Electroanalytical Chemistry
Lecture #6
An Introduction to Electrochemical
Methods (cont’d)
Q,What Experiment is This?
? Name of experiment
? type of excitation
? Response
– i ? ____
– slope
? Deficiency
Excitation
Response
time
time
E
I
to
to
t
What Experiment Is This?
? Name of experiment
? Type of excitation
? Response
– Q ? ____
– intercept
– slope
Excitation
Response
time
time
E
Q
to
to
t
Qdl
Q,What Is This
Experiment?
? Name of experiment
? Excitation
? Response
– i ? ____
– Ep ____ of ?
– E’ = _____________
Time,s
E a
pp
,V
Excitation
E1
E2
Eapp,V
I,A
Ep
E1 E2
Response
Eo
Eo X
X
Cyclic
Voltammetry (CV)
? Important
parameters:
– Epa and Epc
– ipc and iac
– E’
– DE = |Epa - Epc|
Time,s
E a
pp
,V
Excitation
E1
E2
Eapp,V
I,A
Epa
Epc
E1 E2
Response
R - ne- = O
For Nernstian CV
?DEp = |Epa - Epc| = 59/n mV at 250C
– independent of ?
?Eo = (Epa + Epc)/2
?Ipc/Ipa = 1
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]
Criteria for Nernstian Process
?Ep independent of scan rate
?ip ??1/2 (diffusion controlled)
?Ipc/Ipa = 1 (chemically reversible)
Quasi-reversible or Irreversible
?Quasi-reversible:
–DEp > 59 mV and DEp increases with
increasing ?
– iR can mascarade as QR system
?Irreversible:
– chemically - no return wave
– slow ET - 2 waves do not overlap
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.
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.
UME’s,
0.1 ?m
Fe3+
Fast scan rates
30 V/s
planar diffusion
Slow scan rates
5 mV/s
radial diffusion
0.1 ?m
UME’s 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
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.
EXAMPLE 2,Look
Ma,No Electrolyte!
? [S2Mo18O62]4- + e- =
[S2Mo18O62]5- + e- =
[S2Mo18O62]6-
? BAS 100-A
? 3-electrode cell:
– GC macrodisk/Pt wire/ Pt
wire
– ACN with no electrolyteAlan M,Bond,* Darren C,Coomber,Stephen W,Feldberg,Keith B,Oldham,and Truc Vu ; Analytical
Chemistry; 2001; 73(2); 352-359,
20 mV/s
20 mV/s
100 mV/s
Applications of CV
?Many organic functional groups are
reducible:
C=O
C=C
C=N
N=N
S-S
?see Handbook of Organic Compounds
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
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
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 DEp/2 =
90.6 mV/n at 250C
Eapp
I
90 mV
EXAMPLE 2,
Oxidation of
Cysteine at BDD
Nicolae Sp?taru,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.
Stripping Analysis or Stripping
Voltammetry
?2 Flavors:
– Anodic (ASV)
?Good for metal cations
– Cathodic (CSV)
?Good for anions and oxyanions
Stripping Voltammetry - Steps
1,Deposition
2,Concentration
3,Equilibration
4,Stripping
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
Pb ? Pb2+ + 2e-
Ip
Stripping Voltammetry -
Quantitation
?Ip ? Co*
?Concentrations obtained using either
– Standard addition
– Calibration curve
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
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
HDME CSV
?Can study halides,sulfides,selenides,
cyanides,molybdates,vanadates
?EX,FDA 1982-1986 used to confirm
CN- (-0.1 V) in Tylenol Crisis
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
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
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%
Controlled Current Methods -
Chronopotentiometry
? Excitation,I vs,time
– Constant current
(step)
– Linearly increasing
current (ramp)
? Response E vs,time
Excitation
Response
time
time
I
E
to
to t Instrument,galvanostat
Chronopotentiometry
? Experimental
– 3-electrode cell
? Luggin capillary
? Counter isolated with frit
? Working insulated against
convection
– Pt,Au,C,Hg pool
? quiescent solution
CFrit WR
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
?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)
Sand Equation (cont’d)
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:
?(n1FAD11/2 p1/2 C1*/2) + (n2FAD21/2 p1/2
C2*/2) = I (t1+ t2)1/2
?NB,t2 is affected by first reduction
Shape of the Chronopotentiogram
? where
? when Do = DR,
Et/4 = Eo
time
E
t
dl dl
O + e- = R
new rxn
Et/4
?
?
?
?
?
?
?
? ?
??
t
tE
nF
RTE
2/1
2/12/1
4
ln tt
?
?
?
?
?
?
??
D
DEE
o
R
nF
RTo
2/1
4
lnt
Analysis in
Chronopotentiometry
? Test for reversibility
– Plot E vs,ln (…)
– Plot it1/2 vs,I
? useful diagnostic for
adsorption,coupled
reactions
Et/4
?
?
?
?
?
?
?
? ?
t
t
2/1
2/12/1
ln t
Slope,
(RT/nF) =
0.059 V/n
E
i
it1/2 adsorption
preceding
reactions
Adsorption
? Electroactive
Osoln + e- = R (long t)
Oads + e- = R (short t)
? Electroinactive
it1/2 it1/2
i i
Applications
?Adsorption
?Coupled Chemical Electrochemical
Reactions
?Quantitation of mixtures of metals
– Pb2+,Cd2+,Zn2+ (10-2 - 10-4 M)
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
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
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.
