《仪器分析 Instrumental Analysis》课程教学资源（教材讲义）12 Voltammetry

Voltammetry (Chapter 25) Electrochemistry techniques based on current (i)measurement as function of voltage(Eappl) Voltage Supply Variable Resistor M max min Cell Working \Counter Electrode Electrode Reference Electrode Working electrode (microelectrode)place where redox occurs surface area few mm2 to limit current flow Reference electrode constant potential reference (SCE) Counter electrode inert material (Hg,Pt)plays no part in redox but completes circuit Supporting electrolyte alkali metal salt does not react with electrodes but has conductivity CEM 333 page 12.1

Voltammetry (Chapter 25) Electrochemistry techniques based on current (i) measurement as function of voltage (Eappl) - + Voltage Supply Variable Resistor I V Cell Counter Electrode Working Electrode Reference Electrode max min Working electrode (microelectrode) place where redox occurs surface area few mm2 to limit current flow Reference electrode constant potential reference (SCE) Counter electrode inert material (Hg, Pt) plays no part in redox but completes circuit Supporting electrolyte alkali metal salt does not react with electrodes but has conductivity CEM 333 page 12.1

Why not use 2 electrodes? OK in potentiometry-very small currents. Now,want to measure current (larger=better)but potential drops when current is taken from electrode (IR drop) must minimize current withdrawn from reference electrode surface Potentiostat(voltage source)drives cell supplies whatever voltage needed between working and counter electrodes to maintain specific voltage between working and reference electrode NOTE: Almost all current carried between working and counter electrodes Voltage measured between working and reference electrodes Analyte dissolved in cell not at electrode surface! CEM333 page 12.2

Why not use 2 electrodes? OK in potentiometry - very small currents. Now, want to measure current (larger=better) but • potential drops when current is taken from electrode (IR drop) • must minimize current withdrawn from reference electrode surface Potentiostat (voltage source) drives cell • supplies whatever voltage needed between working and counter electrodes to maintain specific voltage between working and reference electrode NOTE: • Almost all current carried between working and counter electrodes • Voltage measured between working and reference electrodes • Analyte dissolved in cell not at electrode surface! CEM 333 page 12.2

Excitation signals (Fig 25-2) Name Waveform (a) E Time- E Peeatw polarography Time 5aac ime d) Current-to-voltage Potentiostatic+ control circuit CEM 333 page 12.3

Excitation signals (Fig 25-2) CEM 333 page 12.3

Microelectrodes C,Au,Pt,Hg each useful in certain solutions/voltage ranges 人 1MH2S04(P) Pt pH 7 buffer(Pt) H1 M NaOH (Pt) 1 M H2SO4 (Hg) H1 M KCI (Hg) Hg H1 M NaOH (Hg) 人 H0.1 M Et NOH (Hg) H 1 M HCIO4 (C) H0.1 M KCI (C) +3+2+10 E(V vs.SCE) Fig 25-4 At-ve limit,oxidation of water 2H20→4H++02(g)+4e At +ve limit,reduction of water 2H20+2e→H2+20H CEM 333 page 12.4

Microelectrodes C, Au, Pt, Hg each useful in certain solutions/voltage ranges Fig 25-4 At -ve limit, oxidation of water 2H2O ® 4H+ + O2 (g) + 4e￾At +ve limit, reduction of water 2H2O + 2e- ® H2 + 2OH￾CEM 333 page 12.4

Varies with material/solution due to different overpotentials Overpotential n always reduces theoretical cell potential when current is flowing n=Ecurrent-Eequilibrium Overpotential (overvoltage)develops as a result of electrode polarization: concentration polarization-mass transport limited adsorption/desorption polarization-rate of surface attach/detachment charge-transfer polarization rate of redox reaction reaction polarization-rate of redox reaction of intermediate in redox reaction Overpotential means must apply greater potential before redox chemistry occurs CEM 333 page 12.5

