1. Chapter 6 Electrochemical Analysis

2. 6.1 Introduction Oxidation – reduction reaction Anode reaction:
Red === Ox + ne -
Cathode reaction:
Ox + ne - === Red
(6r-1)
(6r-2)
Cell reaction expression
Anodesolution,(Ox)solution, (Red) Cathode

3. Zn ZnSO4,(xMol)  CuSO4, (yMol)  Cu
For example:
Zn ZnSO4,(xMol)  CuSO4, (yMol)  Cu
Anode: Zn Zn2+ + 2e-
Cathode: Cu e Cu
(6r-3)
(6r-4)

4. Ecell = Ecathode - Eanode
2. Half-cell Potential
For half – cell reaction :
rAred + ne pAOx
Nernst equation:
For a Cell:
Ecell = Ecathode - Eanode
If, Ecell > 0: Primary Cell
Ecell < 0: Electrolyic Cell
(6r-5)
(6-1)
(6-2)

5. 3.The Types of Electrodes
A metal in Equilibrium with its ions
(Class Ⅰelectrodes)
Ag+ + e Ag
(6r-6)
(6-3)

6. HgHg2Cl2(s)Cl -,(sat’d KCL) Hg2Cl2(s) + 2e- 2Hg + 2Cl –(sat’d KCL)
A metal in equilibrium with a saturated solution of a slightly soluble salt (Class Ⅱelectrodes)
AgAgCl Cl -,(=1)
AgCl(s) + e Ag + Cl –
Reference electrodes
Saturated calomel electrode (SCE)
HgHg2Cl2(s)Cl -,(sat’d KCL)
Hg2Cl2(s) + 2e Hg + 2Cl –(sat’d KCL)
(6r-7)
(6r-8)

7. Ag2S(s) 2Ag++S2- CdS(s) Cd2++S2-
A metal in equilibrium with tow slightly soluble salts with a common Anion (Class Ⅲelectrodes)
AgAg2S,CdSAg+,Cd2+,S2-,
Ag2S(s) Ag++S2-
CdS(s) Cd2++S2-
(6r-9)
(6r-10)

8. 4. The departure of potential
Liquid-junction potential
HCl(0.1M) KCl(salt bridge, xM) KCl(0.1M)
When x>3.6 Eljp<1mV
Polarization
Efact ≠ENernst and Csurf ≠Cbolk
Over-voltage
real potential start a reaction > equilibrium potential
Ohm drop
Ecell = Ecathode - Eanode + IR
R: resistance of solution, I: current
(6-4)

9. 6.2 Potentiometry
Principle
(6-5)
(6-6)
(6-7)
(6-8)

10. 2. Ion selective Membrane Electrode
Structure of ISE
Types
Fig 6-1

11. Ag︱Agcl(s) ︱HCl(inner) ︱glass ︱H+(unknown solution)
(1) The Glass Electrode
Fig 6-2
Ag︱Agcl(s) ︱HCl(inner) ︱glass ︱H+(unknown solution)
(6-9)

12. Glass electrode︱unknown solution ︱SCE
(6-10)
Glass electrode︱unknown solution ︱SCE
(6-11)
(6-12)

13. Selectivity of Glass electrode
H+G-+M+(sol) M+ G- + H+ (sol)
(6r-11)
(6-13)
k: selectivity coefficient
(6-14)

14. (2) The Response Behavior of ISE
Nernst response and Detect limit
(6-15)
Fig 6-3

15. Selectivity
Response time
(6-16)
Fig 6-4

16. 3.Quantitative Analysis
The Prerequisite of Experiments
Ion Intensity Buffer
(6-17)
f_activity coefficient
(6-18)
If Cion,T≈constant, f ≈constant.
(6-19)

17. pH Buffer
MZ+ + xOH M(OH)x (z-x)+
H+ + OH H2O
Complex reagent
M Z+ + nL MLnZ+
(6r-12)
(6r-13)
(6r-14)
(6-20)

18. (6-21)
(6-22)
(6-23)
(6-24)

19. (2)Standard calibration Methods
Fig 6-5
If =1:
E = K + s lgC0
standard concentration series
C0 / molL-1
10-3
3.16x10-4
10-4
3.16x10-5
10-5
lgc -3
-3.500 -4
-4.500 -5

20. (3)Standard Addition Methods
(6-25)
(6-26)
(6-27)
(6-28)

21. (6-29)
assume: f1=f2 , 1=2 , S = /n
(6-30)
(6-31)

22. Introduction 6.3 Polarography (1) Electrolytic cell Cathode:
M+ + e- →M
Hg(l) ∣M+(C)︱SCE
Wkg: Working Electrode
Ref:Reference Electrode(SCE)

23. (2) Polarization M +(Bulk) → M +(Cathode)
Fig 6-7

24. 2. The Dropping Mercury Electrode(DME)
(1) Structure of DME
Fig 6-8

25. (2)Electrolytic current and current density
Fig 6-9

26. 3. Quantitative Analysis (1) Ilkovic Equation
(6-32)
____Average diffusion current
D ____diffusion coefficient
m ____rate of mercury flow
(6-33)

27. (2)The factor of affect diffusion current
Residual current
Changing current
Migrating current
Maximum phenomenon
Oxygen interference

28. 4. Qualitative Analysis
Half wave potential
(6-34)
(6-35)