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.
Lecture #6
An Introduction to Electrochemical
Methods (cont’d)
Q,What Experiment is This?
? Name of experiment
? type of excitation
? Response
– i ? ____
– slope
? Deficiency
Excitation
Response
time
time
E
I
to
to
t
What Experiment Is This?
? Name of experiment
? Type of excitation
? Response
– Q ? ____
– intercept
– slope
Excitation
Response
time
time
E
Q
to
to
t
Qdl
Q,What Is This
Experiment?
? Name of experiment
? Excitation
? Response
– i ? ____
– Ep ____ of ?
– E’ = _____________
Time,s
E a
pp
,V
Excitation
E1
E2
Eapp,V
I,A
Ep
E1 E2
Response
Eo
Eo X
X
Cyclic
Voltammetry (CV)
? Important
parameters:
– Epa and Epc
– ipc and iac
– E’
– DE = |Epa - Epc|
Time,s
E a
pp
,V
Excitation
E1
E2
Eapp,V
I,A
Epa
Epc
E1 E2
Response
R - ne- = O
For Nernstian CV
?DEp = |Epa - Epc| = 59/n mV at 250C
– independent of ?
?Eo = (Epa + Epc)/2
?Ipc/Ipa = 1
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]
Criteria for Nernstian Process
?Ep independent of scan rate
?ip ??1/2 (diffusion controlled)
?Ipc/Ipa = 1 (chemically reversible)
Quasi-reversible or Irreversible
?Quasi-reversible:
–DEp > 59 mV and DEp increases with
increasing ?
– iR can mascarade as QR system
?Irreversible:
– chemically - no return wave
– slow ET - 2 waves do not overlap
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.
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.
UME’s,
0.1 ?m
Fe3+
Fast scan rates
30 V/s
planar diffusion
Slow scan rates
5 mV/s
radial diffusion
0.1 ?m
UME’s 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
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.
EXAMPLE 2,Look
Ma,No Electrolyte!
? [S2Mo18O62]4- + e- =
[S2Mo18O62]5- + e- =
[S2Mo18O62]6-
? BAS 100-A
? 3-electrode cell:
– GC macrodisk/Pt wire/ Pt
wire
– ACN with no electrolyteAlan M,Bond,* Darren C,Coomber,Stephen W,Feldberg,Keith B,Oldham,and Truc Vu ; Analytical
Chemistry; 2001; 73(2); 352-359,
20 mV/s
20 mV/s
100 mV/s
Applications of CV
?Many organic functional groups are
reducible:
C=O
C=C
C=N
N=N
S-S
?see Handbook of Organic Compounds
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
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
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 DEp/2 =
90.6 mV/n at 250C
Eapp
I
90 mV
EXAMPLE 2,
Oxidation of
Cysteine at BDD
Nicolae Sp?taru,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.
Stripping Analysis or Stripping
Voltammetry
?2 Flavors:
– Anodic (ASV)
?Good for metal cations
– Cathodic (CSV)
?Good for anions and oxyanions
Stripping Voltammetry - Steps
1,Deposition
2,Concentration
3,Equilibration
4,Stripping
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
Pb ? Pb2+ + 2e-
Ip
Stripping Voltammetry -
Quantitation
?Ip ? Co*
?Concentrations obtained using either
– Standard addition
– Calibration curve
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
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
HDME CSV
?Can study halides,sulfides,selenides,
cyanides,molybdates,vanadates
?EX,FDA 1982-1986 used to confirm
CN- (-0.1 V) in Tylenol Crisis
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
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
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%
Controlled Current Methods -
Chronopotentiometry
? Excitation,I vs,time
– Constant current
(step)
– Linearly increasing
current (ramp)
? Response E vs,time
Excitation
Response
time
time
I
E
to
to t Instrument,galvanostat
Chronopotentiometry
? Experimental
– 3-electrode cell
? Luggin capillary
? Counter isolated with frit
? Working insulated against
convection
– Pt,Au,C,Hg pool
? quiescent solution
CFrit WR
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
?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)
Sand Equation (cont’d)
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:
?(n1FAD11/2 p1/2 C1*/2) + (n2FAD21/2 p1/2
C2*/2) = I (t1+ t2)1/2
?NB,t2 is affected by first reduction
Shape of the Chronopotentiogram
? where
? when Do = DR,
Et/4 = Eo
time
E
t
dl dl
O + e- = R
new rxn
Et/4
?
?
?
?
?
?
?
? ?
??
t
tE
nF
RTE
2/1
2/12/1
4
ln tt
?
?
?
?
?
?
??
D
DEE
o
R
nF
RTo
2/1
4
lnt
Analysis in
Chronopotentiometry
? Test for reversibility
– Plot E vs,ln (…)
– Plot it1/2 vs,I
? useful diagnostic for
adsorption,coupled
reactions
Et/4
?
?
?
?
?
?
?
? ?
t
t
2/1
2/12/1
ln t
Slope,
(RT/nF) =
0.059 V/n
E
i
it1/2 adsorption
preceding
reactions
Adsorption
? Electroactive
Osoln + e- = R (long t)
Oads + e- = R (short t)
? Electroinactive
it1/2 it1/2
i i
Applications
?Adsorption
?Coupled Chemical Electrochemical
Reactions
?Quantitation of mixtures of metals
– Pb2+,Cd2+,Zn2+ (10-2 - 10-4 M)
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
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
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.
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.
