Transcript Voltametry Seminar

VOLTAMMETRY
A.) Comparison of Voltammetry to Other Electrochemical Methods
1.) Voltammetry: electrochemical method in which information about an analyte is
obtained by measuring current (i) as a function of applied potential
- only a small amount of sample (analyte) is used
Instrumentation – Three electrodes in solution
containing analyte
Working electrode: microelectrode whose potential
is varied with time
Reference electrode: potential remains constant
(Ag/AgCl electrode or calomel)
Counter electrode: electrode (Hg, Pt, C) that
completes circuit, conducts e- from signal source
through solution to the working electrode
Supporting electrolyte: excess of nonreactive
electrolyte to conduct current
−
−
NaCl or KCl in aqueous solvent
Tetrabutylammonium hexafluorophosphate (TBAPF6)
in non-aqueous solvent (e.g., MeCN, DMSO, DMF)
VOLTAMMETRY
Instrumentation – Three electrodes in solution containing analyte
Apply Linear Potential with Time
Observe Current Changes with Applied Potential
2.) Differences from Other Electrochemical Methods
a) Potentiometry: measure potential of sample or system at or near zero
current.
voltammetry – measure current as a change in potential
b) Coulometry: use up all of analyte in process of measurement at fixed current
or potential
voltammetry – use only small amount of analyte while vary potential
3.) Voltammetry first reported in 1922 by Czech Chemist Jaroslav Heyrovsky
(The Father of Polarography) Later given Nobel Prize (1959) for method.
B.) Theory of Voltammetry
1.) Excitation Source: potential set by instrument (working electrode)
- establishes concentration of Reduced and Oxidized Species at electrode
based on Nernst Equation:
0.0592
(aR)r(aS)s …
Eelectrode =
log
n
(aP)p(aQ)q …
- reaction at the surface of the electrode
E0
Apply
Potential
Current is just measure of rate at which species can be brought to electrode surface
Two methods:
Stirred - hydrodynamic voltammetry
Unstirred - polarography (dropping Hg electrode)
Three transport mechanisms:
(i) migration – movement of ions through solution by electrostatic attraction to
charged electrode
(ii) convection – mechanical motion of the solution as a result of stirring or flow
(iii) diffusion – motion of a species caused by a concentration gradient
Voltammetric analysis
 Analyte selectivity is provided by the applied potential on the working electrode.
 Electroactive species in the sample solution are drawn towards the working electrode
where a half-cell redox reaction takes place.
 Another corresponding half-cell redox reaction will also take place at the counter
electrode to complete the electron flow.
 The resultant current flowing through the electrochemical cell reflects the activity (i.e.
 concentration) of the electroactive species involved
Pt working
electrode at -1.0
V vs SCE
Pb2+ + 2e-
Pb
EO = -0.13 V vs. NHE
K+ + e-
K
EO = -2.93 V vs. NHE
AgCl
SCE
X M of PbCl2
0.1M KCl
Ag counter
electrode at
0.0 V
Ag + Cl-
Pb2+ + 2e-
Concentration gradient created
between the surrounding of the
electrode and the bulk solution
-1.0 V vs SCE
Pb
K+
Pb2+
K+
K+
K+
K+
Pb2+
Pb2+
Pb2+
Pb2+
K+
Pb2+
K+
K+
K+
Pb2+
K+
Pb2+
K+
Pb2+
K+
K+
Pb2+
K+
Pb2+
Pb2+
K+
Pb2+
K+
K+
K+
K+
K+
2+
Pb
K+Pb2+ migrate to
the electrode
via diffusion Pb2+
Pb2+ K+
Pb2+
K+
K+
Pb2+
Pb2+
K+
Pb2+
K+
Pb2+
Layers of K+ build up around the electrode stop the
migration of Pb2+ via coulombic attraction
C) Types of Voltammetry
1. Polarography
−
first type of Voltammetry
−
controlled by diffusion, eliminates convection
−
uses dropping Hg electrode (DME) as working electrode; current
varies as drop grows then falls off
a. Advantages of Hg Drop Electrode
−
High overpotential for reduction of H+
2H+ + 2e-
H2 (g)
(0V vs NHE)
• Allows use of Hg electrode at lower potentials than indicated from
thermodynamic potentials
• Example:
Zn2+ and Cd2+ can be reduced in acidic solutions even though E0 vs
NHE = -0.403 (Cd2+/Cd) and -0.763 (Zn2+/Zn)
−
new electrode surface is continuously generated
•
−
Independent of past samples or absorbed impurities
reproducible currents quickly produced
b. Disadvantages of Hg Drop Electrode
−
Hg oxidation
• Around +0.25 V vs. SCE
• Can not be used above a potential of +0.25 V
 Hg undergoes anodic dissolution ~ +0.25 V vs. SCE and is oxidized
to insoluble Hg2Cl2 in presence of Cl- at zero V vs. SCE.
