Chapter 25. Voltammetry

Transcription

1 Chapter 5. Voltammetry Excitation Signal in Voltammetry Voltammetric Instrumentation Hydrodynamic Voltammetry Cyclic Voltammetry Pulse Voltammetry High-Frequency and High-Speed Voltammetry Application of Voltammetry Stripping Methods Voltammetry with Microelectrodes Voltammetry Voltammetry: measurement of current (I) as a function of applied potential (E). Under condition with polarization (η). Negligible consumption of analyte Amperometry: measure I at a fixed E Potentiometry: measure E when I, no polarization Coulometry: measure C, polarization is compensated, all analyte is consumed Polarography: voltammetry at the dropping mercury electrode (DME) DA: Hg (poison), apparatus (cumbersome), better techniques Application: Oxidation and reduction process Adsorption processes on surfaces Electron transfer mechanism Jaroslav Heyrovsky

2 Excitation Signals and Instrumentation WE: E (relative to RE); RE: constant E; CE: Pt wire (current) Supporting electrolyte: a salt added in excess to the analyte solution, like alkali metal salt No reaction at the E region Reduce effect of migration Lower R of the solution E o = E i E o E i Measure I, I-to-E converter E follower, high Z, no I I o R E o = -I i R I i An op amp potentiostat Voltammetric Working Electrode Disk electrode: A small flat disk in a rod of an inert materials like Teflon, glass or Kel-F. HMDE: hanging mercury drop electrode Large negative E, fresh metallic surface, reversible reaction UME: microelectrode, r: < 5 µm, wire in glass, tip polished Flow cell WE: in flowing stream, PEEK (polyethertherketon) E min : reduction of water (H ), E max : oxidation of water (O ) Disk electrode HMDE WE UME Flow electrode

3 Modified Electrode Chemical modification: Irreversibly adsorbing substances: oxidation of electrode (metal or C) surface (O- or OH) electrodeposition Covalent bonding of components : like SAM of thiols with amine or carboxyl group on the other end Organosilanes or amines Coating of polymer films Dip coating, spin coating Application: Electrocatalysis Smart window: electrode changes color upon reaction Analytical sensor Circuit Model of a Working Electrode A. Randles circuit: R Ω, solution resistance C d, double layer capacity Z f, faradaic impedance f dependence B. Faradaic impedance: R s, electron transfer resistance C s, pseudocapacitance, mass transfer C. Faradaic impedance: R ct, charge transfer resistance Z w, Warburg impedance A Bulk electrolyte R Ω Double layer Diffusion layer C d Z f WE C C d B C d R Ω R Ω R ct Z w R s C s 3

4 Concentration Profile in Unstirred Solution Reaction: A + e - P reversible and rapid Mass transfer: 1. Migration: electric field; Supporting electrolyte (1 ).Diffusion: concentration gradient 3.Convection: mechanical Potential vs. surface concentration: Eappl = E A Current:.59 log n cp E ref c A A planar electrode with potential step A c i = nfad A x n: #electron F: Faraday constant A: surface area, cm D: diffusion coefficient, cm /s Concentration distance profile during diffusion controlled reaction P Hydrodynamic Voltammetry the analyte solution is kept in continuous motion stir the solution, flow solution, like in HPLC Flow pattern in a flow stream Flow patter near an electrode.59 c E P appl = EA log Eref n ca 1 ~ 1 µm convection cp = ca ca A + e - P reversible and rapid 4

5 Voltammograms Voltammetric wave: an -shaped wave of I-E Limiting current, i l : the current plateau observed at the top, c A i l = kc A c A = at electrode surface maximum mass transfer rate Current in American way: Reduction current + Oxidation current - Half-wave potential: E 1/ at i = i l /, E Relative to E Identification Linear-sweep voltammogram at slow scan rate E = -.6 V vs. SCE Volumetric Currents A planar electrode: Nernst diffusion layer δ control c ( A nfad A + ne P i = nfad ) A A = ( ca c A ) x δ D A ca : mol Limiting current: c A at the electrode surface = δ :. nfad i A l = ca = kaca δ Reverse current: c P in the bulk solution =. nfadp nfad ( ) P i = cp cp = cp = kpcp δ δ Half-wave potential, E 1/ : i = i l / Eappl = E A E1/ = EA.59 log n.59 log n ka.59 i log Eref kp n il i ka Eref EA Eref kp n :electron / analyte F :96485C/mol electron A :electrode surface area, cm :diffusion coefficient, cm /s 3 / cm Nernst diffuion layer thickness, cm 5

6 Voltammetric I-E Based on the kinetics of the reaction: Reversible systems: obey Nernst equation Totally irreversible system: either the cathodic or anodic reaction is too slow as to be negligible Partially reversible system: the reaction in one direction is much slower than the other one. like organic system, i = kc, E = f(v, c, il) Voltammogram for mixture: E.1 V Anodic/Cathodic Voltammogram: A: oxidation current B: both reaction C: reduction current + E =.1 V E =. V Oxygen Wave and Sensors Clark electrode Oxygen wave: I is proportional to n Sparging: deaerate the solution with inert gas, N, Ne and He Highly depends on the ph of the solution Clark electrode: volumetric sensor Cathodic Pt electrode: O + 4H + + 4e H O Anodic Ag electrode: Ag + Cl - AgCl (s) + e Diffusion across membrane ( ~ 1 µm) Diffusion cross the thin electrolyte solution ( ~ 1 µm) Steady-state current I is dependent on electrochemical equilibrium, [O ] 1 ~ s and d m+s < µm 6

