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Electroanalytical Chemistry

Electroanalytical Chemistry. Lecture #2. An Interfacial Process. For: O + ne - = R 5 separate events must occur: O must be successfully transported from bulk solution (mass transport) O must adsorb transiently onto electrode surface (non-faradaic)

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Electroanalytical Chemistry

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  1. Electroanalytical Chemistry Lecture #2

  2. An Interfacial Process • For: O + ne- = R • 5 separate events must occur: • O must be successfully transported from bulk solution (mass transport) • O must adsorb transiently onto electrode surface (non-faradaic) • CT must occur between electrode and O (faradaic) • R must desorb from electrode surface (non-faradaic) • R must be transported away from electrode surface back into bulk solution (mass transport)

  3. What is an Electrode? • Electrical double layer

  4. Electrode Classification • Based on the nature and number of phases between which electron transfer occurs • 3 Classes: • Electrodes of the First Kind • Electrodes of the Second Kind • Electrodes of the Third Kind

  5. Electrode of the First Kind • Metal in contact with its cations or non-metal in contact with its anions • EXAMPLES: • Cu2+ /Cu(s) • Zn2+/Zn(s) • SHE • Ag+/Ag (nonaqueous reference electrode) • Cl-/Cl2(g)/Pt Electrodes in Daniell cell

  6. Electrode of the First Kind (cont’d) • Electrode response given by Nernst equation (Nernstian): • E = E0 + (RT/nF) ln a(M2+) • NOTE: Fe, Al, and W electrodes are NOT electrodes of the First Kind • these have relatively thick surface oxide coatings

  7. Electrode of the Second Kind • Metal in contact with sparingly soluble salt of the metal • Common name: anion electrodes • EXAMPLES: • Ag/AgCl(s) • Hg/Hg2Cl2(s)/Cl- (saturated calomel electrode; SCE)

  8. Electrode of the Second Kind • Electrode response given by: • E = E0 - (RT/F) ln a(Cl-) • NOTES: • anion activity determines potential • make great reference electrodes because of low solubility of salt (potential very stable)

  9. The Calomel Reference Electrode Note: concentrations typically high  concentrations small  electrode doesn’t become polarized  potential constant

  10. Electrode of the Third Kind • Electrodes that merely serve as sources or sinks for electrons • Common names: redox, inert, unattackable • EXAMPLES: • metals: Pt, Au, GC, graphite, HOPG, Hg • semiconductors: Si, GaAs, In-SnO2/glass • Response: • for Pt in contact with Fe2+, Fe3+ in solution: • E = E0- 0.059 (V) log ([Fe2+]/[Fe3+])

  11. Electrode of the Fourth Kind • Electrodes that cannot be classified as 1-3 • EXAMPLES: • Chemically modified electrodes (CME’s)

  12. Reference Electrodes • Purpose: provide stable potential against which other potentials can be reliably measured • Criteria: • stable (time, temperature) • reproducible (you, me) • potential shouldn’t be altered by passage of small current = not polarizable • easily constructed • convenient for use

  13. Advantages International standard E0 0 V One of most reproducible potentials + 1 mV Disadvantages Convenience Pt black easily poisoned by organics, sulfide, cyanide, etc. Hydrogen explosive Sulfuric and hydrochloric strong acids SHE

  14. Aqueous SCE Ag/AgCl Nonaqueous Ag+/Ag pseudoreferences Pt, Ag wires Ferrocene Practical Reference Electrodes

  15. Cl-(aq)/Hg2Cl2/Hg(l) Hg22+ + 2e- = 2Hg(l) E0 = 0.24 V vs. SHE @ 250C Disadvantages Hg toxic solubility of KCl temperature dependent dE/dT = -0.67 mV/K (must quote temperature) SCE Advantages • Most polarographic data ref’d to SCE From BAS www-site: http://www.bioanalytical.com/

  16. Ag wire coated with AgCl(s), immersed in NaCl or KCl solution Ag+ + e- = Ag(s) E0 = 0.22 V vs. SHE @ 250C Disadvantages solubility of KCl/NaCl temperature dependent dE/dT = -0.73 mV/K (must quote temperature) Ag/AgCl Advantages • chemical processing industry has standardized on this electrode • convenient • rugged/durable From BAS www-site: http://www.bioanalytical.com/

  17. Ag+ + e-= Ag(s) requires use of internal potential standard Disadvantages Potential depends on solvent electrolyte (LiCl, TBAClO4, TBAPF6, TBABF4 Care must be taken to minimize junction potentials Ag+/Ag Advantages • Most widely used • Easily prepared • Works well in all aprotic solvents: • THF, CAN, DMSO, DMF From BAS www-site: http://www.bioanalytical.com/

  18. Pseudo-References • Pt or Ag wire (inert) • Idea:in medium of high resistance, low conductivity, wire will assume reasonably steady, highly reproducible potential (+20 mV) • Advantage: no solution contamination • Limitation: must use internal potential standard (ferrocene)

  19. Can Aqueous References Be Used in Nonaqueous Media? • Yes with caution! • May be significant junction potentials • Requires use of internal standard • May be greater noise • Electrolyte may precipitate/clog electrode frit • Don’t forget about your chemistry • Chemistry may be water sensitive

  20. Electrodes • Metal • solid • Pt, Au, Ag, C • liquid • dropping mercury electrode (DME) • Semiconductors • Si, GaAs • In-SnO2/glass (optically transparent)

  21. Carbon • Paste • With nujol (mineral oil) • Glassy carbon (GC) • Amorphous • Pyrolytic graphite - more ordered than GC • Basal Plane • Edge Plane (more conductive)

