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Electroanalytical Chemistry

Electroanalytical Chemistry. Lecture #6 An Introduction to Electrochemical Methods (cont’d). Excitation. E. t. time. t o. Response. I. time. t o. Q: What Experiment is This?. Name of experiment type of excitation Response i  ____ slope Deficiency. Excitation. E. t. time.

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Electroanalytical Chemistry

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  1. Electroanalytical Chemistry Lecture #6 An Introduction to Electrochemical Methods (cont’d)

  2. Excitation E t time to Response I time to Q: What Experiment is This? • Name of experiment • type of excitation • Response • i  ____ • slope • Deficiency

  3. Excitation E t time to Response Q Qdl to time What Experiment Is This? • Name of experiment • Type of excitation • Response • Q  ____ • intercept • slope

  4. I, A Eapp, V Q: What Is This Experiment? Excitation E2 Eo X Eapp, V • Name of experiment • Excitation • Response • i  ____ • Ep ____ of  • E’ = _____________ E1 Time, s Response Ep X E1 E2 Eo

  5. I, A Eapp, V Cyclic Voltammetry (CV) Excitation E2 forward Eapp, V reverse • Important parameters: • Epa and Epc • ipc and iac • E’ • DE = |Epa - Epc| E1 Time, s Response Epa E1 E2 Epc R - ne- = O

  6. For Nernstian CV • DEp = |Epa - Epc| = 59/n mV at 250C • independent of n • Eo = (Epa + Epc)/2 • Ipc/Ipa = 1

  7. For Nernstian Process • Potential excitation controls [R]/[O] as in Nernst equation:Eapp = E0- 0.059/n log [R]/[O] • if Eapp > E0, [O] ___ [R] and ox occurs • if Eapp < E0, [O] ___ [R] and red occurs • i.e., potential excitation CONTROLS [R]/[O]

  8. Criteria for Nernstian Process • Ep independent of scan rate • ip 1/2 (diffusion controlled) • Ipc/Ipa = 1 (chemically reversible)

  9. Quasi-reversible or Irreversible • Quasi-reversible: • Ep > 59 mV and Ep increases with increasing  • iR can mascarade as QR system • Irreversible: • chemically - no return wave • slow ET - 2 waves do not overlap

  10. EXAMPLE: Electrocatalytic Oxidation of Guanine in DNA • Top: non-faradaic contribution • Bottom: shape and magnitude of redox waves P.M.Armistead; H.H.Thorp Anal. Chem.2000, 72, 3764-70.

  11. EXAMPLE: UME’s in Sol-Gels • Q: Identify the waves in the CV’s shown at left • Top: UME - slow scan rate (sigmoidal shape) • Bottom: UME - fast scan rate Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson* ; Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses, Analytical Chemistry; 2000; 72(21); 5265-5271.

  12. UME’s: Slow scan rates5 mV/s radial diffusion Fast scan rates30 V/s planar diffusion Fe3+ 0.1 m 0.1 m

  13. Radial Diffusion Redox wave: sigmoidal shape Iss = 4nFrDoCo* Iss scan rate independent  DoCo* Planar Diffusion Redox wave: normal shape Ip 1/2  Do1/2 C UME’s Radial vs. Planar Diffusion

  14. EXAMPLE: UME’s in Sol-Gels • Learn Do from CA • Obtain Co*from slow scan rate CV (Iss) Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson* ; Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses, Analytical Chemistry; 2000; 72(21); 5265-5271.

  15. EXAMPLE 2: Look Ma, No Electrolyte! 20 mV/s • [S2Mo18O62]4- + e- = [S2Mo18O62]5- + e- = [S2Mo18O62]6- • BAS 100-A • 3-electrode cell: • GC macrodisk/Pt wire/ Pt wire • ACN with no electrolyte 20 mV/s 100 mV/s Alan M. Bond,* Darren C. Coomber, Stephen W. Feldberg, Keith B. Oldham, and Truc Vu ; Analytical Chemistry; 2001; 73(2); 352-359.

  16. Applications of CV • Many organic functional groups are reducible:C=OC=CC=NN=NS-S • see Handbook of Organic Compounds

  17. Applications of CV • Many functional group are not reducible so we can derivatize these groups • convert them into electroactive groups by chemical modification • EXAMPLES: • alcohols + chromic acid = aldehyde group • phenyl + nitration = nitro group

  18. Adsorption Phenomena • Non-specifically adsorbed • No close-range interaction with electrode • Chemical identity of species not important • Specifically adsorbed • Specific short-range interactions important • Chemical identity of species important

  19. CV and Adsorption • If electroactive adsorbed species: • Ep = Eo - (RT/nF) ln (bo/bR) • ip = (n2F2/4RT) A o*  • If ideal Nernstian,Epa = Epc and Ep/2 = 90.6 mV/n at 250C 90 mV I Eapp

  20. EXAMPLE 2: Oxidation of Cysteine at BDD Nicolae Spãtaru, Bulusu V. Sarada, Elena Popa, Donald A. Tryk, and Akira Fujishima* ; Voltammetric Determination of L-Cysteine at Conductive Diamond Electrodes, Analytical Chemistry; 2001; 73(3); 514-519.