 It cannot be used for anodic oxidation above +0.25 V vs. SCE.
−
−
Non-Faradaic (charging/capacitance) current
•
limits the sensitivity to ~ 10-5 M
•
residual current is > diffusion current at lower concentrations
cumbersome to use
•
−
tends to clog, causing malfunction
Hg disposal problems
•
mercury vapors are also very poisonous
2. Voltammetry (solid working electrode)
Pb2+ + 2eK+ + e-
Pb
K
EO = -0.13 V vs. NHE
EO = -2.93 V vs. NHE
Note:
•
•
•
Reference Electrode: SCE (saturated calomel electrode)
SCE = + 0.24 V vs NHE
Thus, the Eo of Pb2+ = -0.37 V vs SCE
Pt working electrode
Ag counter
AgCl
SCE
X M of PbCl2
0.1M KCl
Ag+ + Cl-
At Electrodes Surface:
Mox + e-  Mred
Eappl = Eo - 0.0592
n
log
[Mred]s
at surface of electrode
[Mox]s
Applied potential
If Eappl = Eo:
0.0592
0=
log
n
[Mred]s
[Mox]s
[Mox]s = [Mred]s
If Eappl << Eo:
Eappl = E0 - 0.0592 log
n
\ [Mred]s >> [Mox]s
[Mred]s
[Mox]s
Current generated at electrode by this process is proportional to concentration at
surface, which in turn is equal to the bulk concentration
For a planar electrode:
measured current (i) = nFADA( dCA )
dx
where:
n = number of electrons in ½ cell reaction
F = Faraday’s constant
A = electrode area (cm2)
D = diffusion coefficient (cm2/s) of A (oxidant)
dCA
dx
= slope of curve between CMox,bulk and CMox,s
dCA
dx
As time increases, push banding further and further out.
Results in a decrease in current with time until reach point where convection of analyte
takes over and diffusion no longer a rate-limiting process.
Thickness of Diffusion Layer (d):
i = nFADox (cox, bulk – cox,s)
d
- largest slope (highest current) will occur if:
Eappl << Eo (cox,s .
then
0)
nFADox
i=
where:
k=
d
(cox, bulk – 0)
nFADox
d
so:
i = kcox,bulk
therefore:
current is proportional to bulk concentration
- also, as solution is stirred, d decreases and i increases
Potential applied on the working electrode is usually swept over (i.e. scan)
a pre-defined range of applied potential
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+
i (A)
+
2e-
Cd
Current starts to be registered at the
electrode
E½
Working electrode is
no yet capable of
reducing Cd2+ 
only small residual
current flow through
the electrode
-0.2
-0.4
All Cd2+ around the electrode has
already been reduced. Current at
the electrode becomes limited by
the diffusion rate of Cd2+ from the
bulk solution to the electrode.
