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: electrode (Hg, Pt, C) that completes circuit, conducts e- from signal source through solution to the working electrode

Supporting electrolyte: excess of nonreactive electrolyte to conduct current

− NaCl or KCl in aqueous solvent− Tetrabutylammonium hexafluorophosphate (TBAPF6)

in non-aqueous solvent (e.g., MeCN, DMSO, DMF)

VOLTAMMETRY

Instrumentation – Three electrodes in solution containing analyte

Apply Linear Potential with Time Observe Current Changes with Applied Potential

2.) Differences from Other Electrochemical Methodsa) 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

3.) Voltammetry first reported in 1922 by Czech Chemist Jaroslav Heyrovsky

(The Father of Polarography) Later given Nobel Prize (1959) 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

Eelectrode = E0 - log0.0592

n(aR)r(aS)s …(aP)p(aQ)q …

Apply

Potential

Current is just measure of rate at which species can be brought to electrode surfaceTwo methods:

Stirred - hydrodynamic voltammetryUnstirred - 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

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

Pb2+ + 2e- Pb EO = -0.13 V vs. NHE

K+ + e- K EO = -2.93 V vs. NHE

Pt working Pt working electrode at -1.0 electrode at -1.0 V vs SCEV vs SCE

SCESCE

Ag counter Ag counter electrode at electrode at 0.0 V0.0 V

X X MM of PbCl of PbCl22

0.10.1MM KCl KCl

AgCl Ag + Cl-

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+ Pb2+K+

K+

K+K+

K+

K+

K+

K+

K+

K+

K+ K+

K+

K+

K+

K+

K+

K+

K+

K+

-1.0 V vs SCE-1.0 V vs SCEPb2+ + 2e- Pb

K+

K+

K+

K+K+ K+

Layers of K+ build up around the electrode stop the migration of Pb2+ via coulombic attraction

Concentration gradient created between the surrounding of the electrode and the bulk solution

PbPb2+2+ migrate to migrate to the electrode the electrode via diffusionvia diffusion

C) Types of Voltammetry

1. Polarography− first type of Voltammetry

− controlled by diffusion, eliminates convection

− uses dropping Hg electrode (DME) as working electrode; current varies as drop grows then falls off

a. Advantages of Hg Drop Electrode− High overpotential for reduction of H+

• Allows use of Hg electrode at lower potentials than indicated from thermodynamic potentials

• Example:

Zn2+ and Cd2+ can be reduced in acidic solutions even though E0 vs NHE = -0.403 (Cd2+/Cd) and -0.763 (Zn2+/Zn)

− new electrode surface is continuously generated

• Independent of past samples or absorbed impurities

− reproducible currents quickly produced

2H+ + 2e- H2 (g) (0V vs NHE)

b. Disadvantages of Hg Drop Electrode− Hg oxidation

• Around +0.25 V vs. SCE

• Can not be used above a potential of +0.25 V

Hg undergoes anodic dissolution ~ +0.25 V vs. SCE and is oxidized to insoluble Hg2Cl2 in presence of Cl- at zero V vs. SCE.

It cannot be used for anodic oxidation above +0.25 V vs. SCE.

− Non-Faradaic (charging/capacitance) current

• limits the sensitivity to ~ 10-5 M

• residual current is > diffusion current at lower concentrations

− cumbersome to use

• tends to clog, causing malfunction

− Hg disposal problems

• mercury vapors are also very poisonous

2. Voltammetry (solid working electrode)

Pb2+ + 2e- Pb EO = -0.13 V vs. NHE

K+ + e- K EO = -2.93 V vs. NHE

Note:Note:• Reference Electrode: SCE (saturated calomel electrode)Reference Electrode: SCE (saturated calomel electrode)• SCE = + 0.24 V vs NHESCE = + 0.24 V vs NHE• Thus, the EThus, the Eoo of Pb of Pb2+ 2+ = -0.37 V vs SCE= -0.37 V vs SCE

Pt working electrodePt working electrode

SCESCE

Ag counterAg counter

X X MM of PbCl of PbCl22

0.10.1MM KCl KCl

AgCl Ag+ + Cl-

At Electrodes Surface:

Eappl = Eo - log 0.0592n

[Mred]s

[Mox]s

at surface of electrode

Applied potential

If Eappl = Eo:

0 = log

ˆ[Mox]s = [Mred]s

0.0592n

[Mred]s

[Mox]s

Mox + e- Mred

If Eappl << Eo:

Eappl = E0 - log

[Mred]s >> [Mox]s

0.0592n

[Mred]s

[Mox]s

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:

measured current (i) = nFADA( )

where:n = number of electrons in ½ cell reactionF = Faraday’s constantA = electrode area (cm2)D = diffusion coefficient (cm2/s) of A (oxidant)

= slope of curve between CMox,bulk and CMox,s

CA

x

CA

x

CA

x

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.

