Chapter 25. VoltammetryPotentiometry : no net current flow Voltammetry(polarography) : resulting current flow measure Polarography differential pulse polarography stripping analysisIntroduction Principally, in voltammetry : vs (voltammograms)Cell checkCell content : Solution of interest Stable reference electrode Indicator electrode

25A Excitation signals in voltammetryIndicator electrode DMC polarography (1922, Heyrovsky )DME Cell Drop rate : constant rate(5-30/) Drop life : 2-12sec Voltage scanning rate : 50-200mV/min(DC polarography) : Reproducibility Every mercury drop surface Electrolytic solu Hg H2 overvoltage + 0.4V vs SCE + Hg anodic dissolution

Fig. 25-1

Fig. 25-2

Fig. 25-4

25B-2 Modified Electrodes:Fig. 25-5

25B-3 VoltammogramsFig. 25-6

25C Hydrodynamic VoltammetryFig. 25-8

Supporting electrolytes : (0.1M-1.0M) : Soln. Electrical field Diffusion .: Strong acid : HCl, H2SO4 Strong based : (NaOH, LiOH) Neutral salt : (chlorides, perchlorates sulfates of alkali metal, tetraalkyloammonium ion) Complexing agents: (tartrate, citrates, cyanides, fluorides, amine)

*Limiting current : ( )Half wave potential : 1/2 : or

: Blank polarogram polarogram : polarogram E1/2: half wave potential ij(id) : limiting current residual plateau current ( )Residual current (charging, condenser current) -

Profiles for Planar Electrodes in Unstirred Solution.Figure 25-9 E-t and I-t curves

Fig 25-10 Concentration distance profiles (diffusion controlled reduction).

Fig 25-11 Flow patterns at the electrode surface

Fig. 25-12

Fig. 25-13 Concentration profiles at an electrode/solution interface.

Fig. 25-14 Voltammograms for Mixtures of Reactions

Fig. 25-15 Mixed Anodic/Cathodic Voltammograms

25C-3 Oxygen WavesFig. 25-16

Removal of Oxygen Dissolved oxygen O2 + 2H+ + 2e- = H2O2 1H2O2 + 2H+ + 2e- = 2H2O 2 pH7 E1/2 = 0--1V vs SCEAir-saturated aqueous solution : 4mM O2 level 5 id : He, N2 5-20 bubble N2 tank O2 acidic V2+ or Cr2+ hot copper

Fig. 25-17 Voltammetric detect in Chromatography

Fig. 25-17 The Flow Cell

Fig. 25-18 The Clark Oxygen SensorCathodic : O2 + 4H+ + 4e- = 2H2OAnodic : Ag + Cl- = AgCl(s) + e-

Fig. 25-20 Amperometric titration curves

Rotating ElectrodesVoltammetry at rotating Pt Elctrode , 600ppm rotating : diffusion & mechanical mixingi1 diffusion only 20 1/2I2...Na2S2O3

Fig. 25-22

25D Cyclic VoltammetryFig. 25-23

Fig. 25-25 Parathion in acetate bufferR-C6H4NO2 + 4e- + 4H+ R-C6H4NHOH + H2O

R-C6H4NHOH R-C6H4NO + 2H+ + 2e-

R-C6H4NO + 2e- + 2H+ R-C6H4NHOH

Potential sweep triangle form1-2 sec sweep(Fig 21-16 )1) A, D =(20.0282/n)V reversibleRate of OX-Red process, mechanism of OX-Red preelec

Adsorption Current ( : available )Adsorption prewave height 2 1 + 2 height

* Pornographic maxima

: stirring : Hg flow chemical nature (gelatin, , tritonx-100) : trial & error

* TemperatureD : temperature senstive 2.5%/1Tests for Current-Limiting processDiffusion, kinetic, adsorpton, catalytic currents (1)-(5) (1) (2) Hg ( )(3) pH(4) Buffer (5) (, kinetic )

1.1. Instrumentation and Apparatus : 1) Cell 2) Electrode 3) Potentiostat or polarograph, 4) O2 system1) Cell() DME ( )() Inert gas (inlet, outlet)() Reference Hg pool cell SCE cell (H-cell) : 5-10ml

* H-cell fig 3.9, 3.10Reference KCl Potentosatats : (3) reference, concenter, indicator Ref. feed back concenter

1.3 Varication of the conventional pornographic method1940 (Breyer, Gutmann, Grahame) 1.4.1 AC polarographyDC + mV , vs detectorCell impedance physical processesa) Ionic migration which, among other things, determines the ohmic resistancesb) Charging of the electrical double layer at the electrode - solution interfacec) The fafrdic process primary subject

polaro peek

q : 2.4F(3m/4d)2/3t2/3

10-6M :

2. Pulse polarography (voltammetry)Charging currunt(non-faraday current) S.w. DME (charging currunt) : rectangular voltage pulse [40msec] 17msec draw out Pulse : voltage pulse amplitude . Fig3.16 (A) (B) (C) Differential pulse : const.amplitude voltage . Fig 3.16 (D) (E) (F)

1) Pulse : drop life 40msec s.w. pulse Pulse 1/2 20msec (charging current min)2) pulse amplitude DC wave 3) Differential pulse : 1) pulse 2) Constant amplitude(5-100mV) pulse voltage 3) pulse pulse . Detection limit : 10-8-10-9M

Fig. 25-29

Fig. 25-30

3. Square wave polarography (voltammetry)Sine wave CH sw : . : 10-8M (100-4) out( )

Fig. 25-31Fig. 25-32

4. Rapid-seen osillographic voltammetryPrinciples : drop time or voltage scan accomplish. I vs E curve At positive more than peak DC polarography mode .At potention more negative than Ep Theory : Ep=E1/2-1.1RT/nF iP=kn3/2AD1/2C1/2A : : scan (V/sec)D : n : ( ) C : k : Randles-sevik 2.72105Dection range : 10-6-10-7M

5. Stripping VoltammetryPrinciple : Hg analgam cation Hg slightly soluble compounds anion : limiting current (Preconcenttration step) 10-500 Stripping : cation + - sweep anion - + sweep

Fig. 25-34

Electrodes : HMDE (hanging mercurry drop elec) Hg-coated pt Glassy carbon(Pre step)HMDE :Hg 2-3 drop teflon spoon pt wire . Carbon elec. Graplite rod, carbon rod, pt wire Hg film(10-100) plating(1step) Coating HMDE Hg (cathonic deposition) Cu, Pb, Cd, Bi, Zn....(2step) (3step) Stripping (cathoic, anodic)

Sensitivity : 1) (Preelctrolysis) 2) Stripping step ex) Linear-sweep voltammetry Differential- pulse voltammetrySens. limit : 0.1 M KCl(0.0002%) -> 10-7M Pb Detection range : 10-7-10-10M

25I Voltammetry with MicroelectrodesFig. 25-37

The Cell Biology of Extracelluar acidificationFig. IA4-1

Fig. IA4-2

*