Controlled potential microelectrode techniques

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Controlled potential microelectrode
techniquespotential sweep methods

1. Potential sweep methods linear sweep
   voltammetry (LSV) and cyclic voltametry (CV).
2. Cyclic voltammetry is a very popular technique
   for initial electrochemical studies of new
   systems and has proven very useful in obtaining
   information about fairly complicated electrode
   reactions.
3. Signal Response

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Linear sweep voltammetry
Signal
Resulting i-E curve
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A typical LSV response curve for the reduction

• At a potential well positive of E0, only
  nonfaradaic currents flow for awhile.
• When the potential reaches the vicinity of E0,
  the reduction begins and current starts to flow.
• As the potential continues to grow more negative,
  the surface concentration of the reactant must
  drop, hence the flux to the surface and the
  current increase.
• As the potential moves past , the surface
  concentration drops to near zero and mass
  transfer of reactant to the surface reaches a
  maximum rate.
• Then it declines as the depletion effect sets in.

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Cyclic voltammetry
Cyclic potential sweep
Resulting cyclic voltammogram
(initial potential and switching potential)
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Sweep voltammogram depends on a number of factors
including

• Scan rate
• Pathway of a general electrode reaction
• Reaction rate of the rate-determining steps)
• Chemical reactivity of the electroactive species

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Scan rate

• In LSV, the potential is scanned from a lower
  limit to an upper limit
• Unit of scan rate(?) V/s or mV/s
• Effects of scan rate on charging current

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Factors affecting electrode reaction rate

• In general, the electrode reaction rate is
  governed by rates of processes such as
• Mass transfer (e.g., from the bulk solution to
  the electrode surface).
• (2) Electron transfer at the electrode surface.
• (3)Chemical reactions preceding or following the
  electron transfer.
• (4)Other surface reactions.
• ? The magnitude of this current is often limited
  by the inherent sluggishness of one or more
  reactions called rate-determining steps.

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Scan rate

• If the scan rate is altered the current
  response also changes.

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Rate-determining steps

• Here we see very clearly that when i0 is much
  greater than the limiting currents, RctltltRmt,c
  Rmt,a and the overpotential, even near Eeq, is a
  concentration overpotential. On the other hand,
  if i0 is much less than the limiting currents,
  then Rmt,c Rmt,altltRct, and the overpotential
  near Eeq is due to activation of charge transfer.

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Peak current and scan rate

• At 25?, ip is

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Nernstian (reversible) systems

• Peak current is linear with square root of scan
  rate
• No effects of scan rate on peak potential
• Reductive peak current is equal to oxidative peak
  current
• Value of peak potential difference is 58 mV/n

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Totally irreversible systems
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Voltammogram and Rate constant

• The figure below shows a series of
  voltammograms recorded at a single voltage sweep
  rate for different values of the reduction rate
  constant (kred)

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Voltammogram and reverbilitity

• The figure below shows the voltammograms for a
  quasi-reversible reaction for different values of
  the reduction and oxidation rate constants.

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Reversal techniques for the reduction

• If E? is at least 35/n mV past the cathodic peak,
  the reversal peaks all have the same general
  shapes.
• If the cathodic sweep is stopped and the current
  is allowed to decay to zero, the resulting anodic
  i-E curve is identical in shape to the cathodic
  one, but is plotted in the opposite direction on
  both the I and E axes.

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Multicomponent systems (1)

• For a two-component system this technique allows
  establishing the baseline for the second wave by
  halting the scan somewhere before the foot of the
  second wave and recording the i-t curve, and then
  repeating the experiment.
• The second run is made at the same rate and
  continues beyond the second peak.

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Multicomponent systems (2)

• For a two-component system, an alternate
  experimental approach involves stopping the sweep
  beyond Ep and allowing the current to decay to a
  small value (the concentration gradient of O is
  essentially zero near the electrode).
• Then one continues the scan and measures ip' from
  the potential axis as a baseline.

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Multistep charge transfers

• For the stepwise reduction of a substance O, the
  situation is similar but more complicated.
• In general the nature of the i-E curve depends on
  ?E0 E02-E01.
• When ? E0 lt-100 mV, two separate waves are
  observed. When ? E0 is between 0 and -100 mV,
  the individual waves are merged into a broad
  wave. When ? E0 0, a single peak with a peak
  current intermediate between those of those of
  single-step 1e and 2e reactions is found. For ?
  E0 180 mV, a single wave characteristic of a
  direct 2e reduction is observed.

