Electroanalytical Chemistry

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 (contd)

• 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 doesnt 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 (CMEs)

12
Reference Electrodes

• Purpose provide stable potential against which
  other potentials can be reliably measured
• Criteria
• stable (time, temperature)
• reproducible (you, me)
• potential shouldnt be altered by passage of
  small current not polarizable
• easily constructed
• convenient for use

13
SHE

• 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

14
Practical Reference Electrodes

• Aqueous
• SCE
• Ag/AgCl

• Nonaqueous
• Ag/Ag
• pseudoreferences
• Pt, Ag wires
• Ferrocene

15
SCE

• Cl-(aq)/Hg2Cl2/Hg(l)
• Hg22 2e- 2Hg(l)
• E0 0.24 V vs. SHE _at_ 250C

• Disadvantages
• Hg toxic
• solubility of KCl temperature dependent dE/dT
  -0.67 mV/K (must quote temperature)

• Advantages
• Most polarographic data refd to SCE

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16
Ag/AgCl

• Ag wire coated with AgCl(s), immersed in NaCl or
  KCl solution
• Ag e- Ag(s)
• E0 0.22 V vs. SHE _at_ 250C

• Disadvantages
• solubility of KCl/NaCl temperature dependent
  dE/dT -0.73 mV/K (must quote temperature)

• Advantages
• chemical processing industry has standardized on
  this electrode
• convenient
• rugged/durable

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17
Ag/Ag

• 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

• Advantages
• Most widely used
• Easily prepared
• Works well in all aprotic solvents
• THF, CAN, DMSO, DMF

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18
Pseudo-References

• Pt or Ag wire (inert)
• Ideain 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
• Dont 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

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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 lt 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

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28
Ew Ecell - iRcell - Epolarization

• When is iR large?
• I is high, I gt 10 ?A
• large electrodes
• solvents with low conductivity
• relatively polar organic solvents

29
Two Common Configurations

• 2-electrode cell
• iR must be small lt 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

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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
Grahame Model

• 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

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 ( gt 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

50
EXAMPLE

• Calculate the molar conductivity of KCl and BaCl2
• LKCl lK lCl- (74 77) x 10-4 S m2/mol
  151 x 10-4 S m2/cm
• LBaCl2 lBa2 2lCl- (127 (277)) x 10-4 S
  m2/mol 281 x 10-4 S m2/mol

51
Transference/Transport Numbers

• Useful measure of how much of the current charge
  is carried by cations vs. anions
• t c L/ LCcAa
• t- a- L-/ LCcAa
• where cL aL- LCcAa
• and t t- 1

52
EXAMPLE

• Calculate transference numbers for ions in KCl
  and BaCl2 (two good electrolytes)
• t (KCl) 74/151 0.49
• t (BaCl2) 127/281 0.45
• Observation cations and anions carry current
  equally well in good electrolytes

53
Reminders

• Solvent and concentration affect ionization and
  therefore ionic conductivity and transference
  numbers
• t (KClaq) 0.49 (just calculated)
• t (KCl/DMF) 0.36in DMF lK 31 lCl- 55

Taken from Table 2.3.1 in Bard, A. Faulkner, L.
Electrochemical Methods Wiley New York, 1980.
54
Ionic Mobility

• Measure of ions velocity in presence of applied
  electric field (units m2/Vs)
• where F is Faradays constant (96,485 C/mol) and
  z is the charge on the ith ion

55
EXAMPLE

• Calculate the ionic mobility of Ba2
• u (Ba2) l/2F (127 x 10-4 S m2/mol)/(2
  96,485 C/mol) 6.6 x 10-8 m2/Vs
• Note on units C J/V

56
Frictional Coefficients

• When ions move through solution they are subject
  to a frictional drag force
• where e is the charge on an electron (1.6 x
  10-19 C)