Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 55

. (II). There.is.a.general.agreement.that.both.cathodic.and.anodic.limits.of.CPEs.are.inluenced.
mainly. by.the. quality of graphite powder .used.. For.instance,. graphite.particles.impreg-
nated.by.ceresin.wax.(i.e.,.protected.against.oxygen).may.yield.a.carbon.paste.with.a.mark-
edly.widened.potential.window.in.the.anodic.range.(up.to.+1.8.V.vs..SCE.[39])..In.addition,.
relative.content.of.graphite.in.the.paste.mixture.is.an.important.factor.[2,3,68,83,175]..In.a.
limited.interval.of.the.carbon-to-pasting.liquid.ratio.(e.g.,.1.g.C:.0.3-0.6.g.PO/SO),.it.can.be.
even.stated.that.the.lesser.the.percentage.of.carbon,.the.wider.the.polarizability.attained..
Furthermore,.the.increasing.content.of.graphite.usually.results.in.enhanced.background,.
involving.increasing.signals.of.both.limiting.electrode.reactions.in.aqueous.solutions,.that.
is,.cathodic.reduction.of.protons.and.the.release.of.hydrogen.or.anodic.oxidation.of.water,.
respectively.[3,61,516,622])..Nevertheless,.changes.in.polarization.through.the.carbon-to-
pasting.liquid.ratio.are.insigniicant,.and.the.proper.selection.of.graphite.material.is.more.
relevant.
(III). With.regard.to.the. type of binder .used,.the.role.of.different.pasting.liquids.in.the.polar-
ization.of.CPEs.is.less.known.and.understood..The.irst.attempts.to.correlate.the.polariza-
tion.characteristics.of.carbon.pastes.with.the.properties.of.a.binder.were.made.in.a.study.
dealing.with.seven.different.liquids.[40],.however,.without.decisive.results.and.addressed.
interpretation..In.other.studies.focused.on.the.function.of.pasting.liquids,.certain.explana-
tions.had.already.been.formulated,.namely,.possible.effect.of.a.more.thorough.coating.of.
the. graphite. particles. by. voluminous. molecules. of. some. binders. such. as. highly. viscous.
silicone.luids.[211,219-221].or.liquid.plasticizers.and.ion.exchangers.[205,233]..Compared.
with.ordinary.carbon.pastes,.the.potential.window.of.these.mixtures.was.extended.mainly.
in. the. cathodic. direction;. for. example,. dioctyl. phthalate-based. carbon. paste. could. be.
polarized.in.0.1.M.HCl.yet.at.a.potential.of.−1.2.V.vs..Ag/AgCl,.whereas.the.Nujol.paste.
was.polarized.at.−0.9.V.[205].
Note: . The. mechanism. assuming. the. “coating. or. packaging. effect”. of. a. TCP-based. carbon.
paste. (see. Figure. 3.1). was. also. applied. to. explain. extreme. polarizability. of. the. TCP-CPE. that.
could. be. operated. under. scanning. from. −2.0. to. +2.0.V. vs.. Ag/AgCl,. that. is,. within. an. interval.
of. 4 V. [96].. However,. the. subsequent. experiments. performed. with. CPEs. made. of. alternate. TCP.
[108,182,638,639].have.revealed.that.the.resultant.polarizability.was.only.slightly.better.than.that.
of.common.CPEs..Then,.abnormal.behavior.of.the.irst.TCP-CPE.should.be.attributed.rather.to.a.
catalytic.action.of.some.impurities.in.the.original.chemical.(of.technical.grade).than.to.a.beneit.of.
the.liquid.itself.[189]..In.addition,.typically.high.currents.observed.during.measurements.with.TCP-
CPE.(e.g.,.[182,254]).have.indicated.a.larger.surface.area,.which.did.not.concur.with.the.previous.
hypothesis..In.other.words,.the.interpretation.through.the.inhibition.of.electrolytic.decomposition.in.
aqueous.solutions.seems.to.explain.better.the.variability.in.the.polarization.of.TCP-CPE,.particu-
larly,.if.the.same.effect.can.be.achieved.by.special.modiiers.or.catalysts.added.into.the.paste.(see.
later.and.in.Sections.4.1.through.4.4).
(IV). Besides.modiications.mentioned.in.the.footnote,.the.polarization.characteristics.of.carbon.
pastes.can.also.be.improved.by.some.surface.treatments.and.alterations,.which.applies,.of.
course,.to.any.electrode.or.sensor..Here,.one.can.quote.an.interesting.approach.based.on.
spraying.the.native.carbon.paste.surface.with.an.additional.graphite.layer.[67,256].or.the.
already.mentioned.family.of.metallic-ilm-plated.CPEs.[103,257,260].
. (V). In.accordance.with.the.information.gathered.in.points.I-IV,.it.can.be.concluded.that. each
carbon paste exhibits more or less speciic polarization characteristics ,.although.the.dif-
ferences.in.both.cathodic.and.anodic.potential.limits.are.usually.minor.or.even.negligible.
[8,9,40,68,83,175,227].. Nevertheless,. it. is. always. useful—and. with. newly. made. pastes.
strongly.advisable—to.examine.the.polarizability.of.the.individual.CPEs.with.the.aid.of.
suitable.testing.experiments.([175,176].and.Section.3.3).