Download Voltammetry A variety of electroanalytical methods rely on the

Voltammetry
A variety of electroanalytical methods rely on the
application of a potential function to an electrode
with the measurement of the resulting current in
the cell.
In contrast with bulk electrolysis methods, the
objective is not to engage all of the analyte in
reaction. Furthermore, voltammetric methods
generally require achievement of polarization
whereas it is ordinarily desirable to avoid polarization
in bulk electrolysis methods.
Bulk electrolysis:
relatively large electrodes (on the order of cm2)
Voltammetry:
relatively small electrodes (on the order of μm2-mm2)
In both cases, relatively high concentrations of bulk
electrolytes (that do not engage in electrochemical
reactions under the conditions used) are used to avoid
migration.
Various excitation waveforms are used for different
purposes.
Linear sweep voltammetry:
Potential of working electrode changed at a rate of
mV/s
Excess of nonreactive supporting electrolyte to
minimize migration (motion of analyte ions due to
external field).
Reference electrode connected via a high impedance
voltmeter. Current flows primarily between working
electrode and counter electrode.
Typical working electrodes are a disk electrode (left)
and a mercury electrode (right).
Voltage ranges associated with Pt, Hg, and C based
working electrodes. Limits established by oxidation
and reduction of water. (Hg gives lower negative
voltages due to large overvoltage of H2 on mercury.)
Current versus applied potential referred to as a
voltammogram:
By convention, reduction (cathodic) currents are
positive and oxidation (anodic) currents are negative.
A + ne- ' P
∫-shaped curve is referred to as a voltammetric wave.
Point Z – limiting current, il – current limited by
polarization
il = kc A
k is a constant, cA = analyte concentration
The point at which i = il/2, E1/2, is referred to as the
half-wave potential and it is approximately equal to
the standard potential for the half-reaction.
This value can be used for identification purposes.
Hence linear sweep voltammetry can be both a
qualitative and quantitative technique.
Two types:
1. High convection: hydrodynamic voltammetry
2. Low convection: polarography
We will discuss only hydrodynamic voltammetry…
Convection supplied by i) vigorous stirring of the
solution in contact with the electrode, ii) rapid
rotation of the electrode, or iii) flowing analyte
solution over the electrode. In all cases, an excess of
supporting electrolyte is used to minimize migration
of the analyte.
Near the electrode:
A + ne- ' P
Eappl
cP0
0.059
=E −
log 0 − Eref
n
cA
0
A
where EA0 is the standard electrode potential for the
half-reaction, cP0 is the product concentration in the
Nernst diffusion layer, δ, and cA0 is the analyte
concentration in the Nernst diffusion layer.
δ is typically 0.01-0.001 cm, depending upon stirring
rate and solvent viscosity.
Current at the electrode is determined by the rate at
which A arrives at the electrode, which is given by:
δcA/δx where x is the distance from the electrode
⎛ ∂c ⎞
i = nFAD A ⎜ A ⎟
where
n
is
the
number
of
e
∂
x
⎝
⎠
transferred, F is Faraday’s constant, A is the
electrode surface area (cm2), DA is the diffusion
coefficient for A (cm2/s), and cA is concentration
(mol/cm3).
⎛ ∂c A ⎞
i = nFAD A ⎜
⎟
⎝ ∂x ⎠
i=
(c
nFADA
δ
)
(
0
0
−
c
=
k
c
−
c
A
A
A A
A
)
At the point where cA0 → 0, i becomes il and
il =
nFAD A
δ
c A = k Ac A
0
plugging il = kAcA into i = k A (c A − c A ) and rearranging
il − i
c =
kA
0
A
i=−
i=−
c P0 =
nFADP
δ
nFADP
i
kP
δ
(c
)
(
0
0
−
c
=
k
c
−
c
P
P
P P
P
cP0 = k P cP0
)
Plugging
c A0 =
il − i
i
c P0 =
k A and
k P into
0
c
0
.
059
= E A0 −
log P0 − Eref
gives
n
cA
Eappl
Eappl = E A0 −
k
i
0.059
0.059
log A −
log
− Eref
n
kP
n
il − i
At E1/2, i = il/2, and
E1 / 2
kA
0.059
=E −
log − Eref
n
kP
0
A
solving for EA0 and plugging into the equation for
Eappl yields:
Eappl
0.059
i
= E1 / 2 −
log
n
il − i
this is the equation for the ∫-shaped curve
if DA and DP are similar, kA/kP ≈ 1 such that
E1/2 ≈ EA0 - Eref
Linear sweep voltammetry can provide qualitative
(via E1/2) and quantitative (via il) information.
Provided E1/2 values are sufficiently different (i.e.,
they can be resolved) species present in mixtures can
be determined in a single expt.
Mixtures of the same species in different oxidation
states can also be determined
Curve C:
10-4 M Fe3+
Curve B:
0.5 x 10-4 M Fe3+, 0.5 x 10-4 M Fe2+
Curve A:
10-4 M Fe2+
Uses of linear sweep hydrodynamic voltammetry:
1.) For oxidizable or reducible species in flowing streams,
as in the outlet of a liquid chromatograph:
e.g., LC/EC – BAS
2.) Sensors – cells established to be selective for particular
species (e.g., oxygen, glucose) usually a fixed V msmt (amperometry)
Cyclic voltammetry:
msmt of current as working electrode potential is
swept first in one direction and then in the other
forward scan: scan in direction of more negative potential
reverse scan: scan in direction of more positive potential
switching potentials: voltages at which scan reversal occurs
Uses: primarily qualitative
study of redox reactions
identification of reaction intermediates
observation of rapid sequential (follow-up) rxns
e.g.: Fe(CN)63- + e- ' Fe(CN)646 mM in Fe(CN)63-, 1 M in KNO3
Points:
A: beginning of scan, small anodic current due to
oxidation of water
B: beginning of reduction of Fe(CN)63- to Fe(CN)64C: rapid increase in current with applied V
D: maximum cathodic current (see equation)
E: current decays due to extension of diffusion layer
(unstirred solution)
F: scan reversal point
F-J: reoxidation of Fe(CN)64- to Fe(CN)63For a rapidly reversible redox reaction
ip = 2.686x105AcD½υ½
where ip = peak current (either cathodic or anodic), A
= electrode area, c is concentration, D = diffusion
coefficient, υ = scan rate (volts/s)
|Epa-Epc| = ΔEp = 0.059/n
if ΔEp > 0.059/n, rate of electron transfer is slow
relative to scan rate, kinetics of electron transfer can
be determined via measurements taken as a function
of scan rate.
Stripping methods:
voltammetry experiments involving a bulk
electrolysis preconcentration step followed by the
removal of the analyte via a potential sweep while
measuring current
Step 1: a fixed electrolysis time during which analyte
is deposited onto an electrode (allows for orders of
magnitude in preconcentration)
Step 2: evolution of the analyte via a voltage sweep
of the electrode (concentrations as low as nano-molar
can be determined)