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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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slide1

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

slide2

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

slide4

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

slide5

Ynet

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.;

slide6

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

slide7

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

slide8

EC mechanism

ECE mechanism

electrode mechanisms
Electrode mechanisms
  • Electrode reaction of an immobilized redox coupe (surface electrode reaction);
  • Electrode mechanism involving formation of an insoluble compound with the electrode material;
slide10

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:

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
slide12

w 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

slide13

DYp

log(w)

Quasireversible maximum and the SW response at the quasireversible maximum

slide14

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!

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

slide16

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

w increases

slide17

The Origin of the Splitting

log(w) = 0.4

log(w) = 0

log(w) = 0.1

slide18

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

slide20

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

slide21

Mo(VI)-phenantroline-fulvic acid system

ks = 8  2 s-1; a= 0.41  0.05

n = 2

slide22

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

slide23

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

Azurin – a blue copper protein

slide24

famotidine

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)

slide25

Square wave voltammetry of 2-guanidinobenzimidazole: another example for the catalytic hydrogen evolution reaction from adsorbed state

slide26

S

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

slide27

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)

slide28

Qvazireversible maximum of the cathodic stripping reaction

Dimensionless current

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

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

precision ± 10 %

slide29

pH = 5.6

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

slide30

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

slide32

S

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

slide33

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-

ad