Thickness of Diffusion Layer ():

i = (cox, bulk – cox,s)

- largest slope (highest current) will occur if:

Eappl << Eo (cox,s 0)

theni = (cox, bulk – 0)

where:k =

so:i = kcox,bulk

therefore:current is proportional to bulk concentration

- also, as solution is stirred, decreases and i increases

nFADox

nFADox

nFADox

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4

i (A)

Potential applied on the working electrode is usually swept over (i.e. scan) Potential applied on the working electrode is usually swept over (i.e. scan) a pre-defined range of applied potentiala pre-defined range of applied potential

0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte

V vs SCE

Working electrode is no yet capable of reducing Cd2+ only small residual current flow through the electrode

Electrode become more and more reducing and capable of reducing Cd2+

Cd2+ + 2e-Cd

Current starts to be registered at the electrode

Current at the working electrode continue to rise as the electrode become more reducing and more Cd2+ around the electrode are being reduced. Diffusion of Cd2+ does not limit the current yet

All Cd2+ around the electrode has already been reduced. Current at the electrode becomes limited by the diffusion rate of Cd2+ from the bulk solution to the electrode. Thus, current stops rising and levels off at a plateau id

E½

Base line of residual current

Combining Potential and Current Together

Half-wave potential : E1/2 = -0.5 E0 - Eref

E0 = -0.5 + SCE for Mn+ + me- ↔ M(n-m)+

E½ at ½ i

Limiting currentRelated to concentration

Voltammograms for Mixtures of Reactants

Two or more species are observed in voltammogram if difference in separate

half-wave potentials are sufficient

0.1V0.2V

Different concentrations result in different currents, but same potential

[Fe2+]=1x10-4M

[Fe2+]=0.5x10-4M[Fe3+]=0.5x10-4M

[Fe3+]=1x10-4M

Amperometric Titrations-Measure equivalence point if analyte or reagent are oxidized or reduced at working electrode- Current is measured at fixed potential as a function of reagent volume

• endpoint is intersection of both lines

Only analyte is reduced

Only reagent is reduced Both analyte and reagent are reduced

endpointendpointendpoint

- Analyte first deposited (reduced) onto the

working electrode from a stirred solution

-“deposit” analyte for a known period of time

- analyte is redissolved or stripped (oxidized)

from the electrode

- analyte “preconcentrated” onto electrode, thus

ASV yields lowest detection limit among all

voltammetric techniques

3. Anodic Stripping Voltammetry (ASV)

a) Instead of linear change in Eappl with time use step changes (pulses in Eappl) with time

b) Measure two currents at each cycle- S1 before pulse & S2 at end of pulse

- plot i vs. E (i = ES2 – ES1)

- peak height ~ concentration

- for reversible reaction, peak potential standard potential for ½ reaction

c) differential-pulse voltammetry

d) Advantages:- can detect peak maxima differing by as little as 0.04 – 0.05 V

0.2V peak separation for normal voltammetry

- decrease limits of detection by 100-1000x compared to normal voltammetry10-7 to 10-8 M

concentration

E0

4. Pulse Voltammetry

a. Method used to look at mechanisms of redox reactions in solution.b. Looks at i vs. E response of small, stationary electrode in unstirred

solution using triangular waveform for excitation

Segm

ent 1 Segm

ent 2

Segment 1

Segment 2

Cyclic voltammogram (solution phase redox species)

5. Cyclic Voltammetry

Mox + ne- Mred

- in forward scan, as E approaches E0’ , current flow due to Mox + ne- Mred

- governed by Nernst equation• concentrations made to meet Nernst equation at surface

- eventually reach i max- solution not stirred, so grows with time, leads to decrease in i max

- in reverse scan - see less current as potential increases until reduction no longer occurs

- then reverse reaction takes place (if reaction is reversible)- important parameters

- Epc – cathodic peak potential- Epa – anodic peak potential

- ipc – cathodic peak current- ipa – anodic peak current

ipc ~ipa (or 1pc/1pa ~ 1)

Δ Ep = (Epa – Epc) = 0.0592 V / n n = number of electrons involved in the reactionFormal reduction potential

Eo’ (E1/2) = = (Epa + Epc ) / 2

Fe3+ + e- Fe2+

ipc ipa

Ep = (Epa – Epc) = 0.0592/n, where n = number of electrons in reaction

E0 = midpoint of Epa Epc

ip = 2.686x105n3/2AcD1/2v1/2 (Randles-Sevcik eqn)

- ip: peak current (A)- n: number of electrons- A: electrode area (cm2)- c: concentration (mol/cm3)- v: scan rate (V/s)- D: diffusion coefficient (cm2/s)

Important Quantitative Information

Thus, - can calculate standard potential for half-reaction- number of electrons involved in half-reaction- diffusion coefficients- if reaction is reversible

Cyclic Voltammogram is a good way to determine diffusion coefficient

ip = 7.422 x 10-6 An = 1A = 0.0314 cm2

C = 1 x 10-6 mol/cm3

D = diffusion coefficient (cm2/s) v = 0.05 V/s

D = 1.55 x 10-5 cm2/s Laser Dye (PM 567)

NB

N

F F

+ _

Oxidation

Reduction

Lai and Bard, J. Phys. Chem. B, 2003, 107, 5036-5042.