Carbon Paste as an Electrode Material - Electroanalysis with Carbon Paste Electrodes - page 70

Practically,. these. two. scenarios. can. occur. successively. or. concomitantly.. As. shown. in. Figure.
3.6,.the.overall.pathway.can.be.illustrated.in.two.sequences,.showing.irst.the.transfer.of.extracted.
objects.into.the.liquid.binder.(schemes.A—2,3).from.a.solution.in.which.they.have.previously.been.
dissolved.(scheme.A—3)..Afterward,.a.reverse.process.of.rediffusion.takes.place.(schemes.B—1,2),.
resulting.in.the.re-release.of.objects.of.interest.out.of.the.paste.and.back.to.the.solution.(schemes.
B—3).where.they.can.be.detected.
The. igure. also. tries. to. illustrate. the. dynamic electrode/solution interphase ,. depicting. that. a.
certain. amount. of. species. are. likely. to. remain. entrapped. in. the. carbon. paste. bulk. although. the.
rediffusion. process. has. been. completed.. Moreover,. Figure. 3.6. depicts. the. actual thickness . of.
the.CP.layer.involved.in.extraction,.which.is.shown.by.different.tone.of.color.used.for.the.binder.
moiety..The.whole.mechanism.sketched.earlier.corresponds.to.a.real.situation.when.extraction.is.
the.principle.of.an.electrochemical.stripping.method.
The.extracted.objects.can.be,.for.example,.ion.pairs.[205,639].or.organic.molecules.[219,221,749];.
any.of.them.must.however.be.neutral.(with.compensated.charges.or.noncharged,.respectively).since.
charged. ions. cannot. be. extracted. [157,748].. The. driving. force. for. the. rediffusion. process. is. then. a.
concentration.gradient.in.the.electrode.bulk.with.the.direction.to.the.electrode.surface.due.to.an.elec-
trochemical.transformation.of.the.species.there.during.the.measurement;.as.this.rediffusion.occurs.
during.the.measurement,.some.broadening.of.the.resulting.signal.may.be.explained.by.this.phenome-
non..Rough.attempts.to.determine.the.thickness.of.the.diffusion.layer.in.the.electrode.bulk.are.the.con-
trolled.removal.of.surface.layers.of.the.paste.before.starting.the.measurement;.through.the.removed.
volume.(as.calculated.by.the.removed.mass,.e.g.,.by.wiping.off.with.a.wet.paper,.and.the.speciic.mass.
of.the.paste).and.the.electrode.area,.it.is.possible.to.estimate.the.thickness.of.the.corresponding.layer.
Nevertheless,. after. rediffusion,. a. certain. portion. of. extracted. species. may. be. irreversibly. cap-
tured.inside.the.paste.[175,220]..This.is.a.typical.phenomenon.for.extractive.accumulations.at.CPEs,.
requiring.a.frequent.or.even.periodical.removal.of.the.used.layer.to.prevent.memory.effects.[175],.
which.is.accomplished.comfortably.with.piston-driven.CPEs.[31,219,243]..Extractive.accumulation.
and.the.subsequent.stripping.step.can.be.performed.in.two.different.solutions,.which.is.advantageous.
for.optimal.adjustment.of.pH.for.both.processes..Using.medium.exchange,.one.can.further.enhance.
the.overall.electroanalytical.performance.of.a.procedure.given,.especially,.its.selectivity.[750,751].
3.4.2.2 Chemical Equilibria in the Bulk and Transport Models
The. effectiveness. of. extraction. processes. is. driven. by. distribution equilibria . (i.e.,. by. the. ratio.
between.the.solubility.of.a.substance.and.its.reaction.product.in.the.pasting.liquid.and.in.the.phase.
used.for.the.measurement.[157,220])..Because.the.latter.is.usually.an.aqueous.solution,.it.becomes.
obvious.that.even.common.carbon.pastes.made.of.Nujol ® ,.oil.of.Uvasol ® .grade,.or.silicone.luids.
offer.nearly.ideal.conditions.for.extractive.accumulation.due.to.a.very.high.hydrophobicity.of.all.
these.binders.
More. exact. deinitions. of. extraction. processes. at. CPEs. have. been. of. continued. interest. (e.g.,.
[20,220,221,226,639,748]).. It. was. conirmed. that. the. eficiency. and. rate. of. extraction. depend. on.
the. mutual. afinity. between. the. binder. and. extracted. substance,. whose. chemical. structure. also.
plays.an.important.role..For.example,.the.presence.of.one.markedly.hydrophilic.functional.group.
(e.g.,.−NH 2 ,.−NO 2 ,.or.−OH).in.the.molecule.resulted.in.lowering.the.eficiency.down.to.a.few.tens.
of.percent,.and.even.partial.protonation.was.able.to.terminate.the.extraction.process.completely.
[20,220,732,748]..The.extraction.can.also.be.controlled.by.varying.the.carbon.paste.composition..
Besides.the.trivial.beneit.of.an.increasing.binder.ratio.in.the.paste,.one.could.observe.somewhat.
lower. extraction. capabilities. of. silicone. luid-based. carbon. pastes. compared. to. mixtures. made.
from.parafin.oil.[220],.which.was.explained.by.a.lower.adherence.of.the.highly.viscous.silicone.
grease.to.the.graphite.particles,.thus.enabling.a.concurrent.adsorption.at.the.uncovered.graphite.
surface.[20].
Based. on. the. experiments. performed. with. a. series. of. CPEs. containing. various. alkane. homo-
logues,.it.was.found.that.the.extraction.process.was.less.satisfactory.when.using.hydrocarbons.with.