  22. Electrode Materials • Different Potential Windows • Can affect electron transfer kinetics

  23. Electrodes • Size • Analytical macro • 1.6 - 3 mm diameter • Micro • 10-100 m diameter From BAS www-site: http://www.bioanalytical.com/

  24. Electrode Geometry Geometry is critical and affects how the data are analyzed and interpreted • Disk • area: r2 • wire (cylinder) • area: l(2 r) r2 • Mesh • optically transparent • Sheet Note: Geometric area < effective surface area

  25. Cleanliness IS Next to Godliness in Electrochemistry • Working electrode must be carefully cleaned before each experiment • Mechanical • Abrasion with alumina or “diamond” polish • Chemical • Sonicate in Alconox • Soak in HNO3 • Electrochemical • Cycle in 0.5 M H2SO4 (Pt)

  26. Electrochemical Cleaning Taken from Table 4-7 in Sawyer, D.T.; Roberts, Jr., J.L. Experimental Electrochemistry for Chemists Wiley: New York, 1976.

  27. Counter Electrode • Area must be greater than that of working • Usually long Pt wire (straight or coiled) or Pt mesh (large surface area) • No special care required for counter From BAS www-site: http://www.bioanalytical.com/

  28. Ew = Ecell - iRcell - Epolarization • When is iR large? • I is high, I > 10 A • large electrodes • solvents with low conductivity • relatively polar organic solvents

  29. Two Common Configurations • 2-electrode cell • iR must be small < 1 mV (microelectrodes) • 3-electrode cell • Avoids internal polarization of reference electrode • Compensates for major potion of cell iR drop From BAS www-site: http://www.bioanalytical.com/

  30. 2-Electrode Cell • 2-electrode cell • Working • Reference electrode • Current passed between working and reference

  31. 3-Electrode Cell • 3-electrode cell • Working • Reference • Counter/auxilliary • Current is passed between working and counter • High impedance placed in front of reference (low current) so ref. Potential constant

  32. Potentiostat/Galvanostat • Potentiostat • Control potential • Cyclic voltammetry, chronoamperometry, etc. • Galvanostat • Control current • Potentiometry From BAS www-site: http://www.bioanalytical.com/

  33. Evolution of the Electrode Double Layer Models • Time-Line: • Helmholtz 1879 • Guoy-Chapman 1910-13 • Stern 1924 • Grahame 1947

  34. + + + + _ _ _ _ Helmholtz Model • Interface between electrolyte solution and electrode behaves like a capacitor in its ability to store charge Potential dies off sharply as we move away from electrode solution  electrode Distance from electrode Double Layer

  35. Helmholtz Model • Double charge layer = electrically neutral interface • Defects: • No interactions occur further away from first layer of adsorbed ions • [Electrolyte] - no effect

  36. Guoy-Chapman Model • Idea: Diffuse double layer - Double layer not compact but of variable thickness with ions free to move • Accounts for effects of applied potential and [electrolyte] Potential dies off exponentially as we move away from electrode  Distance from electrode

  37. + + + + + + + _ _ _ _ _ _ _ Stern Model • Combination of Helmholtz and Guoy Chapman models Bulk solution electrode Compact Layer Diffuse Layer

  38. Specifically adsorbed ions are desolvated, approach electrode surface closer, and feel greater potential 3 region model IHP - Inner Helmholtz Plane passes through center of specifically adsorbed ions OHP - Outer Helmholtz Plane passes through solvated and non-specifically adsorbed ions + + + + + + _ _ _ _ _ _ _ Grahame Model Bulk solution electrode IHP OHP

  39. Au/water • Pzc (potential of zero charge) 0.18 V • E negative of pzc  excess negative charge (electrostatic interactions possible) • Normally hydrophobic • has strong affinity for organic contaminants • Clean surface hydrophilic • wettability

  40. Junction Potential • Electrical potential that develops at the interface between two solutions • Since we isolate reference electrode from working by frit 1 or more junction potentials exist in cell Ew = Ecell - Ejunction

  41. Mass Transport • 3 Modes: • Diffusion • Migration • Convection • Natural • Mechanical • Movement of mass described by Nernst-Planck equation

  42. Diffusion • Movement of mass due to a concentration gradient • Occurs whenever there is chemical change at a surface, e.g., O  R

  43. Migration • Movement of a charged species due to a potential gradient • Opposites attract • Mechanism by which charge passes through electrolyte

  44. Convection • Movement of mass due to a natural or mechanical force • at long times ( > 10 s), diffusing ions set up a natural eddy of matter

  45. Movement of Ions in Solution • Can be described in 3 equivalent ways: • Molar ionic conductivity, i (electrochemistry) • Ionic mobility, ui (separations) • Frictional coefficients, fi (industry/engineering)

  46. Molar Ionic Conductivity • So, short and fat better than long and slender • units: S m2/mol Taken from Table 2.3.2 in Bard, A.; Faulkner, L. Electrochemical Methods Wiley: New York, 1980.

  47. Questions • What size conductivities do electrolyte ions have? • How do the cation and anion conductivities compare in electrolytes? Taken from Table 2.3.2 in Bard, A.; Faulkner, L. Electrochemical Methods Wiley: New York, 1980.

  48. Questions • Have we made any assumptions about concentration and ionization? • Will the conductivity of ions be the same in different solvents? Taken from Table 4-7 in Sawyer, D.T.; Roberts, Jr., J.L. Experimental Electrochemistry for Chemists Wiley: New York, 1976.

  49. Molar Ionic Conductivity • at infinite dilution, no interionic interactions, somolar conductivity of salt =  ion molar conductivity

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