Chapter 22 An Introduction to Electroanalytical Chemistry 22A Electrochemical cells

Chapter 22 An Introduction to Electroanalytical Chemistry22A Electrochemical cells - 2 (1) Electrode - Anode oxidation - Cathode reduction 1) Working and indicator electrode : A reaction take place. 2) Reference electrode : constant potential 3) Counter electrode : internal polarization E(working electrode) = (Ecell - iRcell - Epolarization)- Electrode - Electrolyte

(1) * Galvanic cell : chemical En. electrical En.* Electrolytic cell : electrical En. chemical En. Galvanic cell : Zn Zn2+ + 2e- Cu2+ + 2e- Cu

(2) Zn / Zn2+(aZn2+)Cu2+ (aCu2+) / CuLeft hand electrode : negative pole of cell -- oxidation process occurs Zn Zn2+ + 2eRight hand electrode : Reduction process Cu2+ + 2e Cu* Pt, H2(p=1atom) / H+(0.1M), Cl-(0.1M), AgCl(satd) / Ag

22A-5 Solution StructureFig. 22-3 Electrical double layer1) A compact inner layer (d0 to d1): the potential decreases linearly with distance from the electrode surface.2) A diffuse layer(d1 to d2): the potential decrease is exponential.

22A-6 Faradaic and Nonfaradaic Currents1) Faradaic process : - Faraday's law ( ) .2) Non-faradaic process : - condenser 3) Charging current : non-faradaic process EA EB . charging current.

22B Potentials in Electrochemical Cells Ecell = Eright - Eleft + Elj

22B1. Thermodynamic cell potentialNernst Equation thermodynamic relationship potentiometric measurementG = H - TS = E + PV -TSFrom Vant Hoff reaction equation = dE - TdS -SdT + PdV + VdP

Ion , error A) Standard electrode potential: , l .B) Formal electrode potential: (, , , ) swift formal .Formal potential (, )

@ Effect of complexation on the electrode potential. -> ex) Zn / Zn2+ // Cu2+ / Cu CuSO4 solution EDTA -> Cu2+ + EDTA4- Cu EDTA2-

22B-2 Liquid - Junction Potentials* ionic solution A) Diffusion potentialLiquid junction potentials, different mobilization & concentrations of ions in electrolytes in contact.B) Donnan potential 1 ions . salt bridge, porous glass Kl : salt bridge.C) Liquid junction Transport number Charge Activity of the ions forming the junctions

1) Salt bridge2) Cracked glass bead3) Ceramic frit4) Sleeve5) Gauntly or asbestos fiber, wick6) Platinum wire7) Cellulose pulp8) Glass frit9) Cellophane10) Fine capillary drip : Liquid junction .

22E Currents in an electrochemical cellCell potential ( )

Thermodynamic cell potential Liquid junction potential Ohmic potential Polarization potential { , }

Thermodynamic cell potential Liquid junction potential

Polarization potential concentration polarization overvoltage(kinetic polarization) IR .Fig. 22-6 Curves for an ideal(a) polarized (b) nonpolarized electrodes : , , , , , , , .

* : Concentration polarization, charge transfer polarization, kinetic polarization,( ) * : bulk * : overvoltage

Fig. 22-8 Electrode Surface layer Solution bulk* Change transfer limiting step : activation polarization Mass transfer limiting step : concentration polarization Chemical reaction or adsorption

5) Overvoltage(overpotential)a) Activation overvoltage; Slow electron transfer high activation En b) Resistance overvoltage; iR drop , . : adherent layer c) Concentration overvoltage; Electrode vicinity concentration polarized : stirring 3 . overvoltage = - e : Potential difference across the interface e : Potential difference across the interface at equilibrium

* Non-polarizable electrode : Reference electrode non polarizable electrode .1) Activation overvoltage : slow electron transfer2) Resistance overvoltage iR drop 3) Concentration overvoltage = A + C + R

*** Electrochemical definition and terminology***1) Ideally non-polarized electrode. - (condenser) . , 2) Ideally polarized electrode.- . , KCl (polarography ) K+ + e K (amalgam), 2Hg+ Hg22+ + 2e( )2Cl- Cl2 + 2e ( ), 2H2O + 2e- H2 + 2OH- ( .) * Capacitance of an electrode condenser

3) Depolarizer : polarized electrode 4) Reversible : . :

Modes of Electrochemical Mass TransportMigration, convection, diffusion 3 .

1) Migration : electrical gradient ( ) : or indifferent electrolytes .

2) Convection : gross physical movement

3) Diffusion : most widely studied