, I2
Reductive form: Fe2+, I-
⊖ (Fe3+/Fe2+) = V
⊖ (I2/I) = V
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

6. (Ox)1 + (Red)2 = (Red)1+ (Ox)2
Determination of the reaction direction
Stronger oxidizing species oxidizes stronger reducing species to produce weaker reducing and weaker oxidizing species.
⊖ (Fe3+/Fe2+) = V; ⊖ (I2/I) = V
(Ox)1 + (Red)2 = (Red)1+ (Ox)2
Fe3+ + I = Fe2+ + 1/2I2
When concentration differs far from the standard concentration,  should be used in stead of ⊖.

7. Application of Pourbaix diagram
Cu2+
Cu(OH)2 Cu pH
 / V 2 4 6 8 10 12 14
CuO22
Cu2O
0.0
0.5
1.0
1.5

8. 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.

9. Cl2 + 2NaCl = NaCl + NaClO + H2O
Divergent /Disproportionation reaction
Cl2 + 2NaCl = NaCl + NaClO + H2O
which species can undergo divergent reaction?
Divergent reaction occur when R > L
HIO  IO3 + I2

10. Exercise
Can what species undergo divergent reaction?

11. 7.10.4. Advance of reaction (equilibrium constants)
Fe3+ + I¯  Fe2+ + ½ I2
At equilibrium
1 mol dm-3 iodine solution + Fe2+ (2 mol dm-3)

12. Standard emf and standard equilibrium constant
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

13. Example AgCl(s) Ag+ + Cl¯ The designed cell is
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)

14. 7.10.5 Potentiometric titrations
GEH+(mx)SCE
automatic potential titrator

15. Differential plot HCl-NaOH E / V HAc-NaOH inflexion point 0.00 10.00
20.00
30.00
40.00
50.00
0.300
0.100
0.500
0.700
E / V
HAc-NaOH
HCl-NaOH
0.4
0.2
inflexion point

16. 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)
For combined concentration cell
Using one electrolytic solution with known mean activity coefficient, the mean activity coefficient of another unknown solution can be determined.

17. Example:
Pt(s), H2 (g, p) |HBr(m) | AgBr(s)-Ag(s)
Given E = V, m =  10-4 mol·kg-1, E = V, calculate .
Answer:  =

18. Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s)
Determination of transference number
The relationship between transference number and liquid junction potential can be made use of to determine the transference number of ions.
Zn|ZnSO4(a,1) |ZnSO4(a,2) |Zn
Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s)
Electromotive forces of cell with and without liquid junction potential gives liquid junction potential.

19. 7.10.8 Measurement of pH 1909, Sorensen defined: pH =  log [H+]
Non-operational definition
present definition:
The way to determine pH
1) Hydrogen electrode
Pt(s), H2 (g, p⊖)|H +(x) |SCE
poison of platinized platinum

20. 1:1 quinone: hydroquinone
2) Quinhydrone electrode
Q + 2H + + 2e-  H2Q
supramolecule :
1:1 quinone: hydroquinone
Pt(s)|Q, H2Q, H+(mx) |SCE
Equal concentrations of both species in the solution.
Being nonelectrolytes, activity coefficients of dilute Q and H2Q is unity.

21. 3) Glass electrode
内充液 离子选择性膜
0.1 molkg-1 HCl

22. Ag(s) AgCl(s) HCl(as)  GM H+(ax)(SCE)
Outer
Variable
Inner
fixed 2 4 6 8 10 12 14 16 -2
GE / mV pH
Linear relation of GE and pH exists within pH range from 0 to 14.
membrane potential
 GE = ⊖ GE pH
Test cell:
GE H+(mx)(SCE)
Ag(s) AgCl(s) HCl(as)  GM H+(ax)(SCE)
Reference-2
Reference-1

23. 4) Operational definition of pH
Es = ⊖SCE –(⊖GE pHs )
Calibration
Ex = ⊖SCE –(⊖GE pHx )
Measurement
pH of standard buffer solutions at 25 oC
Buffer A B C D E pH
3.557
4.008
6.865
7.413
9.180
pH meter with standard buffer solution

24. 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.

25. 7.10.9. Determination of ion concentration
Ion-selective electrode
For F- electrode, thin film of LaF3 single crystal is used as ion selective membrane.
Cutaway view of an ion selective electrode
For S2- electrode, compressed thin film of AgCl-Ag2S mixture is used as ion-selective membrane.

26. 7.10.10 Electrochemical sensor
Electrochemical nose
Electroanalytical chip
antigen
antibody
electrochemical sensor of potential type
PbO2
Ion-exchange membrane
amplifier
annunciator
Pt electrode
Gas-permeable membrane
electrochemical sensor of current type