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Square-wave voltammetry: the most advanced electroanalytical technique. Valentin Mir č eski Institute of Chemistry Faculty of Natural Sciences and Mathematics “Ss Cyril and Methodius” University, Skopje Republic of Macedonia. Square-Wave Voltammetry: Potential Modulation. Red  Ox + e.

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Square-wave voltammetry: the most advanced electroanalytical technique

Valentin Mirčeski

Institute of Chemistry

Faculty of Natural Sciences and Mathematics

“Ss Cyril and Methodius” University, Skopje

Republic of Macedonia


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



Faradaic vs. capacitive current in the course of a single potential pulse

Faradaic current I f (due to electrode reaction)

If/ Ic >> 1

(sampling point)

I

Capacitive current, Ic (due to charging - formation of the double layer)

t

0


Y potential pulsenet

Ynet = Yf - Yb

Yf

Yb

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.;


Time window of the voltammetric experiment potential pulse

  • 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


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


EC mechanism application

ECE mechanism


Electrode mechanisms
Electrode mechanisms application

  • Electrode reaction of an immobilized redox coupe (surface electrode reaction);

  • Electrode mechanism involving formation of an insoluble compound with the electrode material;


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)


Toward electrode kinetic measurements modeling and application

Application: redox coupe (surface confined electrode reaction)

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.

Toward electrode kinetic measurements: Modeling and application


w redox coupe (surface confined electrode reaction) increases

Net dimensionless SW voltammograms simulated for different reversibility of the electrode reaction

Dimensionless current

Y = I/(nFAG*f )

w = ks / f

irreversible

quasireversible region

reversible


DY redox coupe (surface confined electrode reaction)p

log(w)

Quasireversible maximum and the SW response at the quasireversible maximum


The origin of the quasireversible maximum redox coupe (surface confined electrode reaction):

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!


Simple methodology for using the quasireversible maximum for redox kinetic measurements
Simple methodology for using the quasireversible maximum for redox kinetic measurements

wmax = ks / fmax

wmaxcalculated by the model

fmax measured in theexperiment

ks = wmaxfmax


Splitting of the net SW response for fast and reversible surface electrode reaction

w increases


The Origin of the surface electrode reactionSplitting

log(w) = 0.4

log(w) = 0

log(w) = 0.1


The surface electrode reactiondependence 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


Examples of surface confined electrode reactions surface electrode reaction

alizarin

vitamin B12

vitamin K2


Comparison of theoretical (□) and experimental (○) net peak currents for alizarin as a function of pH.


Mo(VI)-phenantroline-fulvic acid system peak currents for alizarin as a function of pH.

ks = 8  2 s-1; a= 0.41  0.05

n = 2


Splitting of the net SW response of methylene blue under the influence of the SW amplitude

amplitude increases

methylene blue

3,7-bis(Dimethylamino)-phenothiazin-5-ium chloride


Square wave voltammetry of azurin immobilized on 1-decanethiol-modified gold

Azurin – a blue copper protein


famotidine 1-decanethiol-modified gold

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)


Square wave voltammetry of 1-decanethiol-modified gold2-guanidinobenzimidazole: another example for the catalytic hydrogen evolution reaction from adsorbed state


S 1-decanethiol-modified gold

S

S

S

S

S

S

S

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


Modeling 1-decanethiol-modified gold

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)


Qv 1-decanethiol-modified goldazireversible maximum of the cathodic stripping reaction

Dimensionless current

Y = I / (nFAc*(Df )1/2)

ks = kmaxD1/4fmax3/4rs-1/2rs= 1 cm

precision ± 10 %


pH = 5.6 1-decanethiol-modified gold

ks = 5  0.2 cm s-1

pH = 7.0

Ipf -1 / mA s

pH = 8.5

f / Hz

Cathodic stripping voltammetry of glutathione

-0.35

-0.25

I /m A

-0.05

0.05

-0.200

-0.300

-0.400

-0.500

-0.600

-0.700

E / V


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 < 0.11 cm s-1

ks = 5.22 cm s-1

0.07

0.10

-0.300

-0.500

-0.700

-0.300

-0.500

-0.700

E / V

E / V



S of glutathione

S

S

S

S

S

Mx+

Mx+

Mx+

The influence of the metal ions on the morphology of the film deposited on the electrode

ne-

  • Additional Interactions:

    • attraction

    • repulsion

    • complexation

Electrode


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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