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VOLTAMMETRY

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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 appliedpotential

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

Supporting electrolyte: excess of nonreactive electrolyte (alkali metal) to conduct current

Apply Linear Potential with Time

Observe Current Changes with Applied Potential

2.) Differences from Other Electrochemical Methods

a)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

(polarography). Later given Nobel Prize 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

(aR)r(aS)s …

0.0592

Eelectrode =E0 - log

n

(aP)p(aQ)q …

Apply

Potential

Current is just measure of rate at which species can be brought to electrode surface

Two methods:

Stirred - hydrodynamic voltammetry

Unstirred - 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

Pt working electrode at -1.0 V vs SCE

Ag counter electrode at 0.0 V

AgCl Ag + Cl-

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

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

SCE

X M of PbCl2

0.1M KCl

Pb2+ + 2e-Pb

-1.0 V vs SCE

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

K+

K+

Pb2+

Pb2+

Pb2+

K+

Pb2+

Pb2+

K+

K+

K+

K+

K+

Pb2+

K+

Pb2+

K+

Pb2+

Pb2+

K+

Pb2+

K+

K+

K+

Pb2+

K+

K+

K+

Pb2+

Pb2+

Pb2+

K+

Pb2+ migrate to the electrode via diffusion

K+

Pb2+

K+

Pb2+

Pb2+

K+

Pb2+

Pb2+

K+

Pb2+

Pb2+

K+

K+

K+

K+

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

Mox + e-»Mred

At Electrodes Surface:

Eappl = Eo - log

[Mred]s

0.0592

at surface of electrode

n

[Mox]s

Applied potential

If Eappl = Eo:

0 = log

ˆ [Mox]s = [Mred]s

[Mred]s

0.0592

n

[Mox]s

Apply

Potential

E << Eo

If Eappl << Eo:

Eappl = E0 - log

ˆ [Mred]s >> [Mox]s

[Mred]s

0.0592

n

[Mox]s

2.) 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 reaction

F = Faraday’s constant

A = electrode area (cm2)

D = diffusion coefficient (cm2/s) of A (oxidant)

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

dCA

dx

dCA

dx

dCA

dx

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 (d):

i = (cox, bulk – cox,s)

- largest slope (highest current) will occur if:

Eappl << Eo (cox,s.0)

then

i = (cox, bulk – 0)

where:

k =

so:

i = kcox,bulk

therefore:

current is proportional to bulk concentration

- also, as solution is stirred,ddecreasesand iincreases

nFADox

d

nFADox

d

nFADox

d

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

Cd2+ + 2e-Cd

Current starts to be registered at the electrode

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

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

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

0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte

i (A)

E½

id

Base line of residual current

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

V vs SCE

3.) Combining Potential and Current Together

Limiting current

Related to concentration

E½ at ½ i

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

E0 = -0.5+SCE for Mn+ + me- » M(n-m)+

4.) Types of Voltammetry

a)Polarography

1) first type of Voltammetry

2) controlled by diffusion, eliminates convection

3) uses dropping Hg electrode (DME) as working electrode

current varies as drop grows then falls off

4) Advantages of Hg drop Electrode

a) high overpotential for reduction of H+

2H+ + 2e- » H2 (g)

allows use of Hg electrode at lower potentials than indicated from thermodynamic potentials

eg. Zn2+ and Cd2+ can be reduced in acidic solutions even though Eo’s vs. SHE = -0.403 (Cd2+/Cd) & -0.763 (Zn2+/Zn)

b) new electrode surface is continuously generated

- independent of past samples or absorbed impurities

c) reproducible currents quickly produced

5) Disadvantages of Hg drop Electrode

a) Ease of Hg0 oxidation

Hg0»Hg+ + e-E0 = +0.4V

ˆcan not use above this potential

Occurs at even lower potentials in presence of ions that complex Hg+

ex. Cl-:

