Electroanalytical Chemistry

1
Electroanalytical Chemistry

• Lecture 6
• An Introduction to Electrochemical Methods
  (contd)

2
Q What Experiment is This?

• Name of experiment
• type of excitation
• Response
• i ? ____
• slope
• Deficiency

3
What Experiment Is This?

• Name of experiment
• Type of excitation
• Response
• Q ? ____
• intercept
• slope

4
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
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 equationEapp E0- 0.059/n log R/O
• if Eapp gt E0, O ___ R and ox occurs
• if Eapp lt 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 gt 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 UMEs in Sol-Gels

• Q Identify the waves in the CVs 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
UMEs
Slow scan rates5 mV/s radial diffusion
Fast scan rates30 V/s planar diffusion
Fe3
0.1 ?m
0.1 ?m
13
UMEs Radial vs. Planar Diffusion

• 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

14
EXAMPLE UMEs in Sol-Gels

• Learn Do from CA
• Obtain Cofrom 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

• S2Mo18O624- e- S2Mo18O625- e-
  S2Mo18O626-
• 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
  reducibleCOCCCNNNS-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
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)x0 constant
• Cx0 Co - (2 it1/2/nFA (pDo)1/2)
• So, concentration decreases linearly with t1/2

34
Sand Equation (contd)

• When CXo 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 lt 30 s to minimize convection (natural)

35
The Sand Equation (contd)

• At 250C, a more useful form of the Sand equation
  isi t1/2/Co 85.5 n Do1/2 A (mA s1/2/mM)
• For 2nd component of 2-component mixture
• (n1FAD11/2 p1/2 C1/2) (n2FAD21/2 p1/2 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
Et/4

• Test for reversibility
• Plot E vs. ln ()
• Plot it1/2 vs. I
• useful diagnostic for adsorption, coupled
  reactions

adsorption
it1/2
precedingreactions
i
38
Adsorption

• ElectroactiveOsoln e- R (long t)Oads e-
  R (short t)

• Electroinactive

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.