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measure current (larger=better)but potential drops when current is taken from electrode (IR drop) must minimize current withdrawn from reference electrode surface Potentiostat(voltage source)drives cell supplies whatever voltage needed between working and counter electrodes to maintain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current (larger=better) but • potential drops when current is taken from electrode (IR drop) • must minimize current withdrawn from reference electrode surface Potentiostat (voltage source) drives cell • supplies whatever voltage needed between working and counter electrodes to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) + 4eAt +ve limit, reduction of water 2H2O + 2e- ® H2 + 2OHCEM 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