Chemically Modified Carbon Paste Electrodes - Electroanalysis with Carbon Paste Electrodes - page 105

DC-voltammetry,.(ii).broadening.of.the.peaks.in.DP-and.SW-voltammetry,.and.(iii).widening.the.
distance.between.the.signal.maxima.or.minima.in.cyclic.voltammetry.(CV).
Two.methods.can.be.suggested.to.characterize.and.quantify.the.deviation.from.the.ideal.behavior.
as.well.as.the.inluence.of.the.insulating.matrix..For.ideal.electrochemical.behavior,.the.Nernst.equa-
tion.and.all.derivations.from.it.should.be.somehow.rigorously.valid..As.a.consequence,.the.potential.
difference. of. the. peak. maxima. of. the. oxidation.and. reduction. in. cyclic. voltammetry. should. cor-
respond.to.some.59/n.mV,.where.n.represents.the.number.of.transferred.electrons..Deviations.from.
the.ideal.behavior.by.whatsoever.reasons.could.be.simply.described.by.Equation.4.7:

( E − E ) ∗ n

.

p a , ,

0 059

p c CPE

= (4.7)

c CPE

.

.

where
χ CPE .is.a.dimensionless.factor.describing.the.extent.of.deviation.of.the.carbon.paste.electrode
E p,a .and. E p,c .are.the.anodic.and.cathodic.peak.potentials.of.the.system.(i.e.,.CPE.in.this.case).in.
the.cyclic.voltammograms.in.Volts

In.order.to.quantify.the.inluence.of.the.matrix,.a.similar.approach.can.be.chosen,.where.instead.
of.the.ideal.behavior,.the.voltammetric.responses.of.a.corresponding.homogeneous.solid.electrode.
(graphite.or.glassy.carbon,.GC).are.related.with.the.observed.values.of.the.CPE.(Equation.4.8):

( E −

−

E )

p a , p c CPE ,

.

= (4.8)

c CPE GC

/

( E E )

p a , p c GC ,

.

4.4.2 C heMiCally M odiFied Cpe s Versus b are Cpe s
As.very.often.carbon.pastes.represent.somehow.individuals.changing.their.properties.slightly.from.
each.batch.preparation.to.the.next,.a.simple.tool.to.quantify.the.electrochemical.characteristics.is.
provided. here,. by. distinguishing. the. bare. CPE. (where. sorption. processes. are. expected. to. occur).
and.chemically.modiied.CPEs.(by.choosing.an.illustrative.example,.that.is,.accumulation.at.clay-
modiied. CPE,. which. could. be. extended. to. other. CPEs. modiied. with. solid. particles. exhibiting.
some.afinity.for.target.species.

4.4.2.1 Sorption Processes at the Bare (Unmodiied) Carbon Pastes
Due. to. the. presence. of. carbon. particles. and. a. more. or. less. lipophilic. matrix,. CPE. exhibits.
enhanced.afinity.to.organic.molecules.with.the.tendency.to.be.spontaneously.adsorbed.onto.the.
electrode. surface,. which. can. be. exploited. for. preconcentrating. the. target. compounds,. either. in.
the.actual.form.or.as.various.metal.complexes,.via.which.numerous.single.metal.ions.can.also.be.
accumulated.
When.sorption.takes.place.at.carbon.paste.electrodes,.similar.conditions.can.be.found.as.with.
preconcentration. of. metals. on. liquid. mercury. surfaces.. Depending. on. the. lipophilicity. of. the.
adsorbed.species.and.the.composition.of.the.electrode.material,.sorption.can.be.mainly.restricted.to.
the.surface.of.the.electrode,.or.migration.of.the.sorbed.compound.into.the.electrode.bulk.can.occur.
(“extractive.accumulation”;.see.Section.3.4.and.Figure.3.6)..This.provokes.a.loss.of.accumulated.
species.at.the.electrode.surface.for.long.time.accumulations.and.a.concentration.gradient.of.the.ana-
lyte.of.interest.into.the.electrode.bulk..On.the.other.hand,.during.the.measurement,.the.gradient.is.
reversed.due.to.consumption.of.the.electroactive.analyte.at.the.electrode.surface,.which.will.prompt.
a.migration.of.accumulated.species.back.from.the.interior.of.the.electrode.to.the.solution.interface.
with.the.consequences.of.additional.signal.broadening.and.memory.effects.