2Hg + 2Cl-»Hg2Cl2 (s) + 2 e- starts at .0V

b) Non-Faradaic (charging) current

- limits the sensitivity to ~ 10-5 M

- residual current is > diffusion current at lower concentrations

c) cumbersome to use

- tends to clog causing

malfunction

- causes non-uniform

potential maxima

d) Hg disposal problems

Mn+ + ne- +Hg » M(Hg) amalgam

i max

i avg

The ripples are caused by the constant forming and dropping of the mercury electrode

Half-wave potential

Residual current

6) Classic Polarography

a) ½ wave potential (E½) characteristic of Mn+ E0

b) height of either average current maxima (i avg)

or top current max (i max) is ~ analyte concentration

c) size of i max is governed by

rate of growth of DME -> drop time (t, sec)

rate of mercury flow (m, mg/s)

diffusion coefficient of analyte (D, cm2/s)

number of electrons in process (n)

analyte concentration (c, mol/ml)

Ilkovic equation:

(id)max = 706nD1/2m2/3t1/6c

(id)avg = 607nD1/2m2/3t1/6c

A

d) necessary to do experiment with sample and blank to see analyte signal vs. residual current

- residual current due to impurities and charging (nonfaradaic)

currents

e) can use method to quantitate or identify elements in sample down to ~ 10-5 M

f) can also use method to study reactions involving Mn+ as long as reaction is

reversible

ex.

Mn+ + ne- +Hg » M0(Hg) amalgam

plus reaction:

Mn+ + xA-» MAx(n-x)+

(E1/2)with complex – (E1/2)without complex = -0.0592/n log Kf – 0.0592x/n log [A-]

ˆ plot of(E1/2)with complex – (E1/2)without complex vs. log[A-] gives slope and

intercept that can be used to give Kf and x

Kf

One problem with data detection in normal polarography is that i varies over lifetime of drop, giving variation on i over curve.

One simple way to avoid this is to sample only current at particular time of drop life.

Near end of drop = current sampled polarography

Sample i at same

time interval

Easier to determine iavg, etc but limit of detection only slightly smaller (~3x)

7) Pulse Polarography

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

with time

b) Measure two potentials at each cycle

- S1 before pulse S2 at end of pulse

- plot Di vs. E (Di = ES2 – ES1)

- peak height ~ concentration

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

c) derivative-type polarogram

d) Advantages:

- can detect peak maxima differing by as mucha as 0.044 – 0.05 V

<0.2V peak separatioin for normal polarography

<can do more elements per run

- decrease limits of detection by 100-1000x compared to normal polarography

e) Reasons for Decrease in Limits of Detection

- increase Faradaic current

<surge of current that lowers reactant concentration demanded by new

potential

‚ not seen in classic polarography

‚ timescale of measurments is long compared to lifetime of

momentary surge

<current decays to a level just sufficient to counteract diffusion

< total current >> diffusion current

‚ reducing surface layer to concentration demanded by Nernst equation

- decrease in non-Faradaic current

< surge of non-Faradaic current also occurs as charge on drop increases

‚ current decays exponentially with time

‚ approaches zero near the end of the life of a drop

< measure current at end of drop lifetime significantly reduces non-

Faradaic current

< signal-to-noise increases

f) Can do differential pulse and square wave polarography on other types of electrodes

- called differential or square wave voltammetry

b)Cyclic Voltammetry

1) Method used to look at mechanisms of redox reactions in solution

2) Looks at i vs. E response of small, stationary electrode in unstirred solution using triangular waveform for excitation

Cyclic voltammogram

Start at E >> E0Mox + ne-»Mred

- in forward scan, as E approaches E0 get current due to

Mox + ne-»Mred

<driven by Nernst equation

‚ concentrations made to meet

Nernst equation at surface

<eventually reach i max

< solution not stirred, so d grows with time and

see decrease in i max

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

< then reverse reaction takes place (if

reversible reaction)

< important parameters

‚Epc – cathodic peak potential

‚ Epa – anodic peak potential

‚ipc – cathodic peak current

‚ ipa – anodic peak potential

< ipc.ipa

<d(Epa – Epc) = 0.0592/n,

where n = number of electrons in reaction

<E0 = midpoint of Epa Epc