VOLTAMMETRY

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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 Hg or Pt that completes circuit,
conducts e- from signal source through solution
to the working electrode Supporting electrolyte
excess of nonreactive electrolyte (alkali metal)
to conduct current
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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
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3.) Voltammetry first reported in 1922 by Czech
Chemist Jaroslav Heyrovsky (polarography).
Later given Nobel Prize 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 - reaction at the
surface of the electrode
0.0592
(aR)r(aS)s
Eelectrode E0 - log
n
(aP)p(aQ)q
Apply Potential
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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
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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
Ag counter electrode at 0.0 V
AgCl Ag Cl-
Pb2 2e- Pb EO -0.13 V
vs. NHE K e- K EO -2.93 V vs.
NHE
SCE
X M of PbCl2 0.1M KCl
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-1.0 V vs SCE
Concentration gradient created between the
surrounding of the electrode and the bulk solution
K
K
Pb2
Pb2
Pb2
K
Pb2
Pb2
K
K
K
K
K
Pb2
K
Pb2
Pb2
K
Pb2
Pb2
K
K
Pb2
K
K
K
K
Pb2
Pb2
Pb2
K
K
Pb2 migrate to the electrode via diffusion
K
Pb2
K
Pb2
Pb2
K
Pb2
Pb2
K
Pb2
Pb2
K
K
K
K
Layers of K build up around the electrode stop
the migration of Pb2 via coulombic attraction
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Mox e- Mred
At Electrodes Surface Eappl Eo -
log
Mreds
0.0592
at surface of electrode
n
Moxs
Applied potential
If Eappl Eo 0 log
Moxs Mreds
Mreds
0.0592
n
Moxs
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Apply Potential E ltlt Eo
If Eappl ltlt Eo Eappl E0 - log
Mreds gtgt Moxs
Mreds
0.0592
n
Moxs
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2.) 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 meas
ured current (i) nFADA( )
where n number of electrons in ½
cell reaction F Faradays constant A
electrode area (cm2) D diffusion
coefficient (cm2/s) of A (oxidant)
slope of curve between CMox,bulk and
CMox,s
dCA
dx
dCA
dx
dCA
dx
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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.
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Thickness of Diffusion Layer (d) i
(cox, bulk cox,s) - largest slope
(highest current) will occur if Eappl ltlt Eo
(cox,s . 0) then i (cox,
bulk 0) where k so i
kcox,bulk therefore current is
proportional to bulk concentration - also, as
solution is stirred, d decreases and i
increases
nFADox
d
nFADox
d
nFADox
d
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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
i (?A)
E½
id
Base line of residual current
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
V vs SCE
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3.) Combining Potential and Current Together
Limiting current Related to concentration
E½ at ½ i
Half-wave potential E1/2 -0.5 . E0 -
Eref E0 -0.5SCE for Mn me- M(n-m)
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4.) Types of Voltammetry a)
Polarography 1) first type of Voltammetry 2)
controlled by diffusion, eliminates
convection 3) uses dropping Hg electrode (DME)
as working electrode current varies as drop
grows then falls off
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4) Advantages of Hg drop Electrode a) high
overpotential for reduction of H 2H 2e-
H2 (g) allows use of Hg electrode at
lower potentials than indicated from
thermodynamic potentials eg. Zn2 and
Cd2 can be reduced in acidic solutions even
though Eos vs. SHE -0.403
(Cd2/Cd) -0.763 (Zn2/Zn) b) new electrode
surface is continuously generated -
independent of past samples or absorbed
impurities c) reproducible currents
quickly produced
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5) Disadvantages of Hg drop Electrode a) Ease
of Hg0 oxidation Hg0 Hg e- E0 0.4V
can not use above this potential Occurs at
even lower potentials in presence of ions that
complex Hg ex. Cl- 2Hg 2Cl- Hg2Cl2
(s) 2 e- starts at .0V b) Non-Faradaic
(charging) current - limits the sensitivity to
10-5 M - residual current is gt diffusion
current at lower concentrations c) cumbersome
to use - tends to clog causing
malfunction - causes non-uniform
potential maxima d) Hg disposal problems
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Mn ne- Hg M(Hg) amalgam
i max
i avg
The ripples are caused by the constant forming
and dropping of the mercury electrode
Half-wave potential
Residual current
6) Classic Polarography a) ½ wave potential
(E½) characteristic of Mn E0 b) height of
either average current maxima (i avg) or
top current max (i max) is analyte
concentration c) size of i max is governed by
rate of growth of DME -gt drop time (t,
sec) rate of mercury flow (m, mg/s) diffusion
coefficient of analyte (D, cm2/s) number of
electrons in process (n) analyte concentration
(c, mol/ml)
Ilkovic equation (id)max 706nD1/2m2/3t1/6c (i
d)avg 607nD1/2m2/3t1/6c
A
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d) necessary to do experiment with sample and
blank to see analyte signal vs. residual
current - residual current due to impurities
and charging (nonfaradaic) currents e)
can use method to quantitate or identify elements
in sample down to 10-5 M f) can also use
method to study reactions involving Mn as long
as reaction is reversible ex. Mn
ne- Hg M0(Hg) amalgam plus
reaction Mn xA-
MAx(n-x) (E1/2)with complex (E1/2)without
complex -0.0592/n log Kf 0.0592x/n log
A- plot of (E1/2)with complex
(E1/2)without complex vs. logA- gives slope
and intercept that can be used to give Kf
and x
Kf
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One problem with data detection in normal
polarography is that i varies over lifetime of
drop, giving variation on i over curve. One
simple way to avoid this is to sample only
current at particular time of drop life. Near
end of drop current sampled polarography
Sample i at same time interval
Easier to determine iavg, etc but limit of
detection only slightly smaller (3x)
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• 7) Pulse Polarography
• a) Instead of linear change in Eappl with time
  use step changes (pulses in Eappl)
• with time
• b) Measure two potentials 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 -gt
  standard potential for ½ reaction
• c) derivative-type polarogram
• d) Advantages

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• e) Reasons for Decrease in Limits of Detection
• - increase Faradaic current
• lt surge of current that lowers reactant
  concentration demanded by new
• potential
• not seen in classic polarography
• timescale of measurments is long compared to
  lifetime of
• momentary surge
• lt current decays to a level just sufficient to
  counteract diffusion
• lt total current gtgt diffusion current
• reducing surface layer to concentration
  demanded by Nernst equation
• - decrease in non-Faradaic current
• lt surge of non-Faradaic current also occurs as
  charge on drop increases
• current decays exponentially with time
• approaches zero near the end of the life of
  a drop
• lt measure current at end of drop lifetime
  significantly reduces non-
• Faradaic current
• lt signal-to-noise increases
• f) Can do differential pulse and square wave
  polarography on other types of electrodes

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b) Cyclic Voltammetry 1) Method used to look
at mechanisms of redox reactions in solution 2)
Looks at i vs. E response of small, stationary
electrode in unstirred solution using
triangular waveform for excitation
Cyclic voltammogram
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• Start at E gtgt E0 Mox ne- Mred
• - in forward scan, as E approaches E0 get
  current due to
• Mox ne- Mred
• lt driven by Nernst equation
• concentrations made to meet
• Nernst equation at surface
• lt eventually reach i max
• lt solution not stirred, so d grows with time
  and
• see decrease in i max
• - in reverse scan see less current as potential
  increases until reduction no longer occurs
• lt then reverse reaction takes place (if
• reversible reaction)
• lt important parameters
• Epc cathodic peak potential
• Epa anodic peak potential
• ipc cathodic peak current
• ipa anodic peak potential
• lt ipc . ipa