Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 56

3.2.3 s peCiFiC r eaCTion k ineTiCs aT C arbon p asTes
One.of.the.many.characteristic.features.of.lipophilic.liquid.binders.that.inluence.the.properties.of.
carbon.pastes.is.their.direct.impact.on.the.electrode.reactions.connected.with.the.charge.transfer.
as.well.as.mass.transport.[3,68,83]..This.effect.was.of.permanent.interest.since.carbon.paste-based.
electrodes. had. appeared. in. electrochemical. laboratories. [2,3].. In. a. brief. deinition. derived. from.
Adams's.publications,.atypical.kinetics.at.CPEs.can.be.approached.as.a.phenomenon.arising.from.
the.ability.of.binders.to.moderate.numerous.faradic.processes.taking.place.in.the.proximity.of.the.
active.graphite.sites.[3,528]..Thus,.redox.processes.reversible.or.nearly.reversible.at.compact.elec-
trodes.such.as.GC,.Pt,.or.Au.are.converted.into.a.“pseudo-reversible”.pathway.when.studied.with.a.
CPE..Moreover,.analogously,.those.reactions.that.are.moderated.at.conventional.solid.electrodes.are.
slower.or.even.totally.irreversible.when.employing.CPEs.[3].
The.reasons.for.such.a.broadening.of.the.overpotential.have.been.the.subject.of.speculations.in.
numerous.early.reports.(e.g.,.[13,22,26,29,34,60,640-642]);.nevertheless,.the.ultimate.explanation.
was.given.as.late.as.in.the.early.1980s.by.Adams.et.al..[68]..As.already.mentioned.in.the.historical.
survey,.this.excellent.study—followed.soon.by.some.other.authors.[60,61,63,220]—belongs.among.
the.classics.within.the.ield.(see.survey.of.such.papers.in.[7]).and.comprises.up-to-date.valuable.
discussion.material,.the.most.important.facts.of.which.are.highlighted.as.follows:
. (i). At.the.CP.surface,.hydrophobic.molecules.of.binder.form.numerous. hydrophobic sites and
regions capable of repelling hydrophilic species .from.the.“electrode/solution”.interface,.
having.thus.moderated.their.redox.transformation.and.electron.transfer.as.such.
. (ii). As. a. rule,. the. more. hydrophobic. the. carbon. paste. surface. and. the. more. the. hydrophilic.
species.involved.in.the.reaction.at.the.interface,.the.more.moderated.the.electrode.process.
Note: . Among. typical. hydrophilic. species,. one. can. classify. practically. all. inorganic. ions. and.
molecules,. but. there. are. also. numerous. organic. compounds. whose. molecules. offer. some. frag-
ments. willing. to. participate. in. the. protonation,. hydroxylation,. or. in. some. inductive. and. meso-
meric. transmutations,. which. is. the. case. of. functional. groups. such. as. −OH,. =O,. −NO 2 ,. −N=O,.
or.−NH 2 .[220].
.(iii). At.the.carbon.paste.surface,.the.abundance.and.degree.of.consolidation.of. hydrophobic
regions .are.in.direct.relation.with.the.type.of.liquid.binder.and.its.content.in.the.CP.mix-
ture..Although.the.amount.of.the.binder.increases,.the.surface.becomes.more.hydropho-
bic,. and. the. whole. repelling effect —that. is,. inhibition. of. the. electrode. process—is. yet.
more.pronounced..A. degree of hydrophobicity (lipophilicity) .then.determines.whether.the.
overall.electrode.process.proceeds.reversibly,.“pseudo-reversibly,”.or.irreversibly..(In.elec-
trochemistry,.in.contrast.to.thermodynamics,.the.irreversible.process.does.not.mean.the.
ultimate.status,.and.the.respective.electrode.reaction.can.still.be.converted.to.the.reversible.
process..The.key.parameter.is,.however,.the.actual.reaction.rate.[3,528].)
. (iv). By.treating.the.carbon.paste.surface.by.means.of.special.chemical.or.electrolytic.pro-
cedures,. the. typically. hydrophobic surface of a CPE can be converted into a more
hydrophilic surface state ,. and. thereby. numerous. moderated. and. irreversible. electrode.
processes. may. be. accelerated.. In. favorable. cases,. the. respective. redox. pathways. then.
exhibit.similar.or.even.nearly.identical.reaction.rates.as.those.taking.place.at.ordinary.
solid.electrodes.
Note: . All.the.key.indings.and.conclusions.summarized.earlier.have.been.made.thanks.to.the.stud-
ies.with.carbon.pastes.at.the.limit.of.“dry.graphite”.(see.Section.2.2.1)..In.such.experiments,.the.
hydrophobic.layer.of.liquid.binder.could.be.removed.by.intensive.chemical.oxidation.by.30%.H 2 O 2 .
or. with. Ce(SO 4 ) 2 ,. K 2 CrO 4 ,. and. K 2 S 2 O 8 . in. 1.M. H 2 SO 4 . at. 80°C. for. about. 20.h.. Yet. more. effective.
oxidation.could.then.be.attained.with.the.aid.of.the.so-called. anodization ,.that.is,.controlled.elec-
trolytic.activation.at.very.high.potentials.(see.the.following)..The. alteration of the surface states