Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 59

as.−C-O − .or.=O − ,.accompanied.by.the.effect.of.the.nascent.hydrogen,.H 0 .[173,622]..The.entire.
activation.process.is.usually.performed.in.a.separate.solution.under.quiescent.conditions;.that.is,.
without.stirring..Some.authors.have.recommended.special.“activation.media,”.such.as.a.mixture.
of.0.1.M.KNO 3 .+.0.02.M.HNO 3 .+.0.01.M.Na 2 HPO 4 .[63].or.diluted.solutions.of.mineral.acids.(e.g.,.
0.001.or.0.005.M.HClO 4 .[108,261])..A.very.effective.activation.can.also.be.made.with.the.aid.of.
potential cycling ,.when.the.CP.surface.is.exposed.to.periodical.changes.of.highly.positive.and.
highly.negative.potentials.(e.g.,.[646]).
Alternate. to.electrolytic.activation. and. friendlier.with.respect. to.possible.overloads. in.electric.
circuit.due.to.too.intensively.anodized.electrodes.[188].is.the.chemical.treatment.of.the.CP.surface.
by.“erosion”. with surfactant(s) ,.studied.for.the.irst.time.by.Albahadily.and.Mottola.[64].and.elabo-
rated.in.more.detail.by.Kauffmann.et.al..[243]..The.respective.procedure.relies.on.one.important.condi-
tion—the.solution.with.the.surfactant.(e.g.,.quaternary.ammonium.salt.like.CTAB).should.be. free of
micelles .(not.having.the.characteristics.of.a.colloid)..Otherwise,.the.CP.surface.is.simply.immersed.
into.such.a.solution.and—with.or.without.stirring,.depending.on.experience—left.for.some.minutes.
to.be.treated..The.“alchemy”.behind.this.is.depicted.in.Figure.3.2.(on.the.right.side),.showing.that.
lipophilic. chains. of. surfactant. gradually. withdraw. the. binder. from. the. surface. and. that. this. ero-
sion.continues.until.the.treated.site.gets.“naked”.and.exposes.the.hydrophilic.surface.of.graphite.
particle(s).to.the.solution..In.addition,.this.activation.has.become.popular.in.electroanalysis,.with.
CPEs.applied.in.a.number.of.practical.methods—see.again.Figure.3.2.and.voltammograms.given,.
as.well.as.Chapter.8.and.the.table.surveys.with.inorganic,.organic,.or.even.biological.analytes.
There.is.yet.another.way.of.treating.the.carbon.paste.to.lower.the.undesirable.overpotential..
This.elegant.approach.is.the. addition of a modiier .into.the.paste,.in.this.case,.a.suitable. electro-
catalyst ..Among.substances.with.such.an.effect,.one.can.quote.organo-complexes.with.the.central.
atom.of.a.platinum.metal.(Rh II/III .[526,647],.Ru II/III .[648],.Os III/IV .[649,650].or.all.metals.from.the.
iron.group.(Fe II/III .[618],.Ni II .[651],.Co II .[652,653])..As.conirmed.in.numerous.studies.with.CPEs,.
these.special.modiiers.(discussed.in.detail.in.Section.4.3).may.lower.the.overpotential.down.
to.400.mV.[654],.500.mV.[655],.or.even.600.mV.[656],.which.is.scarcely.attainable.by.electrolytic.
activations..Moreover,.the.use.of.such.modiied.carbon.pastes.eliminates.two.major.drawbacks.of.
electrolytic.activations:.(i).rather.time-consuming.procedures.and.(ii).the.already.mentioned.over-
loading.of.the.apparatus,.especially,.of.some.older.instruments.that.do.not.have.adequate.protection.
against.abrupt.current.changes.and,.in.general,.a.high-current.low.
Finally,. the. activation. procedure. can. be. avoided. and. no. special. modiier. or. electro-catalyst.
needed.if.one.uses. commercial activated carbon .[631,632],.which.is.normally.available.(e.g.,. Norit
R3EX ).and.ready.for.the.preparation.of.the.paste.mixture..(The.chemical.activity.of.such.carbons,.
namely,.the.presence.of.hydrophilic.functional.groups.at.the.“industrially”.preactivated.surface,.can.
additionally.be.characterized.by.means.of.pH-metric.neutralization.titration.in.combination.with.
FT-IR.and.the.low-temperature.nitrogen.adsorption.method.[632].)
3.3 TESTING OF UNMODIFIED CPEs
Electrochemical.characteristics.as.well.as.some.physicochemical.properties.of.carbon.pastes.sur-
veyed.earlier.can.be,.to.a.certain.extent,.deined.using.empirically.proposed.sets.of.experiments—
by. means. of. the. testing of carbon pastes . [83,175,176].. The. corresponding. assays. usually. help. to.
reveal.typical.properties.of.carbon.paste(s).under.test,.thus.allowing.one.the.basic.orientation.in.a.
rather.variable.behavior.of.various.CPEs,.CMCPEs,.CP.biosensors,.and.other.related.sensors..In.
addition,. such. an. approach. often. represents. the. only. way. of. characterizing. freshly. made. carbon.
paste.mixtures,.including.brand.new.formulations.with.constituents.of.hitherto.unknown.properties.
Figure.3.3.illustrates.how.to.arrange.the.strategy.of.such.testing.measurements.in.the.individual.
sequences. in. a. step-by-step. manner.. This. lowchart . has. been. assembled. based. on. previous. expe-
rience. and. recommendations. of. numerous. authors,. namely,. [2,8,9,20,26,28,29,31,40,41,60,61,63,.
68,219,220],.and.styled.for. traditional (binary, unmodiied) carbon pastes ..The.whole.algorithm.has.