Chemistry Reference
In-Depth Information
Current
flow
Sample
solution
Electrode surface
Reduction
Oxidation
Electron transfer
e
-
e
-
Mass transfer
Analyte
Product
FIGURE 6.2
Faradaic.process.with.mass.and.electron.transfer.
With. many. ideal. and. quasi-ideal. systems,. the. electro-transfer. is. much. quicker. than. the. mass.
transfer..In.such.a.case,.the.system.is.called.diffusion.or.mass-transfer.controlled..A.good.indication.
if.diffusion.is.controlling.the.kinetics.is.performing.linear.sweep.or.cyclic.voltammetry.at.different.
scan.rates.and.to.correlate.the.peak.current.with.the.square.root.of.the.scan.rate.according.to.the.
Randles-Sevcik.equation.(Equation.6.4.for.reversible.reactions,.where.the.diffusion.coeficients.for.
oxidized.and.reduced.species,.
D
,.are.practically.identical):
*
2 69 10
5
2 3
/
1 2
/
1 2
.
/
i
p
=
( .
⋅
)
⋅
n
⋅
A D
⋅
⋅
C v
⋅
(6.4)
.
where
i
p
.is.the.peak.current
A
.is.the.electrode.area
D
.is.the.diffusion.coeficient
C
*.is.the.bulk.concentration.of.the.electroactive.species
v
.is.the.scan.rate.(mV.·.s
−1
)
Apparent. sluggish. electron. transfer,. which. will. consequently. control. the. kinetics. rather. than.
mass. transfer,. can. be. described. with. the. Butler-Volmer. equation,. which. considers. forward. and.
backward.transfers.(Equation.6.5):
.
(
)
− ⋅ ⋅
a
n F E E
⋅
(
−
)/
RT
− −
(
1
a
)
⋅
n F E E
⋅
⋅
(
−
)/
RT
i
= ⋅
i
e
−
e
eq
eq
(6.5)
.
0
where
α
.is.the.transfer.coeficient
E
eq
.is.the.equilibrium.potential
i
0
.is.the.exchange.current
The. left. term. is. the. forward. reaction,. the. right. the. backward.. Curve. analysis. can. be. made. from.
voltammetric. sigmoidal. curves.. When. considering. that. far. away. from. the. equilibrium. potential,.
E
eq
.(potential.where.the.net.current.low.is.0,.that.is,.forward.and.backward.current.are.of.the.same.
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