Varies with material/solution due to different overpotentials Overpotential h always reduces theoretical cell potential when current is flowing h = Ecurrent - Eequilibrium Overpotential (overvoltage) develops as a result of electrode polarization: • concentration polarization - mass transport limited • adsorption/desorption polarization - rate of surface attach/detachment • charge-transfer polarization - rate of redox reaction • reaction polarization - rate of redox reaction of intermediate in redox reaction Overpotential means must apply greater potential before redox chemistry occurs CEM 333 page 12.5

Hg particularly useful (i)high overpotential at -ve limit (ii)easy to prepare clean surface Hg Microelectrodes:(Fig 25-3) Me ury Wire 777777777 Hg Plunger Guide bushing Compression spring Polyurethane tip -Valve seat Capillary seal -Ferrule (bonded to capillary) Ferrule support Capillary -Capillary nut -Hg drop Hg drop- Capillary (d) CEM333 page 12.6

Hg particularly useful (i) high overpotential at -ve limit (ii) easy to prepare clean surface Hg Microelectrodes: (Fig 25-3) CEM 333 page 12.6

Voltammograms(voltammetric waves)are graphs of current (i)vs. applied voltage (Eappl) +100.0 Limiting current +80.0- A+ne-= +60.0 +40.0 Y- +20.0- 0.0 -20.0 .0-0.2 -0.4-0.6-0.8-1.0 Eappl (vs.SCE) Fig 25-5 Hg microelectrode is cathode-ve terminal in above A+ne-←→P E0=-0.26V Increase in current at potential at which A can be reduced (reaction demands electrons,supplied by potentiostat) CEM 333 page 12.7

Voltammograms (voltammetric waves) are graphs of current (i) vs. applied voltage (Eappl) Fig 25-5 Hg microelectrode is cathode -ve terminal in above A + ne - « P E 0 = -0.26 V Increase in current at potential at which A can be reduced (reaction demands electrons, supplied by potentiostat) CEM 333 page 12.7

Two important points Half wave potential (E2)is close to Eo for reduction reaction E12≈E0-Eref -0.50=E0-0.24 for SCE E0=-0.26V Limiting current (i)proportional to analyte concentration(really activity) i=k.CA CEM333 page 12.8

Two important points • Half wave potential (E1/2) is close to E0 for reduction reaction E1/2 » E 0 - Eref -0.50 = E 0 - 0.24 E 0 = -0.26 V for SCE • Limiting current (il) proportional to analyte concentration (really, activity) i l = k× cA CEM 333 page 12.8

Current is just measure of rate at which species can be brought to electrode surface Two methods: Stirred-hydrodynamic voltammetry Unstirred-polarography (dropping Hg electrode) Hydrodynamic Voltammetry In stirred solution,diffusion layer (Nernst layer 8 0.1-0.01 mm) forms near electrode (Fig 25-9) Electrode Nernst diffusion laver of stagnant solution Laminar flow region Turbulen CEM 333 page 12.9

Current is just measure of rate at which species can be brought to electrode surface Two methods: Stirred - hydrodynamic voltammetry Unstirred - polarography (dropping Hg electrode) Hydrodynamic Voltammetry In stirred solution, diffusion layer (Nernst layer d 0.1-0.01 mm) forms near electrode (Fig 25-9) CEM 333 page 12.9

Three transport mechanisms (i)migration (ii)convection (iii) diffusion In general A+ne←>P Eind Eotll-E0.0592 logEnr n CA but what are cp and cA near electrode surface (c and c? Ouies Nernst 5- -Convection- Diffusion Convection -layer- CA c唱=cAZ X c8=9a/2Z c唱=ca/2y G 04 10-2to10-3cm 0 Distance xfrom electrode,cm Distance x from electrode,cm (a) (b) Fig25-10 Convection dominates in stirred solution CEM 333 page 12.10

• Three transport mechanisms (i) migration (ii) convection (iii) diffusion In general A + ne - « P Ecell = EA 0 - 0.0592 n log cP cA 6 4 4 4 E7 ind4 4 4 8 - Eref but what are cP and cA near electrode surface cP 0 and cA 0 ( ) ? Fig 25-10 Convection dominates in stirred solution CEM 333 page 12.10