Kompleksni soedinenija

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Square-wave voltammetry the most advanced
electroanalytical technique
Valentin Mirceski Institute of Chemistry
Faculty of Natural Sciences and Mathematics Ss
Cyril and Methodius University, Skopje Republic
of Macedonia
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Square-Wave Voltammetry Potential Modulation
Red ? Ox e
DE

E / V
Esw
E / V

t
Ox e ? Red
t / s
t / s
A single potential cycle consisting of a two
equal potential pulses superimposed on a single
potential tread in two opposite (anodic and
cathodic) directions. The current is measured at
the end of each pulse in order to discriminate
against the capacitate current and to extract
only the faradic response of the electrode
reaction. Properties of the potential modulation
are Esw SW amplitude (pulse height) DE
potential step t duration of a single
potential cycle f - frequency of the pulses.
Square-wave voltammetry (SWV) is a pulsed
voltammetric technique. The potential modulation
consists of a train of equal potential pulses
superimposed on a staircase potential ramp.
f 1/t v DE f
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Variation of the current with the time in the
course of the experiment
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Faradaic vs. capacitive current in the course of
a single potential pulse
Faradaic current I f (due to electrode reaction)
If / Ic gtgt 1 (sampling point)
I
Capacitive current, Ic (due to charging -
formation of the double layer)
t
0
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SW voltammogram
Net component, calculated (not measured!) as a
difference between the forward and backward
components
Forward component measured at the end of each
pulse with odd serial number (i.e., 1st, 3rd,
etc.
Backward component measured at the end of each
pulse with even serial number (i.e., 1st, 3rd,
etc.
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Time window of the voltammetric experiment

• SWV
• Scan rate v f DE
• Example
• DE 0.1 mV, f 200 Hz
• v 0.020 V/s
• t 1/f
• 5 ms
• Example
• DE 0.1 mV, f 500 Hz
• v 0.050 V/s
• t 2 ms

CV For 300 mV potential path v 60 V/s v
150 V/s
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A technique for mechanistic, kinetic and
analytical application
An electrode reaction of a dissolved redox couple
irrevrersible
quasirev.
reversible
Surface confined electrode reaction
irrevrersible
quasirev.
reversible
Y
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EC mechanism
ECE mechanism
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Electrode mechanisms

1. Electrode reaction of an immobilized redox coupe
   (surface electrode reaction)
2. Electrode mechanism involving formation of an
   insoluble compound with the electrode material

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Reaction scheme for the electrode reaction of an
immobilized redox coupe (surface confined
electrode reaction)
?
Oxbulk
Ox(ads)
Electrode
ne-
Diffusion mass transport is neglected
?
Redbulk
Red(ads)
Ox(ads) ne- ? Red(ads)
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Toward electrode kinetic measurements Modeling
and application

• Application
• Protein-film voltammetry
• Electrochemicaly active drugs
• Simple adsorbates (many organic compounds)
• Azodies
• Metal complexes
• Organometalic compounds
• Surface modified electrodes
• Voltammetry of solid micro- particles etc.

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Net dimensionless SW voltammograms simulated for
different reversibility of the electrode reaction
Dimensionless current Y I/(nFAGf )
w ks / f
irreversible
quasireversible region
reversible
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Quasireversible maximum and the SW response at
the quasireversible maximum
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The origin of the quasireversible maximum
Chronoamperometry of the surface eelectrode
reaction
f 250 Hz, a 0.5
ks 500 s-1
ks 375 s-1
dimensionless current
ks f
ks 25 s-1
t
Synchronisation of the rate of the redox
transformation with the SW frequency!
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Simple methodology for using the quasireversible
maximum for redox kinetic measurements

• wmax ks / fmax
• wmax calculated by the model
• fmax measured in the experiment
• ks wmax fmax

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Splitting of the net SW response for fast and
reversible surface electrode reaction
w increases
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The Origin of the Splitting
log(w) 0.4
log(w) 0
log(w) 0.1
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The dependence of the splitting on the SW
amplitude

• Experimental systems that have been analyzed on
  the base of quasireversible maximum and the
  splitting
• Cytochrome C
• Alyzarine red-S
• Probucole
• 2-propylthiouracil
• Fluorouracil
• Molybdenum(VI)-phenantroline-fulvic acid
• Azobenzene
• Methilene blue,.

DEp / mV
Esw / mV
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Examples of surface confined electrode reactions
alizarin
vitamin B12
vitamin K2
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Comparison of theoretical (?) and experimental
(?) net peak currents for alizarin as a function
of pH.
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Mo(VI)-phenantroline-fulvic acid system

• ks 8 ? 2 s-1 a 0.41 ? 0.05
• n 2

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Splitting of the net SW response of methylene
blue under the influence of the SW amplitude
amplitude increases
methylene blue 3,7-bis(Dimethylamino)-phenothiaz
in-5-ium chloride
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Square wave voltammetry of azurin immobilized on
1-decanethiol-modified gold
Azurin a blue copper protein
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Square wave voltammetry of famotidin catalytic
hydrogen evolution reaction from adsorbed state
Electrode mechanism Fam(ads) ?
FamH(ads) FamH(ads) e- ? Fam(ads) H(aq)
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Square wave voltammetry of 2-guanidinobenzimidazol
e another example for the catalytic hydrogen
evolution reaction from adsorbed state
SWV DPV LSV
LOD ?mol L-1 0.035 0.14 0.2
LOQ ?mol L-1 0.1 0.4 0.6
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Reaction scheme of an electrode reaction
involving formation of chemical bonds with the
electrode
ne-

• Application
• Sulfur containing amino acids
• Glutathione and other cysteine containing
  peptides and proteins
• Mercaptans
• Thyroxin
• Thiourea
• Thioethers
• Phorphyrins
• Flavins
• Sulphide
• Iodide etc.

S
Electrode
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Modeling
HgL (s) 2e- ? Hg(l) L2-(aq) HgL2(s) 2e- ?
Hg(l) 2L-(aq) HgL (s) 2e- ? L2-(ads)
Hg(l) ?? L2-(aq)   HgL2(s)
2e- ? 2L-(ads) Hg(l) ??
2L-(aq)
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Qvazireversible maximum of the cathodic stripping
reaction
Dimensionless current Y I / (nFAc(Df )1/2 )
ks kmaxD1/4 fmax3/4 rs-1/2 rs 1 cm
precision 10
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Cathodic stripping voltammetry of glutathione
-0.35
-0.25
I /m A
-0.05
0.05
-0.700
-0.600
-0.500
-0.400
-0.300
-0.200
E / V
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Cathodic stripping voltammetry of glutathione in
the presence of copper
-0.20
-0.23
Without Cu2
With Cu2
-0.10
I / mA
-0.13
I / mA
-0.05
0
-0.03
ks lt 0.11 cm s-1
ks 5.22 cm s-1
0.07
0.10
-0.700
-0.500
-0.300
-0.700
-0.500
-0.300
E / V
E / V
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Influence of different cations on the SW net peak
currents of glutathione
Cu
Mg
Ba
Ca
Zn
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The influence of the metal ions on the morphology
of the film deposited on the electrode
ne-

• Additional Interactions
• attraction
• repulsion
• complexation

Electrode
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Cathodic stripping mechanism coupled with a
chemical reaction
theoretical
experimental
6-mercaptopurine-9-D-riboside in the presence of
nickel(II) ions
A(aq) L(aq) L(aq) Hg(l) HgL (s) 2e-
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Cyclic Square-Wave Voltammetry a technique of
the future
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