7 Enzyme-based Sensors Glucose detection: largest selling chemical instruments A polycarbonates film (glucose permeable, not for protein and other blood constitutes): diffuse through An immobilized enzyme layer (glucose oxidase): glucose reduction H O A cellulose membrane layer for H O diffusion: H O oxidation O Amperometric detection (I c) or volumetric detection (E c) of sucrose, lactose, ethanol and L-Lactate glucose oxidase glu cose + O gluconic acid + HO HO + OH O + HO + e Amperometric Titration At least one species is electrochemical active A WE (rotating Pt) + RE: confined to product either a precipitate or a stable complex. Ag + for X -, Pb + for SO 4 - Exception: Br (BrO 3- ) titration of organics Two WEs: simple instrument, determination of a single specie Karl fisher titration for determining water + BrO3 + 5 Br + 6H 3Br + 3HO produced is reduced Analyte is reduced Both analyte and products are reduced 7

8 Rotating Electrodes O reduction Rotating electrode: RDE: rotating disk electrode, affiliate mass transfer RRDE: rotating ring disk electrode, intermediate detection Levich equation: 1/ 1/ 6 i l =.6nFADω v c n :electron / analyte D :diffusion coefficient, cm /s ω : angular velocity, radians/s v : kinematic viscosity, cm /s 3 ca : mol / cm RDE RRDE Polarography The ripples are caused by the constant forming and dropping of the mercury electrode WE: DME, diffusion control, no convection Residue current: current observed in the absence of an electroactive specie Diffusion current: limiting current which is limited by the diffusion A: DL ~ 1-5 M, Faster equilibrium + new electrode surface reproducible current; High η for H evolution low E window DA: new surface large charging current 1/ / 3 1/ 6 ( i d ) max = 78nD m t c n :electron / analyte D :diffusion coefficient, cm /s m : rate of flow of Hg throug the capillary,mg/s 3 ca : mol / cm t : time, s Polarogram.5 mm Cd + in 1 M HCl 1 M HCl 8

9 Cyclic Voltammetry CV: forward scan, switching potential, reverse scan Application of CV: Study of redox reaction Detection of reaction intermediates Observation of follow-up reactions Reaction: Reversible (+) (reduction) (-) (oxidation) A: H O oxidation O B-H: reduction Irreversible or rapid removal of Red B-D: c A (reduction) D-F: c A =, δ F-H: reduction (+) (-) H-K: oxidation E (vs SCE) 6. mm Fe(CN) 6 3- CV- Fundamental Studies Peak potential: E pc and E pa Reversible: E p =.59 /n Irreversible: E p >.59 /n Peak current: 5 3 / 1/ 1/ i p = n AD v c n :electron / analyte 3 c : mol / cm v : scan rate, V/s Qualitative information in organic and inorganic chemistry first choice reaction intermediate + C : φno + H + e φnhoh E p = Epa Epc D :diffusion coefficient, cm /s A : electrode surface area, cm + A: φno + 4e + 4H φnhoh + HO + B : φnhoh φno + H + e.59 = n Parathion in.5 M acetate buffer in 5% ethanol, ph = 5 9

10 CV of Modified electrode Reversible surface redox couple no mass transfer effect symmetrical peaks + same peak height E E E p = pa pc Digital Simulation of CV Digital simulation: DigiSim, DigiElk Fast implicit finite difference methods 1 st or nd order homogeneous chemical reaction Generate dynamic concentration profiles The exact current may be offset as the nonfaradaic current is not easily simulated 1

11 Differential Pulse Polarography DPP: increasing sensitivity Lower DL: ~ 1-7 to 1-8 M ( ~ 3 order lower than CV) Enhancing faradic current: diffusion current (i d ) + Nernst contribution due to E, several times larger than i d, t is small enough Decrease in nonfaradic current: charging current decays exponentially with time, is small at the late lifetime of the drop, t is large enough Trace heavy metal detection t.36 ppm tatrecylinehcl in.1 M acetate buffer, ph=4 Square-wave Polarography SWP: increasing sensitivity Great speed: step < 1 ms, signal average is possible Lower DL: ~ 1-7 to 1-8 M Enhancing faradic current + Decrease in nonfaradic current I = I f I r, the current difference is plotted 1 mv 5 mv = E SW difference Guanine, adenine, thymine forward reverse SWP generation 11

12 Stripping Methods Stripping methods: Anodic stripping methods: C A for metal Cathodic stripping methods: A C for halides Electrodeposition step: Stirring the solution: mass transfer Only a fraction of analyte is deposited: accumulation process Depends on c, stir rate, deposition time, electrode surface and potential t < 1 min. for c ~ 1-7 M t > 3 min. for c ~ 1-9 M, (higher sensitivity) HMDE or noble metal (Pt, Au, Ag and C) Anodic stripping methods Cd Microelectrodes Microelectrode: r ~ 1 to µm r >> δ, normal electrode, short time δ >> r, UME, long time, steady state Advantage: Small current (I ~ pa to na) small IR drop no RE Capacitor charging current (I nf A) I nf faster scan Faradaic current (I f A/r) bigger contribution from I f lower DL Rate of mass transport increases steady state is established within µs faster kinetic study, higher S/N ration Little disturbance to the system under study Small sample volume Small current system with low dielectric constants, like toluene 5 µm 1 1 i = nfadca +, δ = πdt δ r 1

13 Homework 5- (a, b, c, e),