  21. Stripping Analysis or Stripping Voltammetry • 2 Flavors: • Anodic (ASV) • Good for metal cations • Cathodic (CSV) • Good for anions and oxyanions

  22. Stripping Voltammetry - Steps 1. Deposition 2. Concentration 3. Equilibration 4. Stripping

  23. Example of ASV: Determination of Pb at HDME • Deposition (cathodic) reduce Pb2+ • Stir (maximize convection) • Concentrate analyte • Stop stirring = equilibration/rest period • Scan E in anodic sense and record voltammogram • oxidize analyte (so redissolution occurs) Eapp I Ip Pb  Pb2+ + 2e-

  24. Stripping Voltammetry - Quantitation • Ip Co* • Concentrations obtained using either • Standard addition • Calibration curve

  25. HDME ASV • Usually study M with Eo more negative than Hg • EX: Cd2+, Cu2+, Zn2+, Pb2+ • Study M with Eo more positive than Hg at GC • EX: Ag+, Au+, Hg • Can analyze mixture with DEo 100 mV

  26. CSV • Anodic deposition • Form insoluble, oxidized Hg salt of analyte anion • Stir (maximize convection) • Equilibrate (stop stirring) • Scan potential in opposite sense (cathodic) • Reducing salt/film and forming soluble anion • Record voltammogram

  27. HDME CSV • Can study halides, sulfides, selenides, cyanides, molybdates, vanadates • EX: FDA 1982-1986 used to confirm CN- (-0.1 V) in Tylenol Crisis

  28. Comparison of Potential Methods • Pulse methods • Differential pulse • Good selectivity • Reason: peak shape • Square wave • Good for chromatography • Reason: Rapid response • 3 min diff. pulse expt = 30 s sq. wave expt

  29. Comparison of Potential Methods • LSV • Poorest dl (10-5 M) of any method • Reason: inability to distinguish against charging current • CV • Good for mechanistic study

  30. Comparison of Potential Methods • Stripping Voltammetry • Good for trace analysis • Reasons: lowest dl, most sensitive, good relative precision • EX: 30 min conc. of Ag+ At Hg (ASV) • detection limit = 2 pM • relative precision 2-3%

  31. Controlled Current Methods - Chronopotentiometry Excitation • Excitation: I vs. time • Constant current (step) • Linearly increasing current (ramp) • Response E vs. time I time to Response E Instrument: galvanostat time t to

  32. Frit C R W Chronopotentiometry • Experimental • 3-electrode cell • Luggin capillary • Counter isolated with frit • Working insulated against convection • Pt, Au, C, Hg pool • quiescent solution

  33. Sand Equation • Response: • Boundary condition: • I = i/A = nFD (dC/dx)x=0 = constant • Cx=0 = Co* - (2 it1/2/nFA (pDo)1/2) • So, concentration decreases linearly with t1/2

  34. Sand Equation (cont’d) • When CX=o = 0 (all O reduced):0 = Co* - (2 it1/2/nFA (pDo)1/2) • So, nFA(pDo)1/2Co*/ 2i = t1/2 • Note: 1. The larger i the smaller t 2. t < 30 s to minimize convection (natural)

  35. The Sand Equation (cont’d) • At 250C, a more useful form of the Sand equation is:i t1/2/Co* = 85.5 n Do1/2 A (mA s1/2/mM) • For 2nd component of 2-component mixture: • (n1FAD11/2p1/2 C1*/2) + (n2FAD21/2p1/2 C2*/2) = I (t1+ t2)1/2 • NB: t2 is affected by first reduction

  36. Shape of the Chronopotentiogram O + e- = R • where • when Do = DR,Et/4 = Eo E Et/4 t new rxn time dl dl

  37. Analysis in Chronopotentiometry E Slope: (RT/nF) = 0.059 V/n • Test for reversibility • Plot E vs. ln (…) • Plot it1/2 vs. I • useful diagnostic for adsorption, coupled reactions Et/4 adsorption it1/2 precedingreactions i

  38. ElectroactiveOsoln + e- = R (long t)Oads + e- = R (short t) Electroinactive Adsorption it1/2 it1/2 i i

  39. Applications • Adsorption • Coupled Chemical Electrochemical Reactions • Quantitation of mixtures of metals • Pb2+, Cd2+, Zn2+ (10-2 - 10-4 M)

  40. Advantages of Chronopotentiometry • Simpler instrumentation • No feedback from reference electrode required • Theory simpler and amenable to closed from analytical solution • Can measure higher concentrations - 0.01 M

  41. Disadvantages of Chronopotentiometry • Response waveform less well defined • Electroactive impurities that are reduced before analyte will artificially lengthen transition time and distort wave • Difficult to quantitate at low concentrations • Double layer charging currents • Often larger • Difficult to correct for since E is varying

  42. Comparison: • Which deals with double-layer capacitance and uncompensated resistance better? • LSV • Potential step voltammetry • Chronopotentiometry Jan C. Myland and Keith B. Oldham* ; Which of Three Voltammetric Methods, When Applied to a Reversible Electrode Reaction, Can Best Cope with Double-Layer Capacitance and Severe Uncompensated Resistance?, Analytical Chemistry; 2000; 72(14); 3210-3217.

  43. Comparison: • Which deals with double-layer capacitance and uncompensated resistance better? • LSV • Potential step voltammetry • Chronopotentiometry Jan C. Myland and Keith B. Oldham* ; Which of Three Voltammetric Methods, When Applied to a Reversible Electrode Reaction, Can Best Cope with Double-Layer Capacitance and Severe Uncompensated Resistance?, Analytical Chemistry; 2000; 72(14); 3210-3217.

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