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Electrode reactions with coupled homogeneous
chemical reactions

• If E represents an electron transfer at the
  electrode surface, and C represents a homogeneous
  chemical reaction.
• Classification of reactions CE reaction, EC
  reaction, Catalytic (EC') reaction, ECE reaction.

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Notes

• kf heterogeneous rate constant for oxidation
• kb heterogeous rate constant for reduction
• K equilibrium constant
• ? dimensionless homogeneous kinetic parameter,
  specific to mechanism
• DP diffusion zone, KP pure kinetic region,

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Following reaction-EC

• Note that at small values of ?,essentially
  reversible behavior is found. For large values of
  ? (in the KP region), no current is observed on
  scan reversal and the shape of the curve is
  similar to that of a totally irreversible charge
  transfer.
• In the KP region, Ep is given by

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• The figure below shows a cyclic voltammogram
  recorded for the EC reaction when the chemical
  rate constant kEC is extremely large.

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EC' mechanism
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2-hydroxyacridinone

• Electrochemical oxidation of 2-hydroxyacridinone
  was studied by cyclic voltammetry (CV),
  spectro-electrochemical methods and controlled
  potential electrolysis. The photochemical
  oxidation was also investigated.

Z. Mazerska, S. Zamponi, R. Marassi, P.
Sowinski, J. Konopa. J. Electroanal. Chem. 521
(2002) 144154
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Voltammograms

• Voltammograms were obtained at a glassy-carbon
  electrode (area 0.7 cm2). A conventional
  three-electrode electrochemical cell containing a
  platinum counter electrode (CE) and a saturated
  calomel reference electrode (SCE) was employed.
  All samples were deoxygenated by passing Ar for
  10 min. The electrodes were cleaned between runs
  by polishing with Al2O3 suspension (0.05 µM).

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Voltammograms

• On the first positive sweep one oxidation peak,
  Ia, appeared and three significantly lower peaks,
  Ic, IIc and IIIc, were formed in the reverse
  scan. On the second positive sweep new oxidation
  bands, IIIa and IIa, were observed, which seem to
  form couples with the reduction peaks, IIIc and
  IIc, respectively. The cyclic voltammograms
  recorded under various pH conditions are
  presented.

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Photochemical synthesis

• The 1 mM solution of 2-hydroxyacridinone in the
  quartz flask was exposed to the light emitted
  with the UV lamp and was stirred intensively
  during the respective period of time.
• It is demonstrated, by comparison with the
  voltammogram of the substrate, that photochemical
  product p2 was the species responsible for the
  IIIcIIIa couple.

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Adsorbed intermediates in electrode processes

• Only adsorbed O and R electro-active-nernstian
  reaction
• Only adsorbed O electroactive-irreversible
  reaction

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Electrochemical behavior of riboflavin
immobilized on different matrices
A.C. Pereira, A.S. Santos, L. T. Kubota. J.
Colloid Interface Science 265 (2003) 351358.
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Effects of Scan rate on voltammograms
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Effects of Scan rate on voltammograms
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Cyclic voltammograms of the eletrostaticallyassemb
led iron porphyrin ITO modified electrode in an
aqueous solution containing o.1 mol/L
trifluoromethanesulphonate lithium
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Structural representation of meso-tetra(4-pyridyl)
porphynato iron(III)
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Cyclic voltammograms of the NADH solutions using
(A) a bare glassy carbon electrode and (B) an
electrode modified with tetraruthenated cobalt
porphyrin
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Structural representation of the tetraruthenated
cobalt porphyrin complex
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Cyclic voltammograms of the tetraruthenated
cobalt porphyrin complex (A) and (B) the
corresponding films
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Multiclyclic voltammogram of Ru(tpp)(bpy)2
(tpp 5,10,15,20-tetraphenylporphyrin) at scan
rate of 0.2 V/s in 0.1 mol/L TBAP-dichrolomethane
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Cyclic voltammograms of the poly-Ru(tpp)(bpy)2
(tpp 5,10,15,20-tetraphenylporphyrin) deposited
on the platium electrode in 0.1 mol/L
TBAP-dichrolomethane, scan rate of (a) 100, (b)
80, (c) 60, (d) 40, (e) 20 mV/s