Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 51

The.irst.two.carbon.pastes.(imaged.in.upper.row).that.have.been.prepared.from.the.same.spec-
troscopic. graphite. and. two. most. common. pasting. liquids. “copy”. faithfully. the. original. relief. of.
the.graphite.alone..(Such.structure.morphology.was.nicely.approached.by.Wang.et.al..[172].whose.
“romantic.idyll.at.the.carbon.paste.surface”.is.an.unprecedented.description.and.is.paraphrased.in.
Section.6.5.).In.contrast.to.both.C/Nj.and.C/SO.carbon.pastes,.the.third.mixture.(in.the.central.row,.
on.left).manifests.a.completely.different.structure.and.a.very.intimate.covering.of.graphite.by.liquid,.
giving.rise.to.compact.aggregates.that.might.be.behind.rather.speciic.electrochemical.behavior.of.
the.respective.carbon.paste.(C/TCP;.[205,254])..Another.image.(central.row,.right).is.apparently.the.
best.proof.for.the.legitimacy.of.Adams's.model,.showing.how.the.individual.graphite.spheres.are.
coated.by.the.binder;.many.of.them.are.“glued”.together.into.the.whole.clusters..(This.phenomenon.
is.yet.more.evident.if.one.compares.this.image.with.the.original.surface.relief.of.the.glassy.carbon.
powder.alone;.see.[96].and.SEM.photos.therein.or.Figure.2.2.in.this.text,.images.“A”.and.“B.”)..
Finally,.speciic.structures.of.CNT/Nj.and.C/IL.pastes.have.already.been.commented.
3.1.2 o hMiC r esisTanCe
Attractive.properties.of.carbon.pastes.in.electrochemical.measurements.are.also.given.by.their.high.
conductivity,.especially,.potentiometric.indication.merits.from.this.feature.with.respect.to.the.stable.
and. rapid. response. [157,159].. Ordinary. mixtures. with. parafin. or. silicone. oils. exhibit. the. ohmic.
resistance. of. about. 10. Ω. or. even. less. [7,26,175,228,229],. and. somewhat. higher. values. of. about.
50-200.Ω. can. then. be. measured. with. less. compact. carbon. pastes,. such. as. those. made. of. liquid.
tricresyl. phosphate. (TCP). [205,254]. or. based. on. glassy. carbon. powders. [95,96].. In. experiments.
with. analytical. concentrations. at. normal. levels,. variations. in. ohmic. resistance. can. be. neglected;.
nevertheless,.some.occasional.attempts.to.lower.the.carbon.paste.resistance.have.also.been.made.to.
minimize.undesirable.effects.of.the.ohmic.drop.[26]..In.context.with.this.report,.it.is.worth.men-
tioning.that.a.trend.of.the.increasing.resistance.in.carbon.pastes.has.been.explained.via.the.quality.
of.graphite.powder.used.without.emphasizing.the.role.of.liquid.binder..The.prevailing.effect.of.the.
carbon.moiety.to.the.total.ohmic.resistance.was.also.conirmed.in.later.studies,.again,.when.testing.
glassy.carbon.powders.with.special.surface.characteristics.[96,192,229].
Such.indings.were.quite.surprising.as.one.would.expect.a.more.important.role.of.the.binder..In.
fact,.for.a.long.time,.there.was.no.satisfactory.explanation.on.why.(almost.totally).insulating.binders.
contribute.so.little.to.the.overall.resistance.of.carbon.paste.mixtures,.including.blends.with.a.rather.
high.content.of.liquid.phase..For.example,.some.hypotheses.published.so.far.had.speculated.on.a.
tunnel.effect.known.for.semiconductors.or.on.a.special.role.of.the.binder.acting.as.a.totally.perme-
able.membrane.[82,157].
It.seems.that.these.uncertain.interpretations.were.deinitely.“buried”.recently,.when.two.new.
studies.[228,229].built.upon.an.extensive.database.of.specially.performed.experiments.with.two.
standard.carbon.pastes.(C/Nj.and.C/SO.types).and.one.novel.mixture.(SW-CNTs/SO).revealed.
the.close.relation.between.the.carbon.paste.composition(s).and.the.resultant.ohmic.resistance(s)..
Thus,.a.new.hypothesis.for.the.carbon.paste.microstructure.could.be.postulated.and.veriied,.
explaining. the. resultant. behavior. of. both. CPEs. and. CNTPE. by. means. of. the. model of the
tightest arrangement of solid spheres ,. in. approximation. applicable. also. to. compact. graphite.
particles,.and—rather.surprisingly—to.snake-like.carbon.nanotubes.(for.further.details,.go.into.
Section.3.3).
The. task. of. ohmic. resistance. is. also. closely. associated. with. some. latest. trends. in. the. electro-
chemistry. of. modern. sensors.. It. represents. an. example. when. some. advantages. of. classical. car-
bon.pastes.cannot.be.used.during.the.transformation.of.the.respective.procedures.with.CPEs.to.a.
concept.employing.functionally.identical.screen-printed.electrodes.(SPEs).[75,83]..Compared.with.
carbon.pastes,.solidiied.carbon.inks.in.SPEs.typically.possess.a.much.higher.resistance.(ranging.
from.kΩs.up.to.some.MΩs.[323,327,623]),.which.elucidates.why.some.experiments.with.CPEs.are.
still.superior.to.those.performed.using.SPEs.of.very.similar.or.even.the.same.construction.[295,623].