Chapter 6 Electrochemical Analysis

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

Zn ZnSO4,(xMol)  CuSO4, (yMol)  Cu For example: Zn ZnSO4,(xMol)  CuSO4, (yMol)  Cu Anode: Zn Zn2+ + 2e- Cathode: Cu2 + + 2e- Cu (6r-3) (6r-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)

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

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- 2Hg + 2Cl –(sat’d KCL) (6r-7) (6r-8)

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) 2Ag++S2- CdS(s) Cd2++S2- (6r-9) (6r-10)

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)

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

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

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)

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

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

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

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

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)

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)

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

(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

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

(6-29) assume: f1=f2 , 1=2 , S = 0.0591/n (6-30) (6-31)

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

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

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

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

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

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

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