Thus, current stops rising and
levels off at a plateauid
Current at the working
electrode continue to rise as
the electrode become more
Base line
reducing and more Cd2+
of residual
around the electrode are being
current
reduced. Diffusion of Cd2+
does not limit the current yet
-0.6
V vs SCE
-0.8
-1.0
-1.2
-1.4
Combining Potential and Current Together
Limiting current
Related to concentration
E½ at ½ i
Half-wave potential : E1/2 = -0.5 
E0 - Eref
E0 = -0.5 + SCE for Mn+ + me- ↔ M(n-m)+
Voltammograms for Mixtures of Reactants
[Fe3+]=1x10-4M
[Fe2+]=0.5x10-4M
[Fe3+]=0.5x10-4M
0.1V
0.2V
Two or more species are observed in
voltammogram if difference in separate
half-wave potentials are sufficient
[Fe2+]=1x10-4M
Different concentrations result in
different currents, but same potential
Amperometric Titrations
-Measure equivalence point if analyte or reagent are oxidized or reduced at working electrode
- Current is measured at fixed potential as a function of reagent volume
• endpoint is intersection of both lines
endpoint
Only analyte is reduced
endpoint
Only reagent is reduced
endpoint
Both analyte and reagent
are reduced
3. Anodic Stripping Voltammetry (ASV)
- Analyte first deposited (reduced) onto the
working electrode from a stirred solution
-“deposit” analyte for a known period of time
- analyte is redissolved or stripped (oxidized)
from the electrode
- analyte “preconcentrated” onto electrode, thus
ASV yields lowest detection limit among all
voltammetric techniques
4. Pulse Voltammetry
a) Instead of linear change in Eappl with time use step changes (pulses in Eappl) with time
b) Measure two currents at each cycle
- S1 before pulse & S2 at end of pulse
- plot Di vs. E (Di = ES2 – ES1)
- peak height ~ concentration
- for reversible reaction, peak potential standard potential for ½ reaction
E0
c) differential-pulse voltammetry
concentration
d) Advantages:
- can detect peak maxima differing by as little as 0.04 – 0.05 V
< 0.2V peak separation for normal voltammetry
- decrease limits of detection by 100-1000x compared to normal voltammetry
< 10-7 to 10-8 M
5. Cyclic Voltammetry
a. Method used to look at mechanisms of redox reactions in solution.
b. Looks at i vs. E response of small, stationary electrode in unstirred
solution using triangular waveform for excitation
Cyclic voltammogram
(solution phase redox species)
Segment 1
Segment 2
Mox + ne-
Mred
Fe3+ + e-
Fe2+
- in forward scan, as E approaches E0’ , current flow due to
Mox + neMred
- governed by Nernst equation
• concentrations made to meet Nernst equation at surface
- eventually reach i max
- solution not stirred, so d grows with time, leads to
decrease in i max
- in reverse scan - see less current as potential increases until
reduction no longer occurs
- then reverse reaction takes place (if reaction is
reversible)
- important parameters
- Epc – cathodic peak potential
- Epa – anodic peak potential
- ipc – cathodic peak current
- ipa – anodic peak current
ipc ~ ipa (or
1pc/1pa ~ 1)
Δ Ep = (Epa – Epc) = 0.0592 V / n
n = number of electrons involved in the reaction
Formal reduction potential
Eo’ (E1/2) = = (Epa + Epc ) / 2
Important Quantitative Information
< ipc . ipa
< DEp = (Epa – Epc) = 0.0592/n,
where n = number of electrons in reaction
< E0 = midpoint of Epa  Epc
<ip = 2.686x105n3/2AcD1/2v1/2 (Randles-Sevcik
eqn)
- ip: peak current (A)
- n: number of electrons
- A: electrode area (cm2)
- c: concentration (mol/cm3)
- v: scan rate (V/s)
- D: diffusion coefficient (cm2/s)
Thus,
- can calculate standard potential for half-reaction
- number of electrons involved in half-reaction
- diffusion coefficients
- if reaction is reversible
Cyclic Voltammogram is a good way to determine diffusion coefficient
Reduction
Oxidation
+
N
F
_
B
N
F
Laser Dye (PM 567)
Lai and Bard, J. Phys. Chem. B, 2003, 107, 5036-5042.
ip = 7.422 x 10-6 A
n =1
A = 0.0314 cm2
C = 1 x 10-6 mol/cm3
D = diffusion coefficient
(cm2/s)
v = 0.05 V/s
D = 1.55 x 10-5 cm2/s