Chemistry Reference
In-Depth Information
6.2 VOLTAMMETRY
Voltammetry,. as. a. shortening. of. voltamperometry,. is. in. principle. the. monitoring. of. a. current. in.
dependence.of.an.applied.potential,.which.changes.with.time.(potential.sweep)..The.methods.are.
referred.to.as.controlled.potential.techniques..Different.voltammetric.techniques.differ.in.the.way.
of. establishing. and. modulating. the. basic. linear. potential. ramp.. Electrochemical. transformations.
(oxidations,.reductions).of.electroactive.species.on.the.surface.of.the.electrode.provoke.an.electro-
lytic.current,.named.as.faradic.current..This.is.in.most.cases.the.relevant.component.for.the.voltam-
metric.measurements.though.some.techniques.rely.also.on.the.capacitive.current.
6.2.1 d
ireCT
V
olTaMMeTry
In. linear. potential. sweep. techniques,. a. potential. is. applied. to. the. electrode,. starting. at. an. initial.
potential,.and.changed.linearly.with.time.to.a.inal.value.with.a.constant.scan.rate.
6.2.1.1 Voltammetry at Slow Scan Rates
With. low. scan. rates,. the. shape. of. the. current. response. is. sigmoid,. characterized. by. the. limiting.
current.
i
d
,.which.is.caused.and.usually.controlled.by.diffusion.(“diffusion.current”),.the.half-wave.
potential.
E
1/2
,.where.half.of.the.diffusion.current.is.attained,.and.which.is.somehow.connected.to.
the.normal.reduction.potential,.and.the.steepness.of.the.curve,.which.is.related.to.the.number.of.
electrons.transferred.with.one.molecule.or.ion.of.the.electroactive.component..The.curves.can.be.
described.by.the.Heyrovsky-Ilkovic.equation.(Equation.6.14):
RT
nF
i
−
i
d
E E
=
+
ln
.
(6.14)
1 2
/
i
.
where
E
.is.the.applied.potential.(V)
R
.is.the.general.gas.constant.(8.314.J.K
−1
.mol
−1
)
F
.is.the.
Faraday constant
.(96,485.C.·.mol
−1
)
i
.is.the.current.at.the.potential.
E
i
d
.is.the.limiting.current
n
.is.the.number.of.electrons.transferred
Substituting. the. constant. and. referring. to. the. temperature. for. the. standard. conditions. as. well. as.
conversion.of.the.natural.to.the.decadic.logarithm.yields.Equation.6.15:
0
.059
i
−
i
d
E E
=
+
ln
.
(6.15)
1 2
/
n
i
.
Voltammetric.methods.with.slow.or.moderate.scan.rates.(usually.≤10.mV).are.the.following:
•.
Direct current voltammetry (DCV)
.continuously.monitors.the.current.during.the.potential.
scan..As.the.scan.rate.is.usually.slow,.sigmoidal.curves.are.obtained.as.explained.earlier.
•.
Sampled DCV
. samples. data. points. discontinuously;. otherwise,. the. curve. looks. compa-
rable.to.DCV..It.was.a.tremendous.advantage.in.polarography.because.oscillations.of.the.
response.were.eliminated.
•.
Staircase voltammetry (SCV)
. changes. the. potential. incrementally,. not. continuously;. the.
data.are.sampled.after.a.waiting.period.to.allow.the.capacitive.current